diff --git a/src/bootstrap/doc.rs b/src/bootstrap/doc.rs index 74b13144f2ff0..f4a667141665b 100644 --- a/src/bootstrap/doc.rs +++ b/src/bootstrap/doc.rs @@ -115,10 +115,6 @@ pub fn standalone(build: &Build, target: &str) { .arg("-o").arg(&out) .arg(&path); - if filename == "reference.md" { - cmd.arg("--html-in-header").arg(&full_toc); - } - if filename == "not_found.md" { cmd.arg("--markdown-no-toc") .arg("--markdown-css") diff --git a/src/bootstrap/step.rs b/src/bootstrap/step.rs index ee5b61062fed8..36738b81c189e 100644 --- a/src/bootstrap/step.rs +++ b/src/bootstrap/step.rs @@ -568,6 +568,15 @@ pub fn build_rules<'a>(build: &'a Build) -> Rules { }) .default(build.config.docs) .run(move |s| doc::rustbook(build, s.target, "nomicon")); + rules.doc("doc-reference", "src/doc/reference") + .dep(move |s| { + s.name("tool-rustbook") + .host(&build.config.build) + .target(&build.config.build) + .stage(0) + }) + .default(build.config.docs) + .run(move |s| doc::rustbook(build, s.target, "reference")); rules.doc("doc-standalone", "src/doc") .dep(move |s| { s.name("rustc") diff --git a/src/doc/book/src/attributes.md b/src/doc/book/src/attributes.md index 9e3cdb7ec0978..103ec39aa38a5 100644 --- a/src/doc/book/src/attributes.md +++ b/src/doc/book/src/attributes.md @@ -67,4 +67,4 @@ Rust attributes are used for a number of different things. There is a full list of attributes [in the reference][reference]. Currently, you are not allowed to create your own attributes, the Rust compiler defines them. -[reference]: ../reference.html#attributes +[reference]: ../reference/attributes.html diff --git a/src/doc/book/src/compiler-plugins.md b/src/doc/book/src/compiler-plugins.md index 1b7ce678982af..c05d808a94740 100644 --- a/src/doc/book/src/compiler-plugins.md +++ b/src/doc/book/src/compiler-plugins.md @@ -119,7 +119,7 @@ The advantages over a simple `fn(&str) -> u32` are: a way to define new literal syntax for any data type. In addition to procedural macros, you can define new -[`derive`](../reference.html#derive)-like attributes and other kinds of +[`derive`](../reference/attributes.html#derive)-like attributes and other kinds of extensions. See `Registry::register_syntax_extension` and the `SyntaxExtension` enum. For a more involved macro example, see [`regex_macros`](https://github.com/rust-lang/regex/blob/master/regex_macros/src/lib.rs). @@ -165,8 +165,8 @@ quasiquote as an ordinary plugin library. # Lint plugins Plugins can extend [Rust's lint -infrastructure](../reference.html#lint-check-attributes) with additional checks for -code style, safety, etc. Now let's write a plugin +infrastructure](../reference/attributes.html#lint-check-attributes) with +additional checks for code style, safety, etc. Now let's write a plugin [`lint_plugin_test.rs`](https://github.com/rust-lang/rust/blob/master/src/test/run-pass-fulldeps/auxiliary/lint_plugin_test.rs) that warns about any item named `lintme`. @@ -244,9 +244,10 @@ mostly use the same infrastructure as lint plugins, and provide examples of how to access type information. Lints defined by plugins are controlled by the usual [attributes and compiler -flags](../reference.html#lint-check-attributes), e.g. `#[allow(test_lint)]` or -`-A test-lint`. These identifiers are derived from the first argument to -`declare_lint!`, with appropriate case and punctuation conversion. +flags](../reference/attributes.html#lint-check-attributes), e.g. +`#[allow(test_lint)]` or `-A test-lint`. These identifiers are derived from the +first argument to `declare_lint!`, with appropriate case and punctuation +conversion. You can run `rustc -W help foo.rs` to see a list of lints known to `rustc`, including those provided by plugins loaded by `foo.rs`. diff --git a/src/doc/book/src/macros.md b/src/doc/book/src/macros.md index 861cb4371f9b4..93f63ddc0a562 100644 --- a/src/doc/book/src/macros.md +++ b/src/doc/book/src/macros.md @@ -101,7 +101,7 @@ trees, at compile time. The semicolon is optional on the last (here, only) case. The "pattern" on the left-hand side of `=>` is known as a ‘matcher’. These have [their own little grammar] within the language. -[their own little grammar]: ../reference.html#macros +[their own little grammar]: ../reference/macros.html The matcher `$x:expr` will match any Rust expression, binding that syntax tree to the ‘metavariable’ `$x`. The identifier `expr` is a ‘fragment specifier’; @@ -363,7 +363,7 @@ fn main() { } ``` -[items]: ../reference.html#items +[items]: ../reference/items.html # Recursive macros @@ -490,7 +490,7 @@ be forced to choose between parsing `$i` and parsing `$e`. Changing the invocation syntax to put a distinctive token in front can solve the problem. In this case, you can write `$(I $i:ident)* E $e:expr`. -[item]: ../reference.html#items +[item]: ../reference/items.html # Scoping and macro import/export @@ -565,7 +565,7 @@ When this library is loaded with `#[macro_use] extern crate`, only `m2` will be imported. The Rust Reference has a [listing of macro-related -attributes](../reference.html#macro-related-attributes). +attributes](../reference/attributes.html#macro-related-attributes). # The variable `$crate` diff --git a/src/doc/book/src/syntax-index.md b/src/doc/book/src/syntax-index.md index b2cd59c87af47..a06520f4ac2f3 100644 --- a/src/doc/book/src/syntax-index.md +++ b/src/doc/book/src/syntax-index.md @@ -235,10 +235,10 @@ [Primitive Types (Tuple Indexing)]: primitive-types.html#tuple-indexing [Primitive Types (Tuples)]: primitive-types.html#tuples [Raw Pointers]: raw-pointers.html -[Reference (Byte String Literals)]: ../reference.html#byte-string-literals -[Reference (Integer literals)]: ../reference.html#integer-literals -[Reference (Raw Byte String Literals)]: ../reference.html#raw-byte-string-literals -[Reference (Raw String Literals)]: ../reference.html#raw-string-literals +[Reference (Byte String Literals)]: ../reference/tokens.html/#byte-string-literals +[Reference (Integer literals)]: ../reference/tokens.html#integer-literals +[Reference (Raw Byte String Literals)]: ../reference/tokens.html#raw-byte-string-literals +[Reference (Raw String Literals)]: ../reference/tokens.html#raw-string-literals [References and Borrowing]: references-and-borrowing.html [Strings]: strings.html [Structs (Update syntax)]: structs.html#update-syntax diff --git a/src/doc/grammar.md b/src/doc/grammar.md index c81f2e2282b5e..8e803aff4d6fe 100644 --- a/src/doc/grammar.md +++ b/src/doc/grammar.md @@ -187,7 +187,7 @@ literal : [ string_lit | char_lit | byte_string_lit | byte_lit | num_lit | bool_ The optional `lit_suffix` production is only used for certain numeric literals, but is reserved for future extension. That is, the above gives the lexical grammar, but a Rust parser will reject everything but the 12 special cases -mentioned in [Number literals](reference.html#number-literals) in the +mentioned in [Number literals](reference/tokens.html#number-literals) in the reference. #### Character and string literals diff --git a/src/doc/index.md b/src/doc/index.md index 144b786f58d24..982e24ef69549 100644 --- a/src/doc/index.md +++ b/src/doc/index.md @@ -16,14 +16,6 @@ builds documentation for individual Rust packages. Rust provides a standard library with a number of features; [we host its documentation here][api]. -## Reference Documentation - -Rust does not yet have a formal specification, but we have [a reference document -][ref]. It is guaranteed to be accurate, but not complete. We now have a -policy that all new features must be included in the reference before -stabilization; however, we are still back-filling things that landed before -then. That work is being tracked [here][38643]. - ## Extended Error Documentation Many of Rust's errors come with error codes, and you can request extended @@ -37,11 +29,17 @@ nicknamed 'The Rust Bookshelf.' * [The Rust Programming Language][book] teaches you how to program in Rust. * [The Rustonomicon][nomicon] is your guidebook to the dark arts of unsafe Rust. +* [The Reference][ref] is not a formal spec, but is more detailed and comprehensive than the book. + +Another few words about the reference: it is guaranteed to be accurate, but not +complete. We now have a policy that all new features must be included in the +reference before stabilization; however, we are still back-filling things that +landed before then. That work is being tracked [here][38643]. [Rust Learning]: https://github.com/ctjhoa/rust-learning [Docs.rs]: https://docs.rs/ [api]: std/index.html -[ref]: reference.html +[ref]: reference/index.html [38643]: https://github.com/rust-lang/rust/issues/38643 [err]: error-index.html [book]: book/index.html diff --git a/src/doc/reference.md b/src/doc/reference.md index 15645fa9e31df..fdeea17ed1124 100644 --- a/src/doc/reference.md +++ b/src/doc/reference.md @@ -1,4411 +1,4 @@ -% The Rust Reference +% The Rust Reference has moved -# Introduction - -This document is the primary reference for the Rust programming language. It -provides three kinds of material: - - - Chapters that informally describe each language construct and their use. - - Chapters that informally describe the memory model, concurrency model, - runtime services, linkage model and debugging facilities. - - Appendix chapters providing rationale and references to languages that - influenced the design. - -This document does not serve as an introduction to the language. Background -familiarity with the language is assumed. A separate [book] is available to -help acquire such background familiarity. - -This document also does not serve as a reference to the [standard] library -included in the language distribution. Those libraries are documented -separately by extracting documentation attributes from their source code. Many -of the features that one might expect to be language features are library -features in Rust, so what you're looking for may be there, not here. - -Finally, this document is not normative. It may include details that are -specific to `rustc` itself, and should not be taken as a specification for -the Rust language. We intend to produce such a document someday, but this -is what we have for now. - -You may also be interested in the [grammar]. - -[book]: book/index.html -[standard]: std/index.html -[grammar]: grammar.html - -# Notation - -## Unicode productions - -A few productions in Rust's grammar permit Unicode code points outside the -ASCII range. We define these productions in terms of character properties -specified in the Unicode standard, rather than in terms of ASCII-range code -points. The grammar has a [Special Unicode Productions][unicodeproductions] -section that lists these productions. - -[unicodeproductions]: grammar.html#special-unicode-productions - -## String table productions - -Some rules in the grammar — notably [unary -operators](#unary-operator-expressions), [binary -operators](#binary-operator-expressions), and [keywords][keywords] — are -given in a simplified form: as a listing of a table of unquoted, printable -whitespace-separated strings. These cases form a subset of the rules regarding -the [token](#tokens) rule, and are assumed to be the result of a -lexical-analysis phase feeding the parser, driven by a DFA, operating over the -disjunction of all such string table entries. - -[keywords]: grammar.html#keywords - -When such a string enclosed in double-quotes (`"`) occurs inside the grammar, -it is an implicit reference to a single member of such a string table -production. See [tokens](#tokens) for more information. - -# Lexical structure - -## Input format - -Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8. -Most Rust grammar rules are defined in terms of printable ASCII-range -code points, but a small number are defined in terms of Unicode properties or -explicit code point lists. [^inputformat] - -[^inputformat]: Substitute definitions for the special Unicode productions are - provided to the grammar verifier, restricted to ASCII range, when verifying the - grammar in this document. - -## Identifiers - -An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form: - -[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature - gated. This is expected to improve soon. - -Either - - * The first character has property `XID_start` - * The remaining characters have property `XID_continue` - -Or - - * The first character is `_` - * The identifier is more than one character, `_` alone is not an identifier - * The remaining characters have property `XID_continue` - -that does _not_ occur in the set of [keywords][keywords]. - -> **Note**: `XID_start` and `XID_continue` as character properties cover the -> character ranges used to form the more familiar C and Java language-family -> identifiers. - -## Comments - -Comments in Rust code follow the general C++ style of line (`//`) and -block (`/* ... */`) comment forms. Nested block comments are supported. - -Line comments beginning with exactly _three_ slashes (`///`), and block -comments (`/** ... */`), are interpreted as a special syntax for `doc` -[attributes](#attributes). That is, they are equivalent to writing -`#[doc="..."]` around the body of the comment, i.e., `/// Foo` turns into -`#[doc="Foo"]`. - -Line comments beginning with `//!` and block comments `/*! ... */` are -doc comments that apply to the parent of the comment, rather than the item -that follows. That is, they are equivalent to writing `#![doc="..."]` around -the body of the comment. `//!` comments are usually used to document -modules that occupy a source file. - -Non-doc comments are interpreted as a form of whitespace. - -## Whitespace - -Whitespace is any non-empty string containing only characters that have the -`Pattern_White_Space` Unicode property, namely: - -- `U+0009` (horizontal tab, `'\t'`) -- `U+000A` (line feed, `'\n'`) -- `U+000B` (vertical tab) -- `U+000C` (form feed) -- `U+000D` (carriage return, `'\r'`) -- `U+0020` (space, `' '`) -- `U+0085` (next line) -- `U+200E` (left-to-right mark) -- `U+200F` (right-to-left mark) -- `U+2028` (line separator) -- `U+2029` (paragraph separator) - -Rust is a "free-form" language, meaning that all forms of whitespace serve only -to separate _tokens_ in the grammar, and have no semantic significance. - -A Rust program has identical meaning if each whitespace element is replaced -with any other legal whitespace element, such as a single space character. - -## Tokens - -Tokens are primitive productions in the grammar defined by regular -(non-recursive) languages. "Simple" tokens are given in [string table -production](#string-table-productions) form, and occur in the rest of the -grammar as double-quoted strings. Other tokens have exact rules given. - -### Literals - -A literal is an expression consisting of a single token, rather than a sequence -of tokens, that immediately and directly denotes the value it evaluates to, -rather than referring to it by name or some other evaluation rule. A literal is -a form of constant expression, so is evaluated (primarily) at compile time. - -#### Examples - -##### Characters and strings - -| | Example | `#` sets | Characters | Escapes | -|----------------------------------------------|-----------------|------------|-------------|---------------------| -| [Character](#character-literals) | `'H'` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) | -| [String](#string-literals) | `"hello"` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) | -| [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` | -| [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) | -| [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) | -| [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` | - -##### Byte escapes - -| | Name | -|---|------| -| `\x7F` | 8-bit character code (exactly 2 digits) | -| `\n` | Newline | -| `\r` | Carriage return | -| `\t` | Tab | -| `\\` | Backslash | -| `\0` | Null | - -##### Unicode escapes -| | Name | -|---|------| -| `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) | - -##### Quote escapes -| | Name | -|---|------| -| `\'` | Single quote | -| `\"` | Double quote | - -##### Numbers - -| [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes | -|----------------------------------------|---------|----------------|----------| -| Decimal integer | `98_222` | `N/A` | Integer suffixes | -| Hex integer | `0xff` | `N/A` | Integer suffixes | -| Octal integer | `0o77` | `N/A` | Integer suffixes | -| Binary integer | `0b1111_0000` | `N/A` | Integer suffixes | -| Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes | - -`*` All number literals allow `_` as a visual separator: `1_234.0E+18f64` - -##### Suffixes -| Integer | Floating-point | -|---------|----------------| -| `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, `usize` | `f32`, `f64` | - -#### Character and string literals - -##### Character literals - -A _character literal_ is a single Unicode character enclosed within two -`U+0027` (single-quote) characters, with the exception of `U+0027` itself, -which must be _escaped_ by a preceding `U+005C` character (`\`). - -##### String literals - -A _string literal_ is a sequence of any Unicode characters enclosed within two -`U+0022` (double-quote) characters, with the exception of `U+0022` itself, -which must be _escaped_ by a preceding `U+005C` character (`\`). - -Line-break characters are allowed in string literals. Normally they represent -themselves (i.e. no translation), but as a special exception, when an unescaped -`U+005C` character (`\`) occurs immediately before the newline (`U+000A`), the -`U+005C` character, the newline, and all whitespace at the beginning of the -next line are ignored. Thus `a` and `b` are equal: - -```rust -let a = "foobar"; -let b = "foo\ - bar"; - -assert_eq!(a,b); -``` - -##### Character escapes - -Some additional _escapes_ are available in either character or non-raw string -literals. An escape starts with a `U+005C` (`\`) and continues with one of the -following forms: - -* An _8-bit code point escape_ starts with `U+0078` (`x`) and is - followed by exactly two _hex digits_. It denotes the Unicode code point - equal to the provided hex value. -* A _24-bit code point escape_ starts with `U+0075` (`u`) and is followed - by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D` - (`}`). It denotes the Unicode code point equal to the provided hex value. -* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072` - (`r`), or `U+0074` (`t`), denoting the Unicode values `U+000A` (LF), - `U+000D` (CR) or `U+0009` (HT) respectively. -* The _null escape_ is the character `U+0030` (`0`) and denotes the Unicode - value `U+0000` (NUL). -* The _backslash escape_ is the character `U+005C` (`\`) which must be - escaped in order to denote *itself*. - -##### Raw string literals - -Raw string literals do not process any escapes. They start with the character -`U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a -`U+0022` (double-quote) character. The _raw string body_ can contain any sequence -of Unicode characters and is terminated only by another `U+0022` (double-quote) -character, followed by the same number of `U+0023` (`#`) characters that preceded -the opening `U+0022` (double-quote) character. - -All Unicode characters contained in the raw string body represent themselves, -the characters `U+0022` (double-quote) (except when followed by at least as -many `U+0023` (`#`) characters as were used to start the raw string literal) or -`U+005C` (`\`) do not have any special meaning. - -Examples for string literals: - -``` -"foo"; r"foo"; // foo -"\"foo\""; r#""foo""#; // "foo" - -"foo #\"# bar"; -r##"foo #"# bar"##; // foo #"# bar - -"\x52"; "R"; r"R"; // R -"\\x52"; r"\x52"; // \x52 -``` - -#### Byte and byte string literals - -##### Byte literals - -A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F` -range) or a single _escape_ preceded by the characters `U+0062` (`b`) and -`U+0027` (single-quote), and followed by the character `U+0027`. If the character -`U+0027` is present within the literal, it must be _escaped_ by a preceding -`U+005C` (`\`) character. It is equivalent to a `u8` unsigned 8-bit integer -_number literal_. - -##### Byte string literals - -A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_, -preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and -followed by the character `U+0022`. If the character `U+0022` is present within -the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character. -Alternatively, a byte string literal can be a _raw byte string literal_, defined -below. A byte string literal of length `n` is equivalent to a `&'static [u8; n]` borrowed fixed-sized array -of unsigned 8-bit integers. - -Some additional _escapes_ are available in either byte or non-raw byte string -literals. An escape starts with a `U+005C` (`\`) and continues with one of the -following forms: - -* A _byte escape_ escape starts with `U+0078` (`x`) and is - followed by exactly two _hex digits_. It denotes the byte - equal to the provided hex value. -* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072` - (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF), - `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively. -* The _null escape_ is the character `U+0030` (`0`) and denotes the byte - value `0x00` (ASCII NUL). -* The _backslash escape_ is the character `U+005C` (`\`) which must be - escaped in order to denote its ASCII encoding `0x5C`. - -##### Raw byte string literals - -Raw byte string literals do not process any escapes. They start with the -character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more -of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The -_raw string body_ can contain any sequence of ASCII characters and is terminated -only by another `U+0022` (double-quote) character, followed by the same number of -`U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) -character. A raw byte string literal can not contain any non-ASCII byte. - -All characters contained in the raw string body represent their ASCII encoding, -the characters `U+0022` (double-quote) (except when followed by at least as -many `U+0023` (`#`) characters as were used to start the raw string literal) or -`U+005C` (`\`) do not have any special meaning. - -Examples for byte string literals: - -``` -b"foo"; br"foo"; // foo -b"\"foo\""; br#""foo""#; // "foo" - -b"foo #\"# bar"; -br##"foo #"# bar"##; // foo #"# bar - -b"\x52"; b"R"; br"R"; // R -b"\\x52"; br"\x52"; // \x52 -``` - -#### Number literals - -A _number literal_ is either an _integer literal_ or a _floating-point -literal_. The grammar for recognizing the two kinds of literals is mixed. - -##### Integer literals - -An _integer literal_ has one of four forms: - -* A _decimal literal_ starts with a *decimal digit* and continues with any - mixture of *decimal digits* and _underscores_. -* A _hex literal_ starts with the character sequence `U+0030` `U+0078` - (`0x`) and continues as any mixture of hex digits and underscores. -* An _octal literal_ starts with the character sequence `U+0030` `U+006F` - (`0o`) and continues as any mixture of octal digits and underscores. -* A _binary literal_ starts with the character sequence `U+0030` `U+0062` - (`0b`) and continues as any mixture of binary digits and underscores. - -Like any literal, an integer literal may be followed (immediately, -without any spaces) by an _integer suffix_, which forcibly sets the -type of the literal. The integer suffix must be the name of one of the -integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, -`isize`, or `usize`. - -The type of an _unsuffixed_ integer literal is determined by type inference: - -* If an integer type can be _uniquely_ determined from the surrounding - program context, the unsuffixed integer literal has that type. - -* If the program context under-constrains the type, it defaults to the - signed 32-bit integer `i32`. - -* If the program context over-constrains the type, it is considered a - static type error. - -Examples of integer literals of various forms: - -``` -123i32; // type i32 -123u32; // type u32 -123_u32; // type u32 -0xff_u8; // type u8 -0o70_i16; // type i16 -0b1111_1111_1001_0000_i32; // type i32 -0usize; // type usize -``` - -Note that the Rust syntax considers `-1i8` as an application of the [unary minus -operator](#unary-operator-expressions) to an integer literal `1i8`, rather than -a single integer literal. - -##### Floating-point literals - -A _floating-point literal_ has one of two forms: - -* A _decimal literal_ followed by a period character `U+002E` (`.`). This is - optionally followed by another decimal literal, with an optional _exponent_. -* A single _decimal literal_ followed by an _exponent_. - -Like integer literals, a floating-point literal may be followed by a -suffix, so long as the pre-suffix part does not end with `U+002E` (`.`). -The suffix forcibly sets the type of the literal. There are two valid -_floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point -types), which explicitly determine the type of the literal. - -The type of an _unsuffixed_ floating-point literal is determined by -type inference: - -* If a floating-point type can be _uniquely_ determined from the - surrounding program context, the unsuffixed floating-point literal - has that type. - -* If the program context under-constrains the type, it defaults to `f64`. - -* If the program context over-constrains the type, it is considered a - static type error. - -Examples of floating-point literals of various forms: - -``` -123.0f64; // type f64 -0.1f64; // type f64 -0.1f32; // type f32 -12E+99_f64; // type f64 -let x: f64 = 2.; // type f64 -``` - -This last example is different because it is not possible to use the suffix -syntax with a floating point literal ending in a period. `2.f64` would attempt -to call a method named `f64` on `2`. - -The representation semantics of floating-point numbers are described in -["Machine Types"](#machine-types). - -#### Boolean literals - -The two values of the boolean type are written `true` and `false`. - -### Symbols - -Symbols are a general class of printable [tokens](#tokens) that play structural -roles in a variety of grammar productions. They are a -set of remaining miscellaneous printable tokens that do not -otherwise appear as [unary operators](#unary-operator-expressions), [binary -operators](#binary-operator-expressions), or [keywords][keywords]. -They are catalogued in [the Symbols section][symbols] of the Grammar document. - -[symbols]: grammar.html#symbols - - -## Paths - -A _path_ is a sequence of one or more path components _logically_ separated by -a namespace qualifier (`::`). If a path consists of only one component, it may -refer to either an [item](#items) or a [variable](#variables) in a local control -scope. If a path has multiple components, it refers to an item. - -Every item has a _canonical path_ within its crate, but the path naming an item -is only meaningful within a given crate. There is no global namespace across -crates; an item's canonical path merely identifies it within the crate. - -Two examples of simple paths consisting of only identifier components: - -```{.ignore} -x; -x::y::z; -``` - -Path components are usually [identifiers](#identifiers), but they may -also include angle-bracket-enclosed lists of type arguments. In -[expression](#expressions) context, the type argument list is given -after a `::` namespace qualifier in order to disambiguate it from a -relational expression involving the less-than symbol (`<`). In type -expression context, the final namespace qualifier is omitted. - -Two examples of paths with type arguments: - -``` -# struct HashMap(K,V); -# fn f() { -# fn id(t: T) -> T { t } -type T = HashMap; // Type arguments used in a type expression -let x = id::(10); // Type arguments used in a call expression -# } -``` - -Paths can be denoted with various leading qualifiers to change the meaning of -how it is resolved: - -* Paths starting with `::` are considered to be global paths where the - components of the path start being resolved from the crate root. Each - identifier in the path must resolve to an item. - -```rust -mod a { - pub fn foo() {} -} -mod b { - pub fn foo() { - ::a::foo(); // call a's foo function - } -} -# fn main() {} -``` - -* Paths starting with the keyword `super` begin resolution relative to the - parent module. Each further identifier must resolve to an item. - -```rust -mod a { - pub fn foo() {} -} -mod b { - pub fn foo() { - super::a::foo(); // call a's foo function - } -} -# fn main() {} -``` - -* Paths starting with the keyword `self` begin resolution relative to the - current module. Each further identifier must resolve to an item. - -```rust -fn foo() {} -fn bar() { - self::foo(); -} -# fn main() {} -``` - -Additionally keyword `super` may be repeated several times after the first -`super` or `self` to refer to ancestor modules. - -```rust -mod a { - fn foo() {} - - mod b { - mod c { - fn foo() { - super::super::foo(); // call a's foo function - self::super::super::foo(); // call a's foo function - } - } - } -} -# fn main() {} -``` - -# Macros - -A number of minor features of Rust are not central enough to have their own -syntax, and yet are not implementable as functions. Instead, they are given -names, and invoked through a consistent syntax: `some_extension!(...)`. - -Users of `rustc` can define new macros in two ways: - -* [Macros](book/macros.html) define new syntax in a higher-level, - declarative way. -* [Procedural Macros][procedural macros] can be used to implement custom derive. - -And one unstable way: [compiler plugins][plugin]. - -## Macros - -`macro_rules` allows users to define syntax extension in a declarative way. We -call such extensions "macros by example" or simply "macros". - -Currently, macros can expand to expressions, statements, items, or patterns. - -(A `sep_token` is any token other than `*` and `+`. A `non_special_token` is -any token other than a delimiter or `$`.) - -The macro expander looks up macro invocations by name, and tries each macro -rule in turn. It transcribes the first successful match. Matching and -transcription are closely related to each other, and we will describe them -together. - -### Macro By Example - -The macro expander matches and transcribes every token that does not begin with -a `$` literally, including delimiters. For parsing reasons, delimiters must be -balanced, but they are otherwise not special. - -In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust -syntax named by _designator_. Valid designators are: - -* `item`: an [item](#items) -* `block`: a [block](#block-expressions) -* `stmt`: a [statement](#statements) -* `pat`: a [pattern](#match-expressions) -* `expr`: an [expression](#expressions) -* `ty`: a [type](#types) -* `ident`: an [identifier](#identifiers) -* `path`: a [path](#paths) -* `tt`: a token tree (a single [token](#tokens) or a sequence of token trees surrounded - by matching `()`, `[]`, or `{}`) -* `meta`: the contents of an [attribute](#attributes) - -In the transcriber, the -designator is already known, and so only the name of a matched nonterminal comes -after the dollar sign. - -In both the matcher and transcriber, the Kleene star-like operator indicates -repetition. The Kleene star operator consists of `$` and parentheses, optionally -followed by a separator token, followed by `*` or `+`. `*` means zero or more -repetitions, `+` means at least one repetition. The parentheses are not matched or -transcribed. On the matcher side, a name is bound to _all_ of the names it -matches, in a structure that mimics the structure of the repetition encountered -on a successful match. The job of the transcriber is to sort that structure -out. - -The rules for transcription of these repetitions are called "Macro By Example". -Essentially, one "layer" of repetition is discharged at a time, and all of them -must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident -),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident -),* )` is acceptable (if trivial). - -When Macro By Example encounters a repetition, it examines all of the `$` -_name_ s that occur in its body. At the "current layer", they all must repeat -the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( -($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not -`(a,b,c ; d,e)`. The repetition walks through the choices at that layer in -lockstep, so the former input transcribes to `(a,d), (b,e), (c,f)`. - -Nested repetitions are allowed. - -### Parsing limitations - -The parser used by the macro system is reasonably powerful, but the parsing of -Rust syntax is restricted in two ways: - -1. Macro definitions are required to include suitable separators after parsing - expressions and other bits of the Rust grammar. This implies that - a macro definition like `$i:expr [ , ]` is not legal, because `[` could be part - of an expression. A macro definition like `$i:expr,` or `$i:expr;` would be legal, - however, because `,` and `;` are legal separators. See [RFC 550] for more information. -2. The parser must have eliminated all ambiguity by the time it reaches a `$` - _name_ `:` _designator_. This requirement most often affects name-designator - pairs when they occur at the beginning of, or immediately after, a `$(...)*`; - requiring a distinctive token in front can solve the problem. - -[RFC 550]: https://github.com/rust-lang/rfcs/blob/master/text/0550-macro-future-proofing.md - -## Procedural Macros - -"Procedural macros" are the second way to implement a macro. For now, the only -thing they can be used for is to implement derive on your own types. See -[the book][procedural macros] for a tutorial. - -Procedural macros involve a few different parts of the language and its -standard libraries. First is the `proc_macro` crate, included with Rust, -that defines an interface for building a procedural macro. The -`#[proc_macro_derive(Foo)]` attribute is used to mark the deriving -function. This function must have the type signature: - -```rust,ignore -use proc_macro::TokenStream; - -#[proc_macro_derive(Hello)] -pub fn hello_world(input: TokenStream) -> TokenStream -``` - -Finally, procedural macros must be in their own crate, with the `proc-macro` -crate type. - -# Crates and source files - -Although Rust, like any other language, can be implemented by an interpreter as -well as a compiler, the only existing implementation is a compiler, -and the language has -always been designed to be compiled. For these reasons, this section assumes a -compiler. - -Rust's semantics obey a *phase distinction* between compile-time and -run-time.[^phase-distinction] Semantic rules that have a *static -interpretation* govern the success or failure of compilation, while -semantic rules -that have a *dynamic interpretation* govern the behavior of the program at -run-time. - -[^phase-distinction]: This distinction would also exist in an interpreter. - Static checks like syntactic analysis, type checking, and lints should - happen before the program is executed regardless of when it is executed. - -The compilation model centers on artifacts called _crates_. Each compilation -processes a single crate in source form, and if successful, produces a single -crate in binary form: either an executable or some sort of -library.[^cratesourcefile] - -[^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the - ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit* - in the Owens and Flatt module system, or a *configuration* in Mesa. - -A _crate_ is a unit of compilation and linking, as well as versioning, -distribution and runtime loading. A crate contains a _tree_ of nested -[module](#modules) scopes. The top level of this tree is a module that is -anonymous (from the point of view of paths within the module) and any item -within a crate has a canonical [module path](#paths) denoting its location -within the crate's module tree. - -The Rust compiler is always invoked with a single source file as input, and -always produces a single output crate. The processing of that source file may -result in other source files being loaded as modules. Source files have the -extension `.rs`. - -A Rust source file describes a module, the name and location of which — -in the module tree of the current crate — are defined from outside the -source file: either by an explicit `mod_item` in a referencing source file, or -by the name of the crate itself. Every source file is a module, but not every -module needs its own source file: [module definitions](#modules) can be nested -within one file. - -Each source file contains a sequence of zero or more `item` definitions, and -may optionally begin with any number of [attributes](#items-and-attributes) -that apply to the containing module, most of which influence the behavior of -the compiler. The anonymous crate module can have additional attributes that -apply to the crate as a whole. - -```no_run -// Specify the crate name. -#![crate_name = "projx"] - -// Specify the type of output artifact. -#![crate_type = "lib"] - -// Turn on a warning. -// This can be done in any module, not just the anonymous crate module. -#![warn(non_camel_case_types)] -``` - -A crate that contains a `main` function can be compiled to an executable. If a -`main` function is present, its return type must be `()` -("[unit](#tuple-types)") and it must take no arguments. - -# Items and attributes - -Crates contain [items](#items), each of which may have some number of -[attributes](#attributes) attached to it. - -## Items - -An _item_ is a component of a crate. Items are organized within a crate by a -nested set of [modules](#modules). Every crate has a single "outermost" -anonymous module; all further items within the crate have [paths](#paths) -within the module tree of the crate. - -Items are entirely determined at compile-time, generally remain fixed during -execution, and may reside in read-only memory. - -There are several kinds of item: - -* [`extern crate` declarations](#extern-crate-declarations) -* [`use` declarations](#use-declarations) -* [modules](#modules) -* [function definitions](#functions) -* [`extern` blocks](#external-blocks) -* [type definitions](grammar.html#type-definitions) -* [struct definitions](#structs) -* [enumeration definitions](#enumerations) -* [constant items](#constant-items) -* [static items](#static-items) -* [trait definitions](#traits) -* [implementations](#implementations) - -Some items form an implicit scope for the declaration of sub-items. In other -words, within a function or module, declarations of items can (in many cases) -be mixed with the statements, control blocks, and similar artifacts that -otherwise compose the item body. The meaning of these scoped items is the same -as if the item was declared outside the scope — it is still a static item -— except that the item's *path name* within the module namespace is -qualified by the name of the enclosing item, or is private to the enclosing -item (in the case of functions). The grammar specifies the exact locations in -which sub-item declarations may appear. - -### Type Parameters - -All items except modules, constants and statics may be *parameterized* by type. -Type parameters are given as a comma-separated list of identifiers enclosed in -angle brackets (`<...>`), after the name of the item and before its definition. -The type parameters of an item are considered "part of the name", not part of -the type of the item. A referencing [path](#paths) must (in principle) provide -type arguments as a list of comma-separated types enclosed within angle -brackets, in order to refer to the type-parameterized item. In practice, the -type-inference system can usually infer such argument types from context. There -are no general type-parametric types, only type-parametric items. That is, Rust -has no notion of type abstraction: there are no higher-ranked (or "forall") types -abstracted over other types, though higher-ranked types do exist for lifetimes. - -### Modules - -A module is a container for zero or more [items](#items). - -A _module item_ is a module, surrounded in braces, named, and prefixed with the -keyword `mod`. A module item introduces a new, named module into the tree of -modules making up a crate. Modules can nest arbitrarily. - -An example of a module: - -``` -mod math { - type Complex = (f64, f64); - fn sin(f: f64) -> f64 { - /* ... */ -# panic!(); - } - fn cos(f: f64) -> f64 { - /* ... */ -# panic!(); - } - fn tan(f: f64) -> f64 { - /* ... */ -# panic!(); - } -} -``` - -Modules and types share the same namespace. Declaring a named type with -the same name as a module in scope is forbidden: that is, a type definition, -trait, struct, enumeration, or type parameter can't shadow the name of a module -in scope, or vice versa. - -A module without a body is loaded from an external file, by default with the -same name as the module, plus the `.rs` extension. When a nested submodule is -loaded from an external file, it is loaded from a subdirectory path that -mirrors the module hierarchy. - -```{.ignore} -// Load the `vec` module from `vec.rs` -mod vec; - -mod thread { - // Load the `local_data` module from `thread/local_data.rs` - // or `thread/local_data/mod.rs`. - mod local_data; -} -``` - -The directories and files used for loading external file modules can be -influenced with the `path` attribute. - -```{.ignore} -#[path = "thread_files"] -mod thread { - // Load the `local_data` module from `thread_files/tls.rs` - #[path = "tls.rs"] - mod local_data; -} -``` - -#### Extern crate declarations - -An _`extern crate` declaration_ specifies a dependency on an external crate. -The external crate is then bound into the declaring scope as the `ident` -provided in the `extern_crate_decl`. - -The external crate is resolved to a specific `soname` at compile time, and a -runtime linkage requirement to that `soname` is passed to the linker for -loading at runtime. The `soname` is resolved at compile time by scanning the -compiler's library path and matching the optional `crateid` provided against -the `crateid` attributes that were declared on the external crate when it was -compiled. If no `crateid` is provided, a default `name` attribute is assumed, -equal to the `ident` given in the `extern_crate_decl`. - -Three examples of `extern crate` declarations: - -```{.ignore} -extern crate pcre; - -extern crate std; // equivalent to: extern crate std as std; - -extern crate std as ruststd; // linking to 'std' under another name -``` - -When naming Rust crates, hyphens are disallowed. However, Cargo packages may -make use of them. In such case, when `Cargo.toml` doesn't specify a crate name, -Cargo will transparently replace `-` with `_` (Refer to [RFC 940] for more -details). - -Here is an example: - -```{.ignore} -// Importing the Cargo package hello-world -extern crate hello_world; // hyphen replaced with an underscore -``` - -[RFC 940]: https://github.com/rust-lang/rfcs/blob/master/text/0940-hyphens-considered-harmful.md - -#### Use declarations - -A _use declaration_ creates one or more local name bindings synonymous with -some other [path](#paths). Usually a `use` declaration is used to shorten the -path required to refer to a module item. These declarations may appear in -[modules](#modules) and [blocks](grammar.html#block-expressions), usually at the top. - -> **Note**: Unlike in many languages, -> `use` declarations in Rust do *not* declare linkage dependency with external crates. -> Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies. - -Use declarations support a number of convenient shortcuts: - -* Rebinding the target name as a new local name, using the syntax `use p::q::r as x;` -* Simultaneously binding a list of paths differing only in their final element, - using the glob-like brace syntax `use a::b::{c,d,e,f};` -* Binding all paths matching a given prefix, using the asterisk wildcard syntax - `use a::b::*;` -* Simultaneously binding a list of paths differing only in their final element - and their immediate parent module, using the `self` keyword, such as - `use a::b::{self, c, d};` - -An example of `use` declarations: - -```rust -use std::option::Option::{Some, None}; -use std::collections::hash_map::{self, HashMap}; - -fn foo(_: T){} -fn bar(map1: HashMap, map2: hash_map::HashMap){} - -fn main() { - // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64), - // std::option::Option::None]);' - foo(vec![Some(1.0f64), None]); - - // Both `hash_map` and `HashMap` are in scope. - let map1 = HashMap::new(); - let map2 = hash_map::HashMap::new(); - bar(map1, map2); -} -``` - -Like items, `use` declarations are private to the containing module, by -default. Also like items, a `use` declaration can be public, if qualified by -the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A -public `use` declaration can therefore _redirect_ some public name to a -different target definition: even a definition with a private canonical path, -inside a different module. If a sequence of such redirections form a cycle or -cannot be resolved unambiguously, they represent a compile-time error. - -An example of re-exporting: - -``` -# fn main() { } -mod quux { - pub use quux::foo::{bar, baz}; - - pub mod foo { - pub fn bar() { } - pub fn baz() { } - } -} -``` - -In this example, the module `quux` re-exports two public names defined in -`foo`. - -Also note that the paths contained in `use` items are relative to the crate -root. So, in the previous example, the `use` refers to `quux::foo::{bar, -baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level -module declarations should be at the crate root if direct usage of the declared -modules within `use` items is desired. It is also possible to use `self` and -`super` at the beginning of a `use` item to refer to the current and direct -parent modules respectively. All rules regarding accessing declared modules in -`use` declarations apply to both module declarations and `extern crate` -declarations. - -An example of what will and will not work for `use` items: - -``` -# #![allow(unused_imports)] -use foo::baz::foobaz; // good: foo is at the root of the crate - -mod foo { - - mod example { - pub mod iter {} - } - - use foo::example::iter; // good: foo is at crate root -// use example::iter; // bad: example is not at the crate root - use self::baz::foobaz; // good: self refers to module 'foo' - use foo::bar::foobar; // good: foo is at crate root - - pub mod bar { - pub fn foobar() { } - } - - pub mod baz { - use super::bar::foobar; // good: super refers to module 'foo' - pub fn foobaz() { } - } -} - -fn main() {} -``` - -### Functions - -A _function item_ defines a sequence of [statements](#statements) and a -final [expression](#expressions), along with a name and a set of -parameters. Other than a name, all these are optional. -Functions are declared with the keyword `fn`. Functions may declare a -set of *input* [*variables*](#variables) as parameters, through which the caller -passes arguments into the function, and the *output* [*type*](#types) -of the value the function will return to its caller on completion. - -A function may also be copied into a first-class *value*, in which case the -value has the corresponding [*function type*](#function-types), and can be used -otherwise exactly as a function item (with a minor additional cost of calling -the function indirectly). - -Every control path in a function logically ends with a `return` expression or a -diverging expression. If the outermost block of a function has a -value-producing expression in its final-expression position, that expression is -interpreted as an implicit `return` expression applied to the final-expression. - -An example of a function: - -``` -fn add(x: i32, y: i32) -> i32 { - x + y -} -``` - -As with `let` bindings, function arguments are irrefutable patterns, so any -pattern that is valid in a let binding is also valid as an argument. - -``` -fn first((value, _): (i32, i32)) -> i32 { value } -``` - - -#### Generic functions - -A _generic function_ allows one or more _parameterized types_ to appear in its -signature. Each type parameter must be explicitly declared in an -angle-bracket-enclosed and comma-separated list, following the function name. - -```rust,ignore -// foo is generic over A and B - -fn foo(x: A, y: B) { -``` - -Inside the function signature and body, the name of the type parameter can be -used as a type name. [Trait](#traits) bounds can be specified for type parameters -to allow methods with that trait to be called on values of that type. This is -specified using the `where` syntax: - -```rust,ignore -fn foo(x: T) where T: Debug { -``` - -When a generic function is referenced, its type is instantiated based on the -context of the reference. For example, calling the `foo` function here: - -``` -use std::fmt::Debug; - -fn foo(x: &[T]) where T: Debug { - // details elided - # () -} - -foo(&[1, 2]); -``` - -will instantiate type parameter `T` with `i32`. - -The type parameters can also be explicitly supplied in a trailing -[path](#paths) component after the function name. This might be necessary if -there is not sufficient context to determine the type parameters. For example, -`mem::size_of::() == 4`. - -#### Diverging functions - -A special kind of function can be declared with a `!` character where the -output type would normally be. For example: - -``` -fn my_err(s: &str) -> ! { - println!("{}", s); - panic!(); -} -``` - -We call such functions "diverging" because they never return a value to the -caller. Every control path in a diverging function must end with a `panic!()` or -a call to another diverging function on every control path. The `!` annotation -does *not* denote a type. - -It might be necessary to declare a diverging function because as mentioned -previously, the typechecker checks that every control path in a function ends -with a [`return`](#return-expressions) or diverging expression. So, if `my_err` -were declared without the `!` annotation, the following code would not -typecheck: - -``` -# fn my_err(s: &str) -> ! { panic!() } - -fn f(i: i32) -> i32 { - if i == 42 { - return 42; - } - else { - my_err("Bad number!"); - } -} -``` - -This will not compile without the `!` annotation on `my_err`, since the `else` -branch of the conditional in `f` does not return an `i32`, as required by the -signature of `f`. Adding the `!` annotation to `my_err` informs the -typechecker that, should control ever enter `my_err`, no further type judgments -about `f` need to hold, since control will never resume in any context that -relies on those judgments. Thus the return type on `f` only needs to reflect -the `if` branch of the conditional. - -#### Extern functions - -Extern functions are part of Rust's foreign function interface, providing the -opposite functionality to [external blocks](#external-blocks). Whereas -external blocks allow Rust code to call foreign code, extern functions with -bodies defined in Rust code _can be called by foreign code_. They are defined -in the same way as any other Rust function, except that they have the `extern` -modifier. - -``` -// Declares an extern fn, the ABI defaults to "C" -extern fn new_i32() -> i32 { 0 } - -// Declares an extern fn with "stdcall" ABI -extern "stdcall" fn new_i32_stdcall() -> i32 { 0 } -``` - -Unlike normal functions, extern fns have type `extern "ABI" fn()`. This is the -same type as the functions declared in an extern block. - -``` -# extern fn new_i32() -> i32 { 0 } -let fptr: extern "C" fn() -> i32 = new_i32; -``` - -Extern functions may be called directly from Rust code as Rust uses large, -contiguous stack segments like C. - -### Type aliases - -A _type alias_ defines a new name for an existing [type](#types). Type -aliases are declared with the keyword `type`. Every value has a single, -specific type, but may implement several different traits, or be compatible with -several different type constraints. - -For example, the following defines the type `Point` as a synonym for the type -`(u8, u8)`, the type of pairs of unsigned 8 bit integers: - -``` -type Point = (u8, u8); -let p: Point = (41, 68); -``` - -Currently a type alias to an enum type cannot be used to qualify the -constructors: - -``` -enum E { A } -type F = E; -let _: F = E::A; // OK -// let _: F = F::A; // Doesn't work -``` - -### Structs - -A _struct_ is a nominal [struct type](#struct-types) defined with the -keyword `struct`. - -An example of a `struct` item and its use: - -``` -struct Point {x: i32, y: i32} -let p = Point {x: 10, y: 11}; -let px: i32 = p.x; -``` - -A _tuple struct_ is a nominal [tuple type](#tuple-types), also defined with -the keyword `struct`. For example: - -``` -struct Point(i32, i32); -let p = Point(10, 11); -let px: i32 = match p { Point(x, _) => x }; -``` - -A _unit-like struct_ is a struct without any fields, defined by leaving off -the list of fields entirely. Such a struct implicitly defines a constant of -its type with the same name. For example: - -``` -struct Cookie; -let c = [Cookie, Cookie {}, Cookie, Cookie {}]; -``` - -is equivalent to - -``` -struct Cookie {} -const Cookie: Cookie = Cookie {}; -let c = [Cookie, Cookie {}, Cookie, Cookie {}]; -``` - -The precise memory layout of a struct is not specified. One can specify a -particular layout using the [`repr` attribute](#ffi-attributes). - -### Enumerations - -An _enumeration_ is a simultaneous definition of a nominal [enumerated -type](#enumerated-types) as well as a set of *constructors*, that can be used -to create or pattern-match values of the corresponding enumerated type. - -Enumerations are declared with the keyword `enum`. - -An example of an `enum` item and its use: - -``` -enum Animal { - Dog, - Cat, -} - -let mut a: Animal = Animal::Dog; -a = Animal::Cat; -``` - -Enumeration constructors can have either named or unnamed fields: - -```rust -enum Animal { - Dog (String, f64), - Cat { name: String, weight: f64 }, -} - -let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2); -a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 }; -``` - -In this example, `Cat` is a _struct-like enum variant_, -whereas `Dog` is simply called an enum variant. - -Each enum value has a _discriminant_ which is an integer associated to it. You -can specify it explicitly: - -``` -enum Foo { - Bar = 123, -} -``` - -The right hand side of the specification is interpreted as an `isize` value, -but the compiler is allowed to use a smaller type in the actual memory layout. -The [`repr` attribute](#ffi-attributes) can be added in order to change -the type of the right hand side and specify the memory layout. - -If a discriminant isn't specified, they start at zero, and add one for each -variant, in order. - -You can cast an enum to get its discriminant: - -``` -# enum Foo { Bar = 123 } -let x = Foo::Bar as u32; // x is now 123u32 -``` - -This only works as long as none of the variants have data attached. If -it were `Bar(i32)`, this is disallowed. - -### Constant items - -A *constant item* is a named _constant value_ which is not associated with a -specific memory location in the program. Constants are essentially inlined -wherever they are used, meaning that they are copied directly into the relevant -context when used. References to the same constant are not necessarily -guaranteed to refer to the same memory address. - -Constant values must not have destructors, and otherwise permit most forms of -data. Constants may refer to the address of other constants, in which case the -address will have elided lifetimes where applicable, otherwise – in most cases – -defaulting to the `static` lifetime. (See below on [static lifetime elision].) -The compiler is, however, still at liberty to translate the constant many times, -so the address referred to may not be stable. - -[static lifetime elision]: #static-lifetime-elision - -Constants must be explicitly typed. The type may be `bool`, `char`, a number, or -a type derived from those primitive types. The derived types are references with -the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs. - -```rust -const BIT1: u32 = 1 << 0; -const BIT2: u32 = 1 << 1; - -const BITS: [u32; 2] = [BIT1, BIT2]; -const STRING: &'static str = "bitstring"; - -struct BitsNStrings<'a> { - mybits: [u32; 2], - mystring: &'a str, -} - -const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings { - mybits: BITS, - mystring: STRING, -}; -``` - - - -### Static items - -A *static item* is similar to a *constant*, except that it represents a precise -memory location in the program. A static is never "inlined" at the usage site, -and all references to it refer to the same memory location. Static items have -the `static` lifetime, which outlives all other lifetimes in a Rust program. -Static items may be placed in read-only memory if they do not contain any -interior mutability. - -Statics may contain interior mutability through the `UnsafeCell` language item. -All access to a static is safe, but there are a number of restrictions on -statics: - -* Statics may not contain any destructors. -* The types of static values must ascribe to `Sync` to allow thread-safe access. -* Statics may not refer to other statics by value, only by reference. -* Constants cannot refer to statics. - -Constants should in general be preferred over statics, unless large amounts of -data are being stored, or single-address and mutability properties are required. - -#### Mutable statics - -If a static item is declared with the `mut` keyword, then it is allowed to -be modified by the program. One of Rust's goals is to make concurrency bugs -hard to run into, and this is obviously a very large source of race conditions -or other bugs. For this reason, an `unsafe` block is required when either -reading or writing a mutable static variable. Care should be taken to ensure -that modifications to a mutable static are safe with respect to other threads -running in the same process. - -Mutable statics are still very useful, however. They can be used with C -libraries and can also be bound from C libraries (in an `extern` block). - -```rust -# fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 } - -static mut LEVELS: u32 = 0; - -// This violates the idea of no shared state, and this doesn't internally -// protect against races, so this function is `unsafe` -unsafe fn bump_levels_unsafe1() -> u32 { - let ret = LEVELS; - LEVELS += 1; - return ret; -} - -// Assuming that we have an atomic_add function which returns the old value, -// this function is "safe" but the meaning of the return value may not be what -// callers expect, so it's still marked as `unsafe` -unsafe fn bump_levels_unsafe2() -> u32 { - return atomic_add(&mut LEVELS, 1); -} -``` - -Mutable statics have the same restrictions as normal statics, except that the -type of the value is not required to ascribe to `Sync`. - -#### `'static` lifetime elision - -[Unstable] Both constant and static declarations of reference types have -*implicit* `'static` lifetimes unless an explicit lifetime is specified. As -such, the constant declarations involving `'static` above may be written -without the lifetimes. Returning to our previous example: - -```rust -# #![feature(static_in_const)] -const BIT1: u32 = 1 << 0; -const BIT2: u32 = 1 << 1; - -const BITS: [u32; 2] = [BIT1, BIT2]; -const STRING: &str = "bitstring"; - -struct BitsNStrings<'a> { - mybits: [u32; 2], - mystring: &'a str, -} - -const BITS_N_STRINGS: BitsNStrings = BitsNStrings { - mybits: BITS, - mystring: STRING, -}; -``` - -Note that if the `static` or `const` items include function or closure -references, which themselves include references, the compiler will first try the -standard elision rules ([see discussion in the nomicon][elision-nomicon]). If it -is unable to resolve the lifetimes by its usual rules, it will default to using -the `'static` lifetime. By way of example: - -[elision-nomicon]: https://doc.rust-lang.org/nomicon/lifetime-elision.html - -```rust,ignore -// Resolved as `fn<'a>(&'a str) -> &'a str`. -const RESOLVED_SINGLE: fn(&str) -> &str = .. - -// Resolved as `Fn<'a, 'b, 'c>(&'a Foo, &'b Bar, &'c Baz) -> usize`. -const RESOLVED_MULTIPLE: Fn(&Foo, &Bar, &Baz) -> usize = .. - -// There is insufficient information to bound the return reference lifetime -// relative to the argument lifetimes, so the signature is resolved as -// `Fn(&'static Foo, &'static Bar) -> &'static Baz`. -const RESOLVED_STATIC: Fn(&Foo, &Bar) -> &Baz = .. -``` - -### Traits - -A _trait_ describes an abstract interface that types can -implement. This interface consists of associated items, which come in -three varieties: - -- functions -- constants -- types - -Associated functions whose first parameter is named `self` are called -methods and may be invoked using `.` notation (e.g., `x.foo()`). - -All traits define an implicit type parameter `Self` that refers to -"the type that is implementing this interface". Traits may also -contain additional type parameters. These type parameters (including -`Self`) may be constrained by other traits and so forth as usual. - -Trait bounds on `Self` are considered "supertraits". These are -required to be acyclic. Supertraits are somewhat different from other -constraints in that they affect what methods are available in the -vtable when the trait is used as a [trait object](#trait-objects). - -Traits are implemented for specific types through separate -[implementations](#implementations). - -Consider the following trait: - -``` -# type Surface = i32; -# type BoundingBox = i32; -trait Shape { - fn draw(&self, Surface); - fn bounding_box(&self) -> BoundingBox; -} -``` - -This defines a trait with two methods. All values that have -[implementations](#implementations) of this trait in scope can have their -`draw` and `bounding_box` methods called, using `value.bounding_box()` -[syntax](#method-call-expressions). - -Traits can include default implementations of methods, as in: - -``` -trait Foo { - fn bar(&self); - fn baz(&self) { println!("We called baz."); } -} -``` - -Here the `baz` method has a default implementation, so types that implement -`Foo` need only implement `bar`. It is also possible for implementing types -to override a method that has a default implementation. - -Type parameters can be specified for a trait to make it generic. These appear -after the trait name, using the same syntax used in [generic -functions](#generic-functions). - -``` -trait Seq { - fn len(&self) -> u32; - fn elt_at(&self, n: u32) -> T; - fn iter(&self, F) where F: Fn(T); -} -``` - -It is also possible to define associated types for a trait. Consider the -following example of a `Container` trait. Notice how the type is available -for use in the method signatures: - -``` -trait Container { - type E; - fn empty() -> Self; - fn insert(&mut self, Self::E); -} -``` - -In order for a type to implement this trait, it must not only provide -implementations for every method, but it must specify the type `E`. Here's -an implementation of `Container` for the standard library type `Vec`: - -``` -# trait Container { -# type E; -# fn empty() -> Self; -# fn insert(&mut self, Self::E); -# } -impl Container for Vec { - type E = T; - fn empty() -> Vec { Vec::new() } - fn insert(&mut self, x: T) { self.push(x); } -} -``` - -Generic functions may use traits as _bounds_ on their type parameters. This -will have two effects: - -- Only types that have the trait may instantiate the parameter. -- Within the generic function, the methods of the trait can be - called on values that have the parameter's type. - -For example: - -``` -# type Surface = i32; -# trait Shape { fn draw(&self, Surface); } -fn draw_twice(surface: Surface, sh: T) { - sh.draw(surface); - sh.draw(surface); -} -``` - -Traits also define a [trait object](#trait-objects) with the same -name as the trait. Values of this type are created by coercing from a -pointer of some specific type to a pointer of trait type. For example, -`&T` could be coerced to `&Shape` if `T: Shape` holds (and similarly -for `Box`). This coercion can either be implicit or -[explicit](#type-cast-expressions). Here is an example of an explicit -coercion: - -``` -trait Shape { } -impl Shape for i32 { } -let mycircle = 0i32; -let myshape: Box = Box::new(mycircle) as Box; -``` - -The resulting value is a box containing the value that was cast, along with -information that identifies the methods of the implementation that was used. -Values with a trait type can have [methods called](#method-call-expressions) on -them, for any method in the trait, and can be used to instantiate type -parameters that are bounded by the trait. - -Trait methods may be static, which means that they lack a `self` argument. -This means that they can only be called with function call syntax (`f(x)`) and -not method call syntax (`obj.f()`). The way to refer to the name of a static -method is to qualify it with the trait name, treating the trait name like a -module. For example: - -``` -trait Num { - fn from_i32(n: i32) -> Self; -} -impl Num for f64 { - fn from_i32(n: i32) -> f64 { n as f64 } -} -let x: f64 = Num::from_i32(42); -``` - -Traits may inherit from other traits. Consider the following example: - -``` -trait Shape { fn area(&self) -> f64; } -trait Circle : Shape { fn radius(&self) -> f64; } -``` - -The syntax `Circle : Shape` means that types that implement `Circle` must also -have an implementation for `Shape`. Multiple supertraits are separated by `+`, -`trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a -given type `T`, methods can refer to `Shape` methods, since the typechecker -checks that any type with an implementation of `Circle` also has an -implementation of `Shape`: - -```rust -struct Foo; - -trait Shape { fn area(&self) -> f64; } -trait Circle : Shape { fn radius(&self) -> f64; } -impl Shape for Foo { - fn area(&self) -> f64 { - 0.0 - } -} -impl Circle for Foo { - fn radius(&self) -> f64 { - println!("calling area: {}", self.area()); - - 0.0 - } -} - -let c = Foo; -c.radius(); -``` - -In type-parameterized functions, methods of the supertrait may be called on -values of subtrait-bound type parameters. Referring to the previous example of -`trait Circle : Shape`: - -``` -# trait Shape { fn area(&self) -> f64; } -# trait Circle : Shape { fn radius(&self) -> f64; } -fn radius_times_area(c: T) -> f64 { - // `c` is both a Circle and a Shape - c.radius() * c.area() -} -``` - -Likewise, supertrait methods may also be called on trait objects. - -```{.ignore} -# trait Shape { fn area(&self) -> f64; } -# trait Circle : Shape { fn radius(&self) -> f64; } -# impl Shape for i32 { fn area(&self) -> f64 { 0.0 } } -# impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } } -# let mycircle = 0i32; -let mycircle = Box::new(mycircle) as Box; -let nonsense = mycircle.radius() * mycircle.area(); -``` - -### Implementations - -An _implementation_ is an item that implements a [trait](#traits) for a -specific type. - -Implementations are defined with the keyword `impl`. - -``` -# #[derive(Copy, Clone)] -# struct Point {x: f64, y: f64}; -# type Surface = i32; -# struct BoundingBox {x: f64, y: f64, width: f64, height: f64}; -# trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; } -# fn do_draw_circle(s: Surface, c: Circle) { } -struct Circle { - radius: f64, - center: Point, -} - -impl Copy for Circle {} - -impl Clone for Circle { - fn clone(&self) -> Circle { *self } -} - -impl Shape for Circle { - fn draw(&self, s: Surface) { do_draw_circle(s, *self); } - fn bounding_box(&self) -> BoundingBox { - let r = self.radius; - BoundingBox { - x: self.center.x - r, - y: self.center.y - r, - width: 2.0 * r, - height: 2.0 * r, - } - } -} -``` - -It is possible to define an implementation without referring to a trait. The -methods in such an implementation can only be used as direct calls on the values -of the type that the implementation targets. In such an implementation, the -trait type and `for` after `impl` are omitted. Such implementations are limited -to nominal types (enums, structs, trait objects), and the implementation must -appear in the same crate as the `self` type: - -``` -struct Point {x: i32, y: i32} - -impl Point { - fn log(&self) { - println!("Point is at ({}, {})", self.x, self.y); - } -} - -let my_point = Point {x: 10, y:11}; -my_point.log(); -``` - -When a trait _is_ specified in an `impl`, all methods declared as part of the -trait must be implemented, with matching types and type parameter counts. - -An implementation can take type parameters, which can be different from the -type parameters taken by the trait it implements. Implementation parameters -are written after the `impl` keyword. - -``` -# trait Seq { fn dummy(&self, _: T) { } } -impl Seq for Vec { - /* ... */ -} -impl Seq for u32 { - /* Treat the integer as a sequence of bits */ -} -``` - -### External blocks - -External blocks form the basis for Rust's foreign function interface. -Declarations in an external block describe symbols in external, non-Rust -libraries. - -Functions within external blocks are declared in the same way as other Rust -functions, with the exception that they may not have a body and are instead -terminated by a semicolon. - -Functions within external blocks may be called by Rust code, just like -functions defined in Rust. The Rust compiler automatically translates between -the Rust ABI and the foreign ABI. - -Functions within external blocks may be variadic by specifying `...` after one -or more named arguments in the argument list: - -```ignore -extern { - fn foo(x: i32, ...); -} -``` - -A number of [attributes](#ffi-attributes) control the behavior of external blocks. - -By default external blocks assume that the library they are calling uses the -standard C ABI on the specific platform. Other ABIs may be specified using an -`abi` string, as shown here: - -```ignore -// Interface to the Windows API -extern "stdcall" { } -``` - -There are three ABI strings which are cross-platform, and which all compilers -are guaranteed to support: - -* `extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any - Rust code. -* `extern "C"` -- This is the same as `extern fn foo()`; whatever the default - your C compiler supports. -* `extern "system"` -- Usually the same as `extern "C"`, except on Win32, in - which case it's `"stdcall"`, or what you should use to link to the Windows API - itself - -There are also some platform-specific ABI strings: - -* `extern "cdecl"` -- The default for x86\_32 C code. -* `extern "stdcall"` -- The default for the Win32 API on x86\_32. -* `extern "win64"` -- The default for C code on x86\_64 Windows. -* `extern "sysv64"` -- The default for C code on non-Windows x86\_64. -* `extern "aapcs"` -- The default for ARM. -* `extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's - `__fastcall` and GCC and clang's `__attribute__((fastcall))` -* `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's - `__vectorcall` and clang's `__attribute__((vectorcall))` - -Finally, there are some rustc-specific ABI strings: - -* `extern "rust-intrinsic"` -- The ABI of rustc intrinsics. -* `extern "rust-call"` -- The ABI of the Fn::call trait functions. -* `extern "platform-intrinsic"` -- Specific platform intrinsics -- like, for - example, `sqrt` -- have this ABI. You should never have to deal with it. - -The `link` attribute allows the name of the library to be specified. When -specified the compiler will attempt to link against the native library of the -specified name. - -```{.ignore} -#[link(name = "crypto")] -extern { } -``` - -The type of a function declared in an extern block is `extern "abi" fn(A1, ..., -An) -> R`, where `A1...An` are the declared types of its arguments and `R` is -the declared return type. - -It is valid to add the `link` attribute on an empty extern block. You can use -this to satisfy the linking requirements of extern blocks elsewhere in your code -(including upstream crates) instead of adding the attribute to each extern block. - -## Visibility and Privacy - -These two terms are often used interchangeably, and what they are attempting to -convey is the answer to the question "Can this item be used at this location?" - -Rust's name resolution operates on a global hierarchy of namespaces. Each level -in the hierarchy can be thought of as some item. The items are one of those -mentioned above, but also include external crates. Declaring or defining a new -module can be thought of as inserting a new tree into the hierarchy at the -location of the definition. - -To control whether interfaces can be used across modules, Rust checks each use -of an item to see whether it should be allowed or not. This is where privacy -warnings are generated, or otherwise "you used a private item of another module -and weren't allowed to." - -By default, everything in Rust is *private*, with two exceptions: Associated -items in a `pub` Trait are public by default; Enum variants -in a `pub` enum are also public by default. When an item is declared as `pub`, -it can be thought of as being accessible to the outside world. For example: - -``` -# fn main() {} -// Declare a private struct -struct Foo; - -// Declare a public struct with a private field -pub struct Bar { - field: i32, -} - -// Declare a public enum with two public variants -pub enum State { - PubliclyAccessibleState, - PubliclyAccessibleState2, -} -``` - -With the notion of an item being either public or private, Rust allows item -accesses in two cases: - -1. If an item is public, then it can be used externally through any of its - public ancestors. -2. If an item is private, it may be accessed by the current module and its - descendants. - -These two cases are surprisingly powerful for creating module hierarchies -exposing public APIs while hiding internal implementation details. To help -explain, here's a few use cases and what they would entail: - -* A library developer needs to expose functionality to crates which link - against their library. As a consequence of the first case, this means that - anything which is usable externally must be `pub` from the root down to the - destination item. Any private item in the chain will disallow external - accesses. - -* A crate needs a global available "helper module" to itself, but it doesn't - want to expose the helper module as a public API. To accomplish this, the - root of the crate's hierarchy would have a private module which then - internally has a "public API". Because the entire crate is a descendant of - the root, then the entire local crate can access this private module through - the second case. - -* When writing unit tests for a module, it's often a common idiom to have an - immediate child of the module to-be-tested named `mod test`. This module - could access any items of the parent module through the second case, meaning - that internal implementation details could also be seamlessly tested from the - child module. - -In the second case, it mentions that a private item "can be accessed" by the -current module and its descendants, but the exact meaning of accessing an item -depends on what the item is. Accessing a module, for example, would mean -looking inside of it (to import more items). On the other hand, accessing a -function would mean that it is invoked. Additionally, path expressions and -import statements are considered to access an item in the sense that the -import/expression is only valid if the destination is in the current visibility -scope. - -Here's an example of a program which exemplifies the three cases outlined -above: - -``` -// This module is private, meaning that no external crate can access this -// module. Because it is private at the root of this current crate, however, any -// module in the crate may access any publicly visible item in this module. -mod crate_helper_module { - - // This function can be used by anything in the current crate - pub fn crate_helper() {} - - // This function *cannot* be used by anything else in the crate. It is not - // publicly visible outside of the `crate_helper_module`, so only this - // current module and its descendants may access it. - fn implementation_detail() {} -} - -// This function is "public to the root" meaning that it's available to external -// crates linking against this one. -pub fn public_api() {} - -// Similarly to 'public_api', this module is public so external crates may look -// inside of it. -pub mod submodule { - use crate_helper_module; - - pub fn my_method() { - // Any item in the local crate may invoke the helper module's public - // interface through a combination of the two rules above. - crate_helper_module::crate_helper(); - } - - // This function is hidden to any module which is not a descendant of - // `submodule` - fn my_implementation() {} - - #[cfg(test)] - mod test { - - #[test] - fn test_my_implementation() { - // Because this module is a descendant of `submodule`, it's allowed - // to access private items inside of `submodule` without a privacy - // violation. - super::my_implementation(); - } - } -} - -# fn main() {} -``` - -For a Rust program to pass the privacy checking pass, all paths must be valid -accesses given the two rules above. This includes all use statements, -expressions, types, etc. - -### Re-exporting and Visibility - -Rust allows publicly re-exporting items through a `pub use` directive. Because -this is a public directive, this allows the item to be used in the current -module through the rules above. It essentially allows public access into the -re-exported item. For example, this program is valid: - -``` -pub use self::implementation::api; - -mod implementation { - pub mod api { - pub fn f() {} - } -} - -# fn main() {} -``` - -This means that any external crate referencing `implementation::api::f` would -receive a privacy violation, while the path `api::f` would be allowed. - -When re-exporting a private item, it can be thought of as allowing the "privacy -chain" being short-circuited through the reexport instead of passing through -the namespace hierarchy as it normally would. - -## Attributes - -Any item declaration may have an _attribute_ applied to it. Attributes in Rust -are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 -(C#). An attribute is a general, free-form metadatum that is interpreted -according to name, convention, and language and compiler version. Attributes -may appear as any of: - -* A single identifier, the attribute name -* An identifier followed by the equals sign '=' and a literal, providing a - key/value pair -* An identifier followed by a parenthesized list of sub-attribute arguments - -Attributes with a bang ("!") after the hash ("#") apply to the item that the -attribute is declared within. Attributes that do not have a bang after the hash -apply to the item that follows the attribute. - -An example of attributes: - -```{.rust} -// General metadata applied to the enclosing module or crate. -#![crate_type = "lib"] - -// A function marked as a unit test -#[test] -fn test_foo() { - /* ... */ -} - -// A conditionally-compiled module -#[cfg(target_os="linux")] -mod bar { - /* ... */ -} - -// A lint attribute used to suppress a warning/error -#[allow(non_camel_case_types)] -type int8_t = i8; -``` - -> **Note:** At some point in the future, the compiler will distinguish between -> language-reserved and user-available attributes. Until then, there is -> effectively no difference between an attribute handled by a loadable syntax -> extension and the compiler. - -### Crate-only attributes - -- `crate_name` - specify the crate's crate name. -- `crate_type` - see [linkage](#linkage). -- `feature` - see [compiler features](#compiler-features). -- `no_builtins` - disable optimizing certain code patterns to invocations of - library functions that are assumed to exist -- `no_main` - disable emitting the `main` symbol. Useful when some other - object being linked to defines `main`. -- `no_start` - disable linking to the `native` crate, which specifies the - "start" language item. -- `no_std` - disable linking to the `std` crate. -- `plugin` - load a list of named crates as compiler plugins, e.g. - `#![plugin(foo, bar)]`. Optional arguments for each plugin, - i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's - registrar function. The `plugin` feature gate is required to use - this attribute. -- `recursion_limit` - Sets the maximum depth for potentially - infinitely-recursive compile-time operations like - auto-dereference or macro expansion. The default is - `#![recursion_limit="64"]`. - -### Module-only attributes - -- `no_implicit_prelude` - disable injecting `use std::prelude::*` in this - module. -- `path` - specifies the file to load the module from. `#[path="foo.rs"] mod - bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is - taken relative to the directory that the current module is in. - -### Function-only attributes - -- `main` - indicates that this function should be passed to the entry point, - rather than the function in the crate root named `main`. -- `plugin_registrar` - mark this function as the registration point for - [compiler plugins][plugin], such as loadable syntax extensions. -- `start` - indicates that this function should be used as the entry point, - overriding the "start" language item. See the "start" [language - item](#language-items) for more details. -- `test` - indicates that this function is a test function, to only be compiled - in case of `--test`. -- `should_panic` - indicates that this test function should panic, inverting the success condition. -- `cold` - The function is unlikely to be executed, so optimize it (and calls - to it) differently. -- `naked` - The function utilizes a custom ABI or custom inline ASM that requires - epilogue and prologue to be skipped. - -### Static-only attributes - -- `thread_local` - on a `static mut`, this signals that the value of this - static may change depending on the current thread. The exact consequences of - this are implementation-defined. - -### FFI attributes - -On an `extern` block, the following attributes are interpreted: - -- `link_args` - specify arguments to the linker, rather than just the library - name and type. This is feature gated and the exact behavior is - implementation-defined (due to variety of linker invocation syntax). -- `link` - indicate that a native library should be linked to for the - declarations in this block to be linked correctly. `link` supports an optional - `kind` key with three possible values: `dylib`, `static`, and `framework`. See - [external blocks](#external-blocks) for more about external blocks. Two - examples: `#[link(name = "readline")]` and - `#[link(name = "CoreFoundation", kind = "framework")]`. -- `linked_from` - indicates what native library this block of FFI items is - coming from. This attribute is of the form `#[linked_from = "foo"]` where - `foo` is the name of a library in either `#[link]` or a `-l` flag. This - attribute is currently required to export symbols from a Rust dynamic library - on Windows, and it is feature gated behind the `linked_from` feature. - -On declarations inside an `extern` block, the following attributes are -interpreted: - -- `link_name` - the name of the symbol that this function or static should be - imported as. -- `linkage` - on a static, this specifies the [linkage - type](http://llvm.org/docs/LangRef.html#linkage-types). - -On `enum`s: - -- `repr` - on C-like enums, this sets the underlying type used for - representation. Takes one argument, which is the primitive - type this enum should be represented for, or `C`, which specifies that it - should be the default `enum` size of the C ABI for that platform. Note that - enum representation in C is undefined, and this may be incorrect when the C - code is compiled with certain flags. - -On `struct`s: - -- `repr` - specifies the representation to use for this struct. Takes a list - of options. The currently accepted ones are `C` and `packed`, which may be - combined. `C` will use a C ABI compatible struct layout, and `packed` will - remove any padding between fields (note that this is very fragile and may - break platforms which require aligned access). - -### Macro-related attributes - -- `macro_use` on a `mod` — macros defined in this module will be visible in the - module's parent, after this module has been included. - -- `macro_use` on an `extern crate` — load macros from this crate. An optional - list of names `#[macro_use(foo, bar)]` restricts the import to just those - macros named. The `extern crate` must appear at the crate root, not inside - `mod`, which ensures proper function of the [`$crate` macro - variable](book/macros.html#the-variable-crate). - -- `macro_reexport` on an `extern crate` — re-export the named macros. - -- `macro_export` - export a macro for cross-crate usage. - -- `no_link` on an `extern crate` — even if we load this crate for macros, don't - link it into the output. - -See the [macros section of the -book](book/macros.html#scoping-and-macro-importexport) for more information on -macro scope. - - -### Miscellaneous attributes - -- `deprecated` - mark the item as deprecated; the full attribute is - `#[deprecated(since = "crate version", note = "...")`, where both arguments - are optional. -- `export_name` - on statics and functions, this determines the name of the - exported symbol. -- `link_section` - on statics and functions, this specifies the section of the - object file that this item's contents will be placed into. -- `no_mangle` - on any item, do not apply the standard name mangling. Set the - symbol for this item to its identifier. -- `simd` - on certain tuple structs, derive the arithmetic operators, which - lower to the target's SIMD instructions, if any; the `simd` feature gate - is necessary to use this attribute. -- `unsafe_destructor_blind_to_params` - on `Drop::drop` method, asserts that the - destructor code (and all potential specializations of that code) will - never attempt to read from nor write to any references with lifetimes - that come in via generic parameters. This is a constraint we cannot - currently express via the type system, and therefore we rely on the - programmer to assert that it holds. Adding this to a Drop impl causes - the associated destructor to be considered "uninteresting" by the - Drop-Check rule, and thus it can help sidestep data ordering - constraints that would otherwise be introduced by the Drop-Check - rule. Such sidestepping of the constraints, if done incorrectly, can - lead to undefined behavior (in the form of reading or writing to data - outside of its dynamic extent), and thus this attribute has the word - "unsafe" in its name. To use this, the - `unsafe_destructor_blind_to_params` feature gate must be enabled. -- `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`. -- `rustc_on_unimplemented` - Write a custom note to be shown along with the error - when the trait is found to be unimplemented on a type. - You may use format arguments like `{T}`, `{A}` to correspond to the - types at the point of use corresponding to the type parameters of the - trait of the same name. `{Self}` will be replaced with the type that is supposed - to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate - must be enabled. -- `must_use` - on structs and enums, will warn if a value of this type isn't used or - assigned to a variable. You may also include an optional message by using - `#[must_use = "message"]` which will be given alongside the warning. - -### Conditional compilation - -Sometimes one wants to have different compiler outputs from the same code, -depending on build target, such as targeted operating system, or to enable -release builds. - -Configuration options are boolean (on or off) and are named either with a -single identifier (e.g. `foo`) or an identifier and a string (e.g. `foo = "bar"`; -the quotes are required and spaces around the `=` are unimportant). Note that -similarly-named options, such as `foo`, `foo="bar"` and `foo="baz"` may each be set -or unset independently. - -Configuration options are either provided by the compiler or passed in on the -command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`). -Rust code then checks for their presence using the `#[cfg(...)]` attribute: - -``` -// The function is only included in the build when compiling for OSX -#[cfg(target_os = "macos")] -fn macos_only() { - // ... -} - -// This function is only included when either foo or bar is defined -#[cfg(any(foo, bar))] -fn needs_foo_or_bar() { - // ... -} - -// This function is only included when compiling for a unixish OS with a 32-bit -// architecture -#[cfg(all(unix, target_pointer_width = "32"))] -fn on_32bit_unix() { - // ... -} - -// This function is only included when foo is not defined -#[cfg(not(foo))] -fn needs_not_foo() { - // ... -} -``` - -This illustrates some conditional compilation can be achieved using the -`#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble -arbitrarily complex configurations through nesting. - -The following configurations must be defined by the implementation: - -* `target_arch = "..."` - Target CPU architecture, such as `"x86"`, - `"x86_64"` `"mips"`, `"powerpc"`, `"powerpc64"`, `"arm"`, or - `"aarch64"`. This value is closely related to the first element of - the platform target triple, though it is not identical. -* `target_os = "..."` - Operating system of the target, examples - include `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, - `"freebsd"`, `"dragonfly"`, `"bitrig"` , `"openbsd"` or - `"netbsd"`. This value is closely related to the second and third - element of the platform target triple, though it is not identical. -* `target_family = "..."` - Operating system family of the target, e. g. - `"unix"` or `"windows"`. The value of this configuration option is defined - as a configuration itself, like `unix` or `windows`. -* `unix` - See `target_family`. -* `windows` - See `target_family`. -* `target_env = ".."` - Further disambiguates the target platform with - information about the ABI/libc. Presently this value is either - `"gnu"`, `"msvc"`, `"musl"`, or the empty string. For historical - reasons this value has only been defined as non-empty when needed - for disambiguation. Thus on many GNU platforms this value will be - empty. This value is closely related to the fourth element of the - platform target triple, though it is not identical. For example, - embedded ABIs such as `gnueabihf` will simply define `target_env` as - `"gnu"`. -* `target_endian = "..."` - Endianness of the target CPU, either `"little"` or - `"big"`. -* `target_pointer_width = "..."` - Target pointer width in bits. This is set - to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for - 64-bit pointers. -* `target_has_atomic = "..."` - Set of integer sizes on which the target can perform - atomic operations. Values are `"8"`, `"16"`, `"32"`, `"64"` and `"ptr"`. -* `target_vendor = "..."` - Vendor of the target, for example `apple`, `pc`, or - simply `"unknown"`. -* `test` - Enabled when compiling the test harness (using the `--test` flag). -* `debug_assertions` - Enabled by default when compiling without optimizations. - This can be used to enable extra debugging code in development but not in - production. For example, it controls the behavior of the standard library's - `debug_assert!` macro. - -You can also set another attribute based on a `cfg` variable with `cfg_attr`: - -```rust,ignore -#[cfg_attr(a, b)] -``` - -This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise. - -Lastly, configuration options can be used in expressions by invoking the `cfg!` -macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise. - -### Lint check attributes - -A lint check names a potentially undesirable coding pattern, such as -unreachable code or omitted documentation, for the static entity to which the -attribute applies. - -For any lint check `C`: - -* `allow(C)` overrides the check for `C` so that violations will go - unreported, -* `deny(C)` signals an error after encountering a violation of `C`, -* `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint - level afterwards, -* `warn(C)` warns about violations of `C` but continues compilation. - -The lint checks supported by the compiler can be found via `rustc -W help`, -along with their default settings. [Compiler -plugins](book/compiler-plugins.html#lint-plugins) can provide additional lint checks. - -```{.ignore} -pub mod m1 { - // Missing documentation is ignored here - #[allow(missing_docs)] - pub fn undocumented_one() -> i32 { 1 } - - // Missing documentation signals a warning here - #[warn(missing_docs)] - pub fn undocumented_too() -> i32 { 2 } - - // Missing documentation signals an error here - #[deny(missing_docs)] - pub fn undocumented_end() -> i32 { 3 } -} -``` - -This example shows how one can use `allow` and `warn` to toggle a particular -check on and off: - -```{.ignore} -#[warn(missing_docs)] -pub mod m2{ - #[allow(missing_docs)] - pub mod nested { - // Missing documentation is ignored here - pub fn undocumented_one() -> i32 { 1 } - - // Missing documentation signals a warning here, - // despite the allow above. - #[warn(missing_docs)] - pub fn undocumented_two() -> i32 { 2 } - } - - // Missing documentation signals a warning here - pub fn undocumented_too() -> i32 { 3 } -} -``` - -This example shows how one can use `forbid` to disallow uses of `allow` for -that lint check: - -```{.ignore} -#[forbid(missing_docs)] -pub mod m3 { - // Attempting to toggle warning signals an error here - #[allow(missing_docs)] - /// Returns 2. - pub fn undocumented_too() -> i32 { 2 } -} -``` - -### Language items - -Some primitive Rust operations are defined in Rust code, rather than being -implemented directly in C or assembly language. The definitions of these -operations have to be easy for the compiler to find. The `lang` attribute -makes it possible to declare these operations. For example, the `str` module -in the Rust standard library defines the string equality function: - -```{.ignore} -#[lang = "str_eq"] -pub fn eq_slice(a: &str, b: &str) -> bool { - // details elided -} -``` - -The name `str_eq` has a special meaning to the Rust compiler, and the presence -of this definition means that it will use this definition when generating calls -to the string equality function. - -The set of language items is currently considered unstable. A complete -list of the built-in language items will be added in the future. - -### Inline attributes - -The inline attribute suggests that the compiler should place a copy of -the function or static in the caller, rather than generating code to -call the function or access the static where it is defined. - -The compiler automatically inlines functions based on internal heuristics. -Incorrectly inlining functions can actually make the program slower, so it -should be used with care. - -`#[inline]` and `#[inline(always)]` always cause the function to be serialized -into the crate metadata to allow cross-crate inlining. - -There are three different types of inline attributes: - -* `#[inline]` hints the compiler to perform an inline expansion. -* `#[inline(always)]` asks the compiler to always perform an inline expansion. -* `#[inline(never)]` asks the compiler to never perform an inline expansion. - -### `derive` - -The `derive` attribute allows certain traits to be automatically implemented -for data structures. For example, the following will create an `impl` for the -`PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given -the `PartialEq` or `Clone` constraints for the appropriate `impl`: - -``` -#[derive(PartialEq, Clone)] -struct Foo { - a: i32, - b: T, -} -``` - -The generated `impl` for `PartialEq` is equivalent to - -``` -# struct Foo { a: i32, b: T } -impl PartialEq for Foo { - fn eq(&self, other: &Foo) -> bool { - self.a == other.a && self.b == other.b - } - - fn ne(&self, other: &Foo) -> bool { - self.a != other.a || self.b != other.b - } -} -``` - -You can implement `derive` for your own type through [procedural -macros](#procedural-macros). - -### Compiler Features - -Certain aspects of Rust may be implemented in the compiler, but they're not -necessarily ready for every-day use. These features are often of "prototype -quality" or "almost production ready", but may not be stable enough to be -considered a full-fledged language feature. - -For this reason, Rust recognizes a special crate-level attribute of the form: - -```{.ignore} -#![feature(feature1, feature2, feature3)] -``` - -This directive informs the compiler that the feature list: `feature1`, -`feature2`, and `feature3` should all be enabled. This is only recognized at a -crate-level, not at a module-level. Without this directive, all features are -considered off, and using the features will result in a compiler error. - -The currently implemented features of the reference compiler are: - -* `advanced_slice_patterns` - See the [match expressions](#match-expressions) - section for discussion; the exact semantics of - slice patterns are subject to change, so some types - are still unstable. - -* `slice_patterns` - OK, actually, slice patterns are just scary and - completely unstable. - -* `asm` - The `asm!` macro provides a means for inline assembly. This is often - useful, but the exact syntax for this feature along with its - semantics are likely to change, so this macro usage must be opted - into. - -* `associated_consts` - Allows constants to be defined in `impl` and `trait` - blocks, so that they can be associated with a type or - trait in a similar manner to methods and associated - types. - -* `box_patterns` - Allows `box` patterns, the exact semantics of which - is subject to change. - -* `box_syntax` - Allows use of `box` expressions, the exact semantics of which - is subject to change. - -* `cfg_target_vendor` - Allows conditional compilation using the `target_vendor` - matcher which is subject to change. - -* `cfg_target_has_atomic` - Allows conditional compilation using the `target_has_atomic` - matcher which is subject to change. - -* `concat_idents` - Allows use of the `concat_idents` macro, which is in many - ways insufficient for concatenating identifiers, and may be - removed entirely for something more wholesome. - -* `custom_attribute` - Allows the usage of attributes unknown to the compiler - so that new attributes can be added in a backwards compatible - manner (RFC 572). - -* `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for - `#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax - extensions. - -* `inclusive_range_syntax` - Allows use of the `a...b` and `...b` syntax for inclusive ranges. - -* `inclusive_range` - Allows use of the types that represent desugared inclusive ranges. - -* `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics - are inherently unstable and no promise about them is made. - -* `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`, - lang items are inherently unstable and no promise about them - is made. - -* `link_args` - This attribute is used to specify custom flags to the linker, - but usage is strongly discouraged. The compiler's usage of the - system linker is not guaranteed to continue in the future, and - if the system linker is not used then specifying custom flags - doesn't have much meaning. - -* `link_llvm_intrinsics` – Allows linking to LLVM intrinsics via - `#[link_name="llvm.*"]`. - -* `linkage` - Allows use of the `linkage` attribute, which is not portable. - -* `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a - nasty hack that will certainly be removed. - -* `main` - Allows use of the `#[main]` attribute, which changes the entry point - into a Rust program. This capability is subject to change. - -* `macro_reexport` - Allows macros to be re-exported from one crate after being imported - from another. This feature was originally designed with the sole - use case of the Rust standard library in mind, and is subject to - change. - -* `non_ascii_idents` - The compiler supports the use of non-ascii identifiers, - but the implementation is a little rough around the - edges, so this can be seen as an experimental feature - for now until the specification of identifiers is fully - fleshed out. - -* `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit - `extern crate std`. This typically requires use of the unstable APIs - behind the libstd "facade", such as libcore and libcollections. It - may also cause problems when using syntax extensions, including - `#[derive]`. - -* `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows - trait definitions to add specialized notes to error messages - when an implementation was expected but not found. - -* `optin_builtin_traits` - Allows the definition of default and negative trait - implementations. Experimental. - -* `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions. - These depend on compiler internals and are subject to change. - -* `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin]. - -* `quote` - Allows use of the `quote_*!` family of macros, which are - implemented very poorly and will likely change significantly - with a proper implementation. - -* `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be - for internal use only or have meaning added to them in the future. - -* `rustc_diagnostic_macros`- A mysterious feature, used in the implementation - of rustc, not meant for mortals. - -* `simd` - Allows use of the `#[simd]` attribute, which is overly simple and - not the SIMD interface we want to expose in the long term. - -* `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions. - The SIMD interface is subject to change. - -* `start` - Allows use of the `#[start]` attribute, which changes the entry point - into a Rust program. This capability, especially the signature for the - annotated function, is subject to change. - -* `thread_local` - The usage of the `#[thread_local]` attribute is experimental - and should be seen as unstable. This attribute is used to - declare a `static` as being unique per-thread leveraging - LLVM's implementation which works in concert with the kernel - loader and dynamic linker. This is not necessarily available - on all platforms, and usage of it is discouraged. - -* `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty - hack that will certainly be removed. - -* `unboxed_closures` - Rust's new closure design, which is currently a work in - progress feature with many known bugs. - -* `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the - `#[allow_internal_unstable]` attribute, designed - to allow `std` macros to call - `#[unstable]`/feature-gated functionality - internally without imposing on callers - (i.e. making them behave like function calls in - terms of encapsulation). - -* `default_type_parameter_fallback` - Allows type parameter defaults to - influence type inference. - -* `stmt_expr_attributes` - Allows attributes on expressions. - -* `type_ascription` - Allows type ascription expressions `expr: Type`. - -* `abi_vectorcall` - Allows the usage of the vectorcall calling convention - (e.g. `extern "vectorcall" func fn_();`) - -* `abi_sysv64` - Allows the usage of the system V AMD64 calling convention - (e.g. `extern "sysv64" func fn_();`) - -If a feature is promoted to a language feature, then all existing programs will -start to receive compilation warnings about `#![feature]` directives which enabled -the new feature (because the directive is no longer necessary). However, if a -feature is decided to be removed from the language, errors will be issued (if -there isn't a parser error first). The directive in this case is no longer -necessary, and it's likely that existing code will break if the feature isn't -removed. - -If an unknown feature is found in a directive, it results in a compiler error. -An unknown feature is one which has never been recognized by the compiler. - -# Statements and expressions - -Rust is _primarily_ an expression language. This means that most forms of -value-producing or effect-causing evaluation are directed by the uniform syntax -category of _expressions_. Each kind of expression can typically _nest_ within -each other kind of expression, and rules for evaluation of expressions involve -specifying both the value produced by the expression and the order in which its -sub-expressions are themselves evaluated. - -In contrast, statements in Rust serve _mostly_ to contain and explicitly -sequence expression evaluation. - -## Statements - -A _statement_ is a component of a block, which is in turn a component of an -outer [expression](#expressions) or [function](#functions). - -Rust has two kinds of statement: [declaration -statements](#declaration-statements) and [expression -statements](#expression-statements). - -### Declaration statements - -A _declaration statement_ is one that introduces one or more *names* into the -enclosing statement block. The declared names may denote new variables or new -items. - -#### Item declarations - -An _item declaration statement_ has a syntactic form identical to an -[item](#items) declaration within a module. Declaring an item — a -function, enumeration, struct, type, static, trait, implementation or module -— locally within a statement block is simply a way of restricting its -scope to a narrow region containing all of its uses; it is otherwise identical -in meaning to declaring the item outside the statement block. - -> **Note**: there is no implicit capture of the function's dynamic environment when -> declaring a function-local item. - -#### `let` statements - -A _`let` statement_ introduces a new set of variables, given by a pattern. The -pattern may be followed by a type annotation, and/or an initializer expression. -When no type annotation is given, the compiler will infer the type, or signal -an error if insufficient type information is available for definite inference. -Any variables introduced by a variable declaration are visible from the point of -declaration until the end of the enclosing block scope. - -### Expression statements - -An _expression statement_ is one that evaluates an [expression](#expressions) -and ignores its result. The type of an expression statement `e;` is always -`()`, regardless of the type of `e`. As a rule, an expression statement's -purpose is to trigger the effects of evaluating its expression. - -## Expressions - -An expression may have two roles: it always produces a *value*, and it may have -*effects* (otherwise known as "side effects"). An expression *evaluates to* a -value, and has effects during *evaluation*. Many expressions contain -sub-expressions (operands). The meaning of each kind of expression dictates -several things: - -* Whether or not to evaluate the sub-expressions when evaluating the expression -* The order in which to evaluate the sub-expressions -* How to combine the sub-expressions' values to obtain the value of the expression - -In this way, the structure of expressions dictates the structure of execution. -Blocks are just another kind of expression, so blocks, statements, expressions, -and blocks again can recursively nest inside each other to an arbitrary depth. - -#### Lvalues, rvalues and temporaries - -Expressions are divided into two main categories: _lvalues_ and _rvalues_. -Likewise within each expression, sub-expressions may occur in _lvalue context_ -or _rvalue context_. The evaluation of an expression depends both on its own -category and the context it occurs within. - -An lvalue is an expression that represents a memory location. These expressions -are [paths](#path-expressions) (which refer to local variables, function and -method arguments, or static variables), dereferences (`*expr`), [indexing -expressions](#index-expressions) (`expr[expr]`), and [field -references](#field-expressions) (`expr.f`). All other expressions are rvalues. - -The left operand of an [assignment](#assignment-expressions) or -[compound-assignment](#compound-assignment-expressions) expression is -an lvalue context, as is the single operand of a unary -[borrow](#unary-operator-expressions). The discriminant or subject of -a [match expression](#match-expressions) may be an lvalue context, if -ref bindings are made, but is otherwise an rvalue context. All other -expression contexts are rvalue contexts. - -When an lvalue is evaluated in an _lvalue context_, it denotes a memory -location; when evaluated in an _rvalue context_, it denotes the value held _in_ -that memory location. - -##### Temporary lifetimes - -When an rvalue is used in an lvalue context, a temporary un-named -lvalue is created and used instead. The lifetime of temporary values -is typically the innermost enclosing statement; the tail expression of -a block is considered part of the statement that encloses the block. - -When a temporary rvalue is being created that is assigned into a `let` -declaration, however, the temporary is created with the lifetime of -the enclosing block instead, as using the enclosing statement (the -`let` declaration) would be a guaranteed error (since a pointer to the -temporary would be stored into a variable, but the temporary would be -freed before the variable could be used). The compiler uses simple -syntactic rules to decide which values are being assigned into a `let` -binding, and therefore deserve a longer temporary lifetime. - -Here are some examples: - -- `let x = foo(&temp())`. The expression `temp()` is an rvalue. As it - is being borrowed, a temporary is created which will be freed after - the innermost enclosing statement (the `let` declaration, in this case). -- `let x = temp().foo()`. This is the same as the previous example, - except that the value of `temp()` is being borrowed via autoref on a - method-call. Here we are assuming that `foo()` is an `&self` method - defined in some trait, say `Foo`. In other words, the expression - `temp().foo()` is equivalent to `Foo::foo(&temp())`. -- `let x = &temp()`. Here, the same temporary is being assigned into - `x`, rather than being passed as a parameter, and hence the - temporary's lifetime is considered to be the enclosing block. -- `let x = SomeStruct { foo: &temp() }`. As in the previous case, the - temporary is assigned into a struct which is then assigned into a - binding, and hence it is given the lifetime of the enclosing block. -- `let x = [ &temp() ]`. As in the previous case, the - temporary is assigned into an array which is then assigned into a - binding, and hence it is given the lifetime of the enclosing block. -- `let ref x = temp()`. In this case, the temporary is created using a ref binding, - but the result is the same: the lifetime is extended to the enclosing block. - -#### Moved and copied types - -When a [local variable](#variables) is used as an -[rvalue](#lvalues-rvalues-and-temporaries), the variable will be copied -if its type implements `Copy`. All others are moved. - -### Literal expressions - -A _literal expression_ consists of one of the [literal](#literals) forms -described earlier. It directly describes a number, character, string, boolean -value, or the unit value. - -```{.literals} -(); // unit type -"hello"; // string type -'5'; // character type -5; // integer type -``` - -### Path expressions - -A [path](#paths) used as an expression context denotes either a local variable -or an item. Path expressions are [lvalues](#lvalues-rvalues-and-temporaries). - -### Tuple expressions - -Tuples are written by enclosing zero or more comma-separated expressions in -parentheses. They are used to create [tuple-typed](#tuple-types) values. - -```{.tuple} -(0.0, 4.5); -("a", 4usize, true); -``` - -You can disambiguate a single-element tuple from a value in parentheses with a -comma: - -``` -(0,); // single-element tuple -(0); // zero in parentheses -``` - -### Struct expressions - -There are several forms of struct expressions. A _struct expression_ -consists of the [path](#paths) of a [struct item](#structs), followed by -a brace-enclosed list of zero or more comma-separated name-value pairs, -providing the field values of a new instance of the struct. A field name -can be any identifier, and is separated from its value expression by a colon. -The location denoted by a struct field is mutable if and only if the -enclosing struct is mutable. - -A _tuple struct expression_ consists of the [path](#paths) of a [struct -item](#structs), followed by a parenthesized list of one or more -comma-separated expressions (in other words, the path of a struct item -followed by a tuple expression). The struct item must be a tuple struct -item. - -A _unit-like struct expression_ consists only of the [path](#paths) of a -[struct item](#structs). - -The following are examples of struct expressions: - -``` -# struct Point { x: f64, y: f64 } -# struct NothingInMe { } -# struct TuplePoint(f64, f64); -# mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } } -# struct Cookie; fn some_fn(t: T) {} -Point {x: 10.0, y: 20.0}; -NothingInMe {}; -TuplePoint(10.0, 20.0); -let u = game::User {name: "Joe", age: 35, score: 100_000}; -some_fn::(Cookie); -``` - -A struct expression forms a new value of the named struct type. Note -that for a given *unit-like* struct type, this will always be the same -value. - -A struct expression can terminate with the syntax `..` followed by an -expression to denote a functional update. The expression following `..` (the -base) must have the same struct type as the new struct type being formed. -The entire expression denotes the result of constructing a new struct (with -the same type as the base expression) with the given values for the fields that -were explicitly specified and the values in the base expression for all other -fields. - -``` -# struct Point3d { x: i32, y: i32, z: i32 } -let base = Point3d {x: 1, y: 2, z: 3}; -Point3d {y: 0, z: 10, .. base}; -``` - -#### Struct field init shorthand - -When initializing a data structure (struct, enum, union) with named fields, -it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`. -This allows a compact syntax with less duplication. - -Example: - -``` -# struct Point3d { x: i32, y: i32, z: i32 } -# let x = 0; -# let y_value = 0; -# let z = 0; -Point3d { x: x, y: y_value, z: z }; -Point3d { x, y: y_value, z }; -``` - -### Block expressions - -A _block expression_ is similar to a module in terms of the declarations that -are possible. Each block conceptually introduces a new namespace scope. Use -items can bring new names into scopes and declared items are in scope for only -the block itself. - -A block will execute each statement sequentially, and then execute the -expression (if given). If the block ends in a statement, its value is `()`: - -``` -let x: () = { println!("Hello."); }; -``` - -If it ends in an expression, its value and type are that of the expression: - -``` -let x: i32 = { println!("Hello."); 5 }; - -assert_eq!(5, x); -``` - -### Method-call expressions - -A _method call_ consists of an expression followed by a single dot, an -identifier, and a parenthesized expression-list. Method calls are resolved to -methods on specific traits, either statically dispatching to a method if the -exact `self`-type of the left-hand-side is known, or dynamically dispatching if -the left-hand-side expression is an indirect [trait object](#trait-objects). - -### Field expressions - -A _field expression_ consists of an expression followed by a single dot and an -identifier, when not immediately followed by a parenthesized expression-list -(the latter is a [method call expression](#method-call-expressions)). A field -expression denotes a field of a [struct](#struct-types). - -```{.ignore .field} -mystruct.myfield; -foo().x; -(Struct {a: 10, b: 20}).a; -``` - -A field access is an [lvalue](#lvalues-rvalues-and-temporaries) referring to -the value of that field. When the type providing the field inherits mutability, -it can be [assigned](#assignment-expressions) to. - -Also, if the type of the expression to the left of the dot is a -pointer, it is automatically dereferenced as many times as necessary -to make the field access possible. In cases of ambiguity, we prefer -fewer autoderefs to more. - -### Array expressions - -An [array](#array-and-slice-types) _expression_ is written by enclosing zero -or more comma-separated expressions of uniform type in square brackets. - -In the `[expr ';' expr]` form, the expression after the `';'` must be a -constant expression that can be evaluated at compile time, such as a -[literal](#literals) or a [static item](#static-items). - -``` -[1, 2, 3, 4]; -["a", "b", "c", "d"]; -[0; 128]; // array with 128 zeros -[0u8, 0u8, 0u8, 0u8]; -``` - -### Index expressions - -[Array](#array-and-slice-types)-typed expressions can be indexed by -writing a square-bracket-enclosed expression (the index) after them. When the -array is mutable, the resulting [lvalue](#lvalues-rvalues-and-temporaries) can -be assigned to. - -Indices are zero-based, and may be of any integral type. Vector access is -bounds-checked at compile-time for constant arrays being accessed with a constant index value. -Otherwise a check will be performed at run-time that will put the thread in a _panicked state_ if it fails. - -```{should-fail} -([1, 2, 3, 4])[0]; - -let x = (["a", "b"])[10]; // compiler error: const index-expr is out of bounds - -let n = 10; -let y = (["a", "b"])[n]; // panics - -let arr = ["a", "b"]; -arr[10]; // panics -``` - -Also, if the type of the expression to the left of the brackets is a -pointer, it is automatically dereferenced as many times as necessary -to make the indexing possible. In cases of ambiguity, we prefer fewer -autoderefs to more. - -### Range expressions - -The `..` operator will construct an object of one of the `std::ops::Range` variants. - -``` -1..2; // std::ops::Range -3..; // std::ops::RangeFrom -..4; // std::ops::RangeTo -..; // std::ops::RangeFull -``` - -The following expressions are equivalent. - -``` -let x = std::ops::Range {start: 0, end: 10}; -let y = 0..10; - -assert_eq!(x, y); -``` - -Similarly, the `...` operator will construct an object of one of the -`std::ops::RangeInclusive` variants. - -``` -# #![feature(inclusive_range_syntax)] -1...2; // std::ops::RangeInclusive -...4; // std::ops::RangeToInclusive -``` - -The following expressions are equivalent. - -``` -# #![feature(inclusive_range_syntax, inclusive_range)] -let x = std::ops::RangeInclusive::NonEmpty {start: 0, end: 10}; -let y = 0...10; - -assert_eq!(x, y); -``` - -### Unary operator expressions - -Rust defines the following unary operators. With the exception of `?`, they are -all written as prefix operators, before the expression they apply to. - -* `-` - : Negation. Signed integer types and floating-point types support negation. It - is an error to apply negation to unsigned types; for example, the compiler - rejects `-1u32`. -* `*` - : Dereference. When applied to a [pointer](#pointer-types) it denotes the - pointed-to location. For pointers to mutable locations, the resulting - [lvalue](#lvalues-rvalues-and-temporaries) can be assigned to. - On non-pointer types, it calls the `deref` method of the `std::ops::Deref` - trait, or the `deref_mut` method of the `std::ops::DerefMut` trait (if - implemented by the type and required for an outer expression that will or - could mutate the dereference), and produces the result of dereferencing the - `&` or `&mut` borrowed pointer returned from the overload method. -* `!` - : Logical negation. On the boolean type, this flips between `true` and - `false`. On integer types, this inverts the individual bits in the - two's complement representation of the value. -* `&` and `&mut` - : Borrowing. When applied to an lvalue, these operators produce a - reference (pointer) to the lvalue. The lvalue is also placed into - a borrowed state for the duration of the reference. For a shared - borrow (`&`), this implies that the lvalue may not be mutated, but - it may be read or shared again. For a mutable borrow (`&mut`), the - lvalue may not be accessed in any way until the borrow expires. - If the `&` or `&mut` operators are applied to an rvalue, a - temporary value is created; the lifetime of this temporary value - is defined by [syntactic rules](#temporary-lifetimes). -* `?` - : Propagating errors if applied to `Err(_)` and unwrapping if - applied to `Ok(_)`. Only works on the `Result` type, - and written in postfix notation. - -### Binary operator expressions - -Binary operators expressions are given in terms of [operator -precedence](#operator-precedence). - -#### Arithmetic operators - -Binary arithmetic expressions are syntactic sugar for calls to built-in traits, -defined in the `std::ops` module of the `std` library. This means that -arithmetic operators can be overridden for user-defined types. The default -meaning of the operators on standard types is given here. - -* `+` - : Addition and array/string concatenation. - Calls the `add` method on the `std::ops::Add` trait. -* `-` - : Subtraction. - Calls the `sub` method on the `std::ops::Sub` trait. -* `*` - : Multiplication. - Calls the `mul` method on the `std::ops::Mul` trait. -* `/` - : Quotient. - Calls the `div` method on the `std::ops::Div` trait. -* `%` - : Remainder. - Calls the `rem` method on the `std::ops::Rem` trait. - -#### Bitwise operators - -Like the [arithmetic operators](#arithmetic-operators), bitwise operators are -syntactic sugar for calls to methods of built-in traits. This means that -bitwise operators can be overridden for user-defined types. The default -meaning of the operators on standard types is given here. Bitwise `&`, `|` and -`^` applied to boolean arguments are equivalent to logical `&&`, `||` and `!=` -evaluated in non-lazy fashion. - -* `&` - : Bitwise AND. - Calls the `bitand` method of the `std::ops::BitAnd` trait. -* `|` - : Bitwise inclusive OR. - Calls the `bitor` method of the `std::ops::BitOr` trait. -* `^` - : Bitwise exclusive OR. - Calls the `bitxor` method of the `std::ops::BitXor` trait. -* `<<` - : Left shift. - Calls the `shl` method of the `std::ops::Shl` trait. -* `>>` - : Right shift (arithmetic). - Calls the `shr` method of the `std::ops::Shr` trait. - -#### Lazy boolean operators - -The operators `||` and `&&` may be applied to operands of boolean type. The -`||` operator denotes logical 'or', and the `&&` operator denotes logical -'and'. They differ from `|` and `&` in that the right-hand operand is only -evaluated when the left-hand operand does not already determine the result of -the expression. That is, `||` only evaluates its right-hand operand when the -left-hand operand evaluates to `false`, and `&&` only when it evaluates to -`true`. - -#### Comparison operators - -Comparison operators are, like the [arithmetic -operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators), -syntactic sugar for calls to built-in traits. This means that comparison -operators can be overridden for user-defined types. The default meaning of the -operators on standard types is given here. - -* `==` - : Equal to. - Calls the `eq` method on the `std::cmp::PartialEq` trait. -* `!=` - : Unequal to. - Calls the `ne` method on the `std::cmp::PartialEq` trait. -* `<` - : Less than. - Calls the `lt` method on the `std::cmp::PartialOrd` trait. -* `>` - : Greater than. - Calls the `gt` method on the `std::cmp::PartialOrd` trait. -* `<=` - : Less than or equal. - Calls the `le` method on the `std::cmp::PartialOrd` trait. -* `>=` - : Greater than or equal. - Calls the `ge` method on the `std::cmp::PartialOrd` trait. - -#### Type cast expressions - -A type cast expression is denoted with the binary operator `as`. - -Executing an `as` expression casts the value on the left-hand side to the type -on the right-hand side. - -An example of an `as` expression: - -``` -# fn sum(values: &[f64]) -> f64 { 0.0 } -# fn len(values: &[f64]) -> i32 { 0 } - -fn average(values: &[f64]) -> f64 { - let sum: f64 = sum(values); - let size: f64 = len(values) as f64; - sum / size -} -``` - -Some of the conversions which can be done through the `as` operator -can also be done implicitly at various points in the program, such as -argument passing and assignment to a `let` binding with an explicit -type. Implicit conversions are limited to "harmless" conversions that -do not lose information and which have minimal or no risk of -surprising side-effects on the dynamic execution semantics. - -#### Assignment expressions - -An _assignment expression_ consists of an -[lvalue](#lvalues-rvalues-and-temporaries) expression followed by an equals -sign (`=`) and an [rvalue](#lvalues-rvalues-and-temporaries) expression. - -Evaluating an assignment expression [either copies or -moves](#moved-and-copied-types) its right-hand operand to its left-hand -operand. - -``` -# let mut x = 0; -# let y = 0; -x = y; -``` - -#### Compound assignment expressions - -The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be -composed with the `=` operator. The expression `lval OP= val` is equivalent to -`lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`. - -Any such expression always has the [`unit`](#tuple-types) type. - -#### Operator precedence - -The precedence of Rust binary operators is ordered as follows, going from -strong to weak: - -```{.text .precedence} -as : -* / % -+ - -<< >> -& -^ -| -== != < > <= >= -&& -|| -.. ... -<- -= -``` - -Operators at the same precedence level are evaluated left-to-right. [Unary -operators](#unary-operator-expressions) have the same precedence level and are -stronger than any of the binary operators. - -### Grouped expressions - -An expression enclosed in parentheses evaluates to the result of the enclosed -expression. Parentheses can be used to explicitly specify evaluation order -within an expression. - -An example of a parenthesized expression: - -``` -let x: i32 = (2 + 3) * 4; -``` - - -### Call expressions - -A _call expression_ invokes a function, providing zero or more input variables -and an optional location to move the function's output into. If the function -eventually returns, then the expression completes. - -Some examples of call expressions: - -``` -# fn add(x: i32, y: i32) -> i32 { 0 } - -let x: i32 = add(1i32, 2i32); -let pi: Result = "3.14".parse(); -``` - -### Lambda expressions - -A _lambda expression_ (sometimes called an "anonymous function expression") -defines a function and denotes it as a value, in a single expression. A lambda -expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an -expression. - -A lambda expression denotes a function that maps a list of parameters -(`ident_list`) onto the expression that follows the `ident_list`. The -identifiers in the `ident_list` are the parameters to the function. These -parameters' types need not be specified, as the compiler infers them from -context. - -Lambda expressions are most useful when passing functions as arguments to other -functions, as an abbreviation for defining and capturing a separate function. - -Significantly, lambda expressions _capture their environment_, which regular -[function definitions](#functions) do not. The exact type of capture depends -on the [function type](#function-types) inferred for the lambda expression. In -the simplest and least-expensive form (analogous to a ```|| { }``` expression), -the lambda expression captures its environment by reference, effectively -borrowing pointers to all outer variables mentioned inside the function. -Alternately, the compiler may infer that a lambda expression should copy or -move values (depending on their type) from the environment into the lambda -expression's captured environment. A lambda can be forced to capture its -environment by moving values by prefixing it with the `move` keyword. - -In this example, we define a function `ten_times` that takes a higher-order -function argument, and we then call it with a lambda expression as an argument, -followed by a lambda expression that moves values from its environment. - -``` -fn ten_times(f: F) where F: Fn(i32) { - for index in 0..10 { - f(index); - } -} - -ten_times(|j| println!("hello, {}", j)); - -let word = "konnichiwa".to_owned(); -ten_times(move |j| println!("{}, {}", word, j)); -``` - -### Infinite loops - -A `loop` expression denotes an infinite loop. - -A `loop` expression may optionally have a _label_. The label is written as -a lifetime preceding the loop expression, as in `'foo: loop{ }`. If a -label is present, then labeled `break` and `continue` expressions nested -within this loop may exit out of this loop or return control to its head. -See [break expressions](#break-expressions) and [continue -expressions](#continue-expressions). - -### `break` expressions - -A `break` expression has an optional _label_. If the label is absent, then -executing a `break` expression immediately terminates the innermost loop -enclosing it. It is only permitted in the body of a loop. If the label is -present, then `break 'foo` terminates the loop with label `'foo`, which need not -be the innermost label enclosing the `break` expression, but must enclose it. - -### `continue` expressions - -A `continue` expression has an optional _label_. If the label is absent, then -executing a `continue` expression immediately terminates the current iteration -of the innermost loop enclosing it, returning control to the loop *head*. In -the case of a `while` loop, the head is the conditional expression controlling -the loop. In the case of a `for` loop, the head is the call-expression -controlling the loop. If the label is present, then `continue 'foo` returns -control to the head of the loop with label `'foo`, which need not be the -innermost label enclosing the `continue` expression, but must enclose it. - -A `continue` expression is only permitted in the body of a loop. - -### `while` loops - -A `while` loop begins by evaluating the boolean loop conditional expression. -If the loop conditional expression evaluates to `true`, the loop body block -executes and control returns to the loop conditional expression. If the loop -conditional expression evaluates to `false`, the `while` expression completes. - -An example: - -``` -let mut i = 0; - -while i < 10 { - println!("hello"); - i = i + 1; -} -``` - -Like `loop` expressions, `while` loops can be controlled with `break` or -`continue`, and may optionally have a _label_. See [infinite -loops](#infinite-loops), [break expressions](#break-expressions), and -[continue expressions](#continue-expressions) for more information. - -### `for` expressions - -A `for` expression is a syntactic construct for looping over elements provided -by an implementation of `std::iter::IntoIterator`. - -An example of a `for` loop over the contents of an array: - -``` -# type Foo = i32; -# fn bar(f: &Foo) { } -# let a = 0; -# let b = 0; -# let c = 0; - -let v: &[Foo] = &[a, b, c]; - -for e in v { - bar(e); -} -``` - -An example of a for loop over a series of integers: - -``` -# fn bar(b:usize) { } -for i in 0..256 { - bar(i); -} -``` - -Like `loop` expressions, `for` loops can be controlled with `break` or -`continue`, and may optionally have a _label_. See [infinite -loops](#infinite-loops), [break expressions](#break-expressions), and -[continue expressions](#continue-expressions) for more information. - -### `if` expressions - -An `if` expression is a conditional branch in program control. The form of an -`if` expression is a condition expression, followed by a consequent block, any -number of `else if` conditions and blocks, and an optional trailing `else` -block. The condition expressions must have type `bool`. If a condition -expression evaluates to `true`, the consequent block is executed and any -subsequent `else if` or `else` block is skipped. If a condition expression -evaluates to `false`, the consequent block is skipped and any subsequent `else -if` condition is evaluated. If all `if` and `else if` conditions evaluate to -`false` then any `else` block is executed. - -### `match` expressions - -A `match` expression branches on a *pattern*. The exact form of matching that -occurs depends on the pattern. Patterns consist of some combination of -literals, destructured arrays or enum constructors, structs and tuples, -variable binding specifications, wildcards (`..`), and placeholders (`_`). A -`match` expression has a *head expression*, which is the value to compare to -the patterns. The type of the patterns must equal the type of the head -expression. - -In a pattern whose head expression has an `enum` type, a placeholder (`_`) -stands for a *single* data field, whereas a wildcard `..` stands for *all* the -fields of a particular variant. - -A `match` behaves differently depending on whether or not the head expression -is an [lvalue or an rvalue](#lvalues-rvalues-and-temporaries). If the head -expression is an rvalue, it is first evaluated into a temporary location, and -the resulting value is sequentially compared to the patterns in the arms until -a match is found. The first arm with a matching pattern is chosen as the branch -target of the `match`, any variables bound by the pattern are assigned to local -variables in the arm's block, and control enters the block. - -When the head expression is an lvalue, the match does not allocate a temporary -location (however, a by-value binding may copy or move from the lvalue). When -possible, it is preferable to match on lvalues, as the lifetime of these -matches inherits the lifetime of the lvalue, rather than being restricted to -the inside of the match. - -An example of a `match` expression: - -``` -let x = 1; - -match x { - 1 => println!("one"), - 2 => println!("two"), - 3 => println!("three"), - 4 => println!("four"), - 5 => println!("five"), - _ => println!("something else"), -} -``` - -Patterns that bind variables default to binding to a copy or move of the -matched value (depending on the matched value's type). This can be changed to -bind to a reference by using the `ref` keyword, or to a mutable reference using -`ref mut`. - -Subpatterns can also be bound to variables by the use of the syntax `variable @ -subpattern`. For example: - -``` -let x = 1; - -match x { - e @ 1 ... 5 => println!("got a range element {}", e), - _ => println!("anything"), -} -``` - -Patterns can also dereference pointers by using the `&`, `&mut` and `box` -symbols, as appropriate. For example, these two matches on `x: &i32` are -equivalent: - -``` -# let x = &3; -let y = match *x { 0 => "zero", _ => "some" }; -let z = match x { &0 => "zero", _ => "some" }; - -assert_eq!(y, z); -``` - -Multiple match patterns may be joined with the `|` operator. A range of values -may be specified with `...`. For example: - -``` -# let x = 2; - -let message = match x { - 0 | 1 => "not many", - 2 ... 9 => "a few", - _ => "lots" -}; -``` - -Range patterns only work on scalar types (like integers and characters; not -like arrays and structs, which have sub-components). A range pattern may not -be a sub-range of another range pattern inside the same `match`. - -Finally, match patterns can accept *pattern guards* to further refine the -criteria for matching a case. Pattern guards appear after the pattern and -consist of a bool-typed expression following the `if` keyword. A pattern guard -may refer to the variables bound within the pattern they follow. - -``` -# let maybe_digit = Some(0); -# fn process_digit(i: i32) { } -# fn process_other(i: i32) { } - -let message = match maybe_digit { - Some(x) if x < 10 => process_digit(x), - Some(x) => process_other(x), - None => panic!(), -}; -``` - -### `if let` expressions - -An `if let` expression is semantically identical to an `if` expression but in -place of a condition expression it expects a `let` statement with a refutable -pattern. If the value of the expression on the right hand side of the `let` -statement matches the pattern, the corresponding block will execute, otherwise -flow proceeds to the first `else` block that follows. - -``` -let dish = ("Ham", "Eggs"); - -// this body will be skipped because the pattern is refuted -if let ("Bacon", b) = dish { - println!("Bacon is served with {}", b); -} - -// this body will execute -if let ("Ham", b) = dish { - println!("Ham is served with {}", b); -} -``` - -### `while let` loops - -A `while let` loop is semantically identical to a `while` loop but in place of -a condition expression it expects `let` statement with a refutable pattern. If -the value of the expression on the right hand side of the `let` statement -matches the pattern, the loop body block executes and control returns to the -pattern matching statement. Otherwise, the while expression completes. - -### `return` expressions - -Return expressions are denoted with the keyword `return`. Evaluating a `return` -expression moves its argument into the designated output location for the -current function call, destroys the current function activation frame, and -transfers control to the caller frame. - -An example of a `return` expression: - -``` -fn max(a: i32, b: i32) -> i32 { - if a > b { - return a; - } - return b; -} -``` - -# Type system - -## Types - -Every variable, item and value in a Rust program has a type. The _type_ of a -*value* defines the interpretation of the memory holding it. - -Built-in types and type-constructors are tightly integrated into the language, -in nontrivial ways that are not possible to emulate in user-defined types. -User-defined types have limited capabilities. - -### Primitive types - -The primitive types are the following: - -* The boolean type `bool` with values `true` and `false`. -* The machine types (integer and floating-point). -* The machine-dependent integer types. -* Arrays -* Tuples -* Slices -* Function pointers - -#### Machine types - -The machine types are the following: - -* The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from - the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and - [0, 2^64 - 1] respectively. - -* The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with - values drawn from the integer intervals [-(2^(7)), 2^7 - 1], - [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1] - respectively. - -* The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and - `f64`, respectively. - -#### Machine-dependent integer types - -The `usize` type is an unsigned integer type with the same number of bits as the -platform's pointer type. It can represent every memory address in the process. - -The `isize` type is a signed integer type with the same number of bits as the -platform's pointer type. The theoretical upper bound on object and array size -is the maximum `isize` value. This ensures that `isize` can be used to calculate -differences between pointers into an object or array and can address every byte -within an object along with one byte past the end. - -### Textual types - -The types `char` and `str` hold textual data. - -A value of type `char` is a [Unicode scalar value]( -http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that -is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to -0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 / -UTF-32 string. - -A value of type `str` is a Unicode string, represented as an array of 8-bit -unsigned bytes holding a sequence of UTF-8 code points. Since `str` is of -unknown size, it is not a _first-class_ type, but can only be instantiated -through a pointer type, such as `&str`. - -### Tuple types - -A tuple *type* is a heterogeneous product of other types, called the *elements* -of the tuple. It has no nominal name and is instead structurally typed. - -Tuple types and values are denoted by listing the types or values of their -elements, respectively, in a parenthesized, comma-separated list. - -Because tuple elements don't have a name, they can only be accessed by -pattern-matching or by using `N` directly as a field to access the -`N`th element. - -An example of a tuple type and its use: - -``` -type Pair<'a> = (i32, &'a str); -let p: Pair<'static> = (10, "ten"); -let (a, b) = p; - -assert_eq!(a, 10); -assert_eq!(b, "ten"); -assert_eq!(p.0, 10); -assert_eq!(p.1, "ten"); -``` - -For historical reasons and convenience, the tuple type with no elements (`()`) -is often called ‘unit’ or ‘the unit type’. - -### Array, and Slice types - -Rust has two different types for a list of items: - -* `[T; N]`, an 'array' -* `&[T]`, a 'slice' - -An array has a fixed size, and can be allocated on either the stack or the -heap. - -A slice is a 'view' into an array. It doesn't own the data it points -to, it borrows it. - -Examples: - -```{rust} -// A stack-allocated array -let array: [i32; 3] = [1, 2, 3]; - -// A heap-allocated array -let vector: Vec = vec![1, 2, 3]; - -// A slice into an array -let slice: &[i32] = &vector[..]; -``` - -As you can see, the `vec!` macro allows you to create a `Vec` easily. The -`vec!` macro is also part of the standard library, rather than the language. - -All in-bounds elements of arrays and slices are always initialized, and access -to an array or slice is always bounds-checked. - -### Struct types - -A `struct` *type* is a heterogeneous product of other types, called the -*fields* of the type.[^structtype] - -[^structtype]: `struct` types are analogous to `struct` types in C, - the *record* types of the ML family, - or the *struct* types of the Lisp family. - -New instances of a `struct` can be constructed with a [struct -expression](#struct-expressions). - -The memory layout of a `struct` is undefined by default to allow for compiler -optimizations like field reordering, but it can be fixed with the -`#[repr(...)]` attribute. In either case, fields may be given in any order in -a corresponding struct *expression*; the resulting `struct` value will always -have the same memory layout. - -The fields of a `struct` may be qualified by [visibility -modifiers](#visibility-and-privacy), to allow access to data in a -struct outside a module. - -A _tuple struct_ type is just like a struct type, except that the fields are -anonymous. - -A _unit-like struct_ type is like a struct type, except that it has no -fields. The one value constructed by the associated [struct -expression](#struct-expressions) is the only value that inhabits such a -type. - -### Enumerated types - -An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted -by the name of an [`enum` item](#enumerations). [^enumtype] - -[^enumtype]: The `enum` type is analogous to a `data` constructor declaration in - ML, or a *pick ADT* in Limbo. - -An [`enum` item](#enumerations) declares both the type and a number of *variant -constructors*, each of which is independently named and takes an optional tuple -of arguments. - -New instances of an `enum` can be constructed by calling one of the variant -constructors, in a [call expression](#call-expressions). - -Any `enum` value consumes as much memory as the largest variant constructor for -its corresponding `enum` type. - -Enum types cannot be denoted *structurally* as types, but must be denoted by -named reference to an [`enum` item](#enumerations). - -### Recursive types - -Nominal types — [enumerations](#enumerated-types) and -[structs](#struct-types) — may be recursive. That is, each `enum` -constructor or `struct` field may refer, directly or indirectly, to the -enclosing `enum` or `struct` type itself. Such recursion has restrictions: - -* Recursive types must include a nominal type in the recursion - (not mere [type definitions](grammar.html#type-definitions), - or other structural types such as [arrays](#array-and-slice-types) or [tuples](#tuple-types)). -* A recursive `enum` item must have at least one non-recursive constructor - (in order to give the recursion a basis case). -* The size of a recursive type must be finite; - in other words the recursive fields of the type must be [pointer types](#pointer-types). -* Recursive type definitions can cross module boundaries, but not module *visibility* boundaries, - or crate boundaries (in order to simplify the module system and type checker). - -An example of a *recursive* type and its use: - -``` -enum List { - Nil, - Cons(T, Box>) -} - -let a: List = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil)))); -``` - -### Pointer types - -All pointers in Rust are explicit first-class values. They can be copied, -stored into data structs, and returned from functions. There are two -varieties of pointer in Rust: - -* References (`&`) - : These point to memory _owned by some other value_. - A reference type is written `&type`, - or `&'a type` when you need to specify an explicit lifetime. - Copying a reference is a "shallow" operation: - it involves only copying the pointer itself. - Releasing a reference has no effect on the value it points to, - but a reference of a temporary value will keep it alive during the scope - of the reference itself. - -* Raw pointers (`*`) - : Raw pointers are pointers without safety or liveness guarantees. - Raw pointers are written as `*const T` or `*mut T`, - for example `*const i32` means a raw pointer to a 32-bit integer. - Copying or dropping a raw pointer has no effect on the lifecycle of any - other value. Dereferencing a raw pointer or converting it to any other - pointer type is an [`unsafe` operation](#unsafe-functions). - Raw pointers are generally discouraged in Rust code; - they exist to support interoperability with foreign code, - and writing performance-critical or low-level functions. - -The standard library contains additional 'smart pointer' types beyond references -and raw pointers. - -### Function types - -The function type constructor `fn` forms new function types. A function type -consists of a possibly-empty set of function-type modifiers (such as `unsafe` -or `extern`), a sequence of input types and an output type. - -An example of a `fn` type: - -``` -fn add(x: i32, y: i32) -> i32 { - x + y -} - -let mut x = add(5,7); - -type Binop = fn(i32, i32) -> i32; -let bo: Binop = add; -x = bo(5,7); -``` - -#### Function types for specific items - -Internal to the compiler, there are also function types that are specific to a particular -function item. In the following snippet, for example, the internal types of the functions -`foo` and `bar` are different, despite the fact that they have the same signature: - -``` -fn foo() { } -fn bar() { } -``` - -The types of `foo` and `bar` can both be implicitly coerced to the fn -pointer type `fn()`. There is currently no syntax for unique fn types, -though the compiler will emit a type like `fn() {foo}` in error -messages to indicate "the unique fn type for the function `foo`". - -### Closure types - -A [lambda expression](#lambda-expressions) produces a closure value with -a unique, anonymous type that cannot be written out. - -Depending on the requirements of the closure, its type implements one or -more of the closure traits: - -* `FnOnce` - : The closure can be called once. A closure called as `FnOnce` - can move out values from its environment. - -* `FnMut` - : The closure can be called multiple times as mutable. A closure called as - `FnMut` can mutate values from its environment. `FnMut` inherits from - `FnOnce` (i.e. anything implementing `FnMut` also implements `FnOnce`). - -* `Fn` - : The closure can be called multiple times through a shared reference. - A closure called as `Fn` can neither move out from nor mutate values - from its environment. `Fn` inherits from `FnMut`, which itself - inherits from `FnOnce`. - - -### Trait objects - -In Rust, a type like `&SomeTrait` or `Box` is called a _trait object_. -Each instance of a trait object includes: - - - a pointer to an instance of a type `T` that implements `SomeTrait` - - a _virtual method table_, often just called a _vtable_, which contains, for - each method of `SomeTrait` that `T` implements, a pointer to `T`'s - implementation (i.e. a function pointer). - -The purpose of trait objects is to permit "late binding" of methods. Calling a -method on a trait object results in virtual dispatch at runtime: that is, a -function pointer is loaded from the trait object vtable and invoked indirectly. -The actual implementation for each vtable entry can vary on an object-by-object -basis. - -Note that for a trait object to be instantiated, the trait must be -_object-safe_. Object safety rules are defined in [RFC 255]. - -[RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md - -Given a pointer-typed expression `E` of type `&T` or `Box`, where `T` -implements trait `R`, casting `E` to the corresponding pointer type `&R` or -`Box` results in a value of the _trait object_ `R`. This result is -represented as a pair of pointers: the vtable pointer for the `T` -implementation of `R`, and the pointer value of `E`. - -An example of a trait object: - -``` -trait Printable { - fn stringify(&self) -> String; -} - -impl Printable for i32 { - fn stringify(&self) -> String { self.to_string() } -} - -fn print(a: Box) { - println!("{}", a.stringify()); -} - -fn main() { - print(Box::new(10) as Box); -} -``` - -In this example, the trait `Printable` occurs as a trait object in both the -type signature of `print`, and the cast expression in `main`. - -### Type parameters - -Within the body of an item that has type parameter declarations, the names of -its type parameters are types: - -```ignore -fn to_vec(xs: &[A]) -> Vec { - if xs.is_empty() { - return vec![]; - } - let first: A = xs[0].clone(); - let mut rest: Vec = to_vec(&xs[1..]); - rest.insert(0, first); - rest -} -``` - -Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest` -has type `Vec`, a vector with element type `A`. - -### Self types - -The special type `Self` has a meaning within traits and impls. In a trait definition, it refers -to an implicit type parameter representing the "implementing" type. In an impl, -it is an alias for the implementing type. For example, in: - -``` -pub trait From { - fn from(T) -> Self; -} - -impl From for String { - fn from(x: i32) -> Self { - x.to_string() - } -} -``` - -The notation `Self` in the impl refers to the implementing type: `String`. In another -example: - -``` -trait Printable { - fn make_string(&self) -> String; -} - -impl Printable for String { - fn make_string(&self) -> String { - (*self).clone() - } -} -``` - -The notation `&self` is a shorthand for `self: &Self`. In this case, -in the impl, `Self` refers to the value of type `String` that is the -receiver for a call to the method `make_string`. - -## Subtyping - -Subtyping is implicit and can occur at any stage in type checking or -inference. Subtyping in Rust is very restricted and occurs only due to -variance with respect to lifetimes and between types with higher ranked -lifetimes. If we were to erase lifetimes from types, then the only subtyping -would be due to type equality. - -Consider the following example: string literals always have `'static` -lifetime. Nevertheless, we can assign `s` to `t`: - -``` -fn bar<'a>() { - let s: &'static str = "hi"; - let t: &'a str = s; -} -``` -Since `'static` "lives longer" than `'a`, `&'static str` is a subtype of -`&'a str`. - -## Type coercions - -Coercions are defined in [RFC 401]. A coercion is implicit and has no syntax. - -[RFC 401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md - -### Coercion sites - -A coercion can only occur at certain coercion sites in a program; these are -typically places where the desired type is explicit or can be derived by -propagation from explicit types (without type inference). Possible coercion -sites are: - -* `let` statements where an explicit type is given. - - For example, `42` is coerced to have type `i8` in the following: - - ```rust - let _: i8 = 42; - ``` - -* `static` and `const` statements (similar to `let` statements). - -* Arguments for function calls - - The value being coerced is the actual parameter, and it is coerced to - the type of the formal parameter. - - For example, `42` is coerced to have type `i8` in the following: - - ```rust - fn bar(_: i8) { } - - fn main() { - bar(42); - } - ``` - -* Instantiations of struct or variant fields - - For example, `42` is coerced to have type `i8` in the following: - - ```rust - struct Foo { x: i8 } - - fn main() { - Foo { x: 42 }; - } - ``` - -* Function results, either the final line of a block if it is not - semicolon-terminated or any expression in a `return` statement - - For example, `42` is coerced to have type `i8` in the following: - - ```rust - fn foo() -> i8 { - 42 - } - ``` - -If the expression in one of these coercion sites is a coercion-propagating -expression, then the relevant sub-expressions in that expression are also -coercion sites. Propagation recurses from these new coercion sites. -Propagating expressions and their relevant sub-expressions are: - -* Array literals, where the array has type `[U; n]`. Each sub-expression in -the array literal is a coercion site for coercion to type `U`. - -* Array literals with repeating syntax, where the array has type `[U; n]`. The -repeated sub-expression is a coercion site for coercion to type `U`. - -* Tuples, where a tuple is a coercion site to type `(U_0, U_1, ..., U_n)`. -Each sub-expression is a coercion site to the respective type, e.g. the -zeroth sub-expression is a coercion site to type `U_0`. - -* Parenthesized sub-expressions (`(e)`): if the expression has type `U`, then -the sub-expression is a coercion site to `U`. - -* Blocks: if a block has type `U`, then the last expression in the block (if -it is not semicolon-terminated) is a coercion site to `U`. This includes -blocks which are part of control flow statements, such as `if`/`else`, if -the block has a known type. - -### Coercion types - -Coercion is allowed between the following types: - -* `T` to `U` if `T` is a subtype of `U` (*reflexive case*) - -* `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to `T_3` -(*transitive case*) - - Note that this is not fully supported yet - -* `&mut T` to `&T` - -* `*mut T` to `*const T` - -* `&T` to `*const T` - -* `&mut T` to `*mut T` - -* `&T` to `&U` if `T` implements `Deref`. For example: - - ```rust - use std::ops::Deref; - - struct CharContainer { - value: char, - } - - impl Deref for CharContainer { - type Target = char; - - fn deref<'a>(&'a self) -> &'a char { - &self.value - } - } - - fn foo(arg: &char) {} - - fn main() { - let x = &mut CharContainer { value: 'y' }; - foo(x); //&mut CharContainer is coerced to &char. - } - ``` - -* `&mut T` to `&mut U` if `T` implements `DerefMut`. - -* TyCtor(`T`) to TyCtor(coerce_inner(`T`)), where TyCtor(`T`) is one of - - `&T` - - `&mut T` - - `*const T` - - `*mut T` - - `Box` - - and where - - coerce_inner(`[T, ..n]`) = `[T]` - - coerce_inner(`T`) = `U` where `T` is a concrete type which implements the - trait `U`. - - In the future, coerce_inner will be recursively extended to tuples and - structs. In addition, coercions from sub-traits to super-traits will be - added. See [RFC 401] for more details. - -# Special traits - -Several traits define special evaluation behavior. - -## The `Copy` trait - -The `Copy` trait changes the semantics of a type implementing it. Values whose -type implements `Copy` are copied rather than moved upon assignment. - -## The `Sized` trait - -The `Sized` trait indicates that the size of this type is known at compile-time. - -## The `Drop` trait - -The `Drop` trait provides a destructor, to be run whenever a value of this type -is to be destroyed. - -## The `Deref` trait - -The `Deref` trait allows a type to implicitly implement all the methods -of the type `U`. When attempting to resolve a method call, the compiler will search -the top-level type for the implementation of the called method. If no such method is -found, `.deref()` is called and the compiler continues to search for the method -implementation in the returned type `U`. - -## The `Send` trait - -The `Send` trait indicates that a value of this type is safe to send from one -thread to another. - -## The `Sync` trait - -The `Sync` trait indicates that a value of this type is safe to share between -multiple threads. - -# Memory model - -A Rust program's memory consists of a static set of *items* and a *heap*. -Immutable portions of the heap may be safely shared between threads, mutable -portions may not be safely shared, but several mechanisms for effectively-safe -sharing of mutable values, built on unsafe code but enforcing a safe locking -discipline, exist in the standard library. - -Allocations in the stack consist of *variables*, and allocations in the heap -consist of *boxes*. - -### Memory allocation and lifetime - -The _items_ of a program are those functions, modules and types that have their -value calculated at compile-time and stored uniquely in the memory image of the -rust process. Items are neither dynamically allocated nor freed. - -The _heap_ is a general term that describes boxes. The lifetime of an -allocation in the heap depends on the lifetime of the box values pointing to -it. Since box values may themselves be passed in and out of frames, or stored -in the heap, heap allocations may outlive the frame they are allocated within. -An allocation in the heap is guaranteed to reside at a single location in the -heap for the whole lifetime of the allocation - it will never be relocated as -a result of moving a box value. - -### Memory ownership - -When a stack frame is exited, its local allocations are all released, and its -references to boxes are dropped. - -### Variables - -A _variable_ is a component of a stack frame, either a named function parameter, -an anonymous [temporary](#lvalues-rvalues-and-temporaries), or a named local -variable. - -A _local variable_ (or *stack-local* allocation) holds a value directly, -allocated within the stack's memory. The value is a part of the stack frame. - -Local variables are immutable unless declared otherwise like: `let mut x = ...`. - -Function parameters are immutable unless declared with `mut`. The `mut` keyword -applies only to the following parameter (so `|mut x, y|` and `fn f(mut x: -Box, y: Box)` declare one mutable variable `x` and one immutable -variable `y`). - -Methods that take either `self` or `Box` can optionally place them in a -mutable variable by prefixing them with `mut` (similar to regular arguments): - -``` -trait Changer: Sized { - fn change(mut self) {} - fn modify(mut self: Box) {} -} -``` - -Local variables are not initialized when allocated; the entire frame worth of -local variables are allocated at once, on frame-entry, in an uninitialized -state. Subsequent statements within a function may or may not initialize the -local variables. Local variables can be used only after they have been -initialized; this is enforced by the compiler. - -# Linkage - -The Rust compiler supports various methods to link crates together both -statically and dynamically. This section will explore the various methods to -link Rust crates together, and more information about native libraries can be -found in the [FFI section of the book][ffi]. - -In one session of compilation, the compiler can generate multiple artifacts -through the usage of either command line flags or the `crate_type` attribute. -If one or more command line flags are specified, all `crate_type` attributes will -be ignored in favor of only building the artifacts specified by command line. - -* `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be - produced. This requires that there is a `main` function in the crate which - will be run when the program begins executing. This will link in all Rust and - native dependencies, producing a distributable binary. - -* `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced. - This is an ambiguous concept as to what exactly is produced because a library - can manifest itself in several forms. The purpose of this generic `lib` option - is to generate the "compiler recommended" style of library. The output library - will always be usable by rustc, but the actual type of library may change from - time-to-time. The remaining output types are all different flavors of - libraries, and the `lib` type can be seen as an alias for one of them (but the - actual one is compiler-defined). - -* `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will - be produced. This is different from the `lib` output type in that this forces - dynamic library generation. The resulting dynamic library can be used as a - dependency for other libraries and/or executables. This output type will - create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on - windows. - -* `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system - library will be produced. This is different from other library outputs in that - the Rust compiler will never attempt to link to `staticlib` outputs. The - purpose of this output type is to create a static library containing all of - the local crate's code along with all upstream dependencies. The static - library is actually a `*.a` archive on linux and osx and a `*.lib` file on - windows. This format is recommended for use in situations such as linking - Rust code into an existing non-Rust application because it will not have - dynamic dependencies on other Rust code. - -* `--crate-type=cdylib`, `#[crate_type = "cdylib"]` - A dynamic system - library will be produced. This is used when compiling Rust code as - a dynamic library to be loaded from another language. This output type will - create `*.so` files on Linux, `*.dylib` files on OSX, and `*.dll` files on - Windows. - -* `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be - produced. This is used as an intermediate artifact and can be thought of as a - "static Rust library". These `rlib` files, unlike `staticlib` files, are - interpreted by the Rust compiler in future linkage. This essentially means - that `rustc` will look for metadata in `rlib` files like it looks for metadata - in dynamic libraries. This form of output is used to produce statically linked - executables as well as `staticlib` outputs. - -* `--crate-type=proc-macro`, `#[crate_type = "proc-macro"]` - The output - produced is not specified, but if a `-L` path is provided to it then the - compiler will recognize the output artifacts as a macro and it can be loaded - for a program. If a crate is compiled with the `proc-macro` crate type it - will forbid exporting any items in the crate other than those functions - tagged `#[proc_macro_derive]` and those functions must also be placed at the - crate root. Finally, the compiler will automatically set the - `cfg(proc_macro)` annotation whenever any crate type of a compilation is the - `proc-macro` crate type. - -Note that these outputs are stackable in the sense that if multiple are -specified, then the compiler will produce each form of output at once without -having to recompile. However, this only applies for outputs specified by the -same method. If only `crate_type` attributes are specified, then they will all -be built, but if one or more `--crate-type` command line flags are specified, -then only those outputs will be built. - -With all these different kinds of outputs, if crate A depends on crate B, then -the compiler could find B in various different forms throughout the system. The -only forms looked for by the compiler, however, are the `rlib` format and the -dynamic library format. With these two options for a dependent library, the -compiler must at some point make a choice between these two formats. With this -in mind, the compiler follows these rules when determining what format of -dependencies will be used: - -1. If a static library is being produced, all upstream dependencies are - required to be available in `rlib` formats. This requirement stems from the - reason that a dynamic library cannot be converted into a static format. - - Note that it is impossible to link in native dynamic dependencies to a static - library, and in this case warnings will be printed about all unlinked native - dynamic dependencies. - -2. If an `rlib` file is being produced, then there are no restrictions on what - format the upstream dependencies are available in. It is simply required that - all upstream dependencies be available for reading metadata from. - - The reason for this is that `rlib` files do not contain any of their upstream - dependencies. It wouldn't be very efficient for all `rlib` files to contain a - copy of `libstd.rlib`! - -3. If an executable is being produced and the `-C prefer-dynamic` flag is not - specified, then dependencies are first attempted to be found in the `rlib` - format. If some dependencies are not available in an rlib format, then - dynamic linking is attempted (see below). - -4. If a dynamic library or an executable that is being dynamically linked is - being produced, then the compiler will attempt to reconcile the available - dependencies in either the rlib or dylib format to create a final product. - - A major goal of the compiler is to ensure that a library never appears more - than once in any artifact. For example, if dynamic libraries B and C were - each statically linked to library A, then a crate could not link to B and C - together because there would be two copies of A. The compiler allows mixing - the rlib and dylib formats, but this restriction must be satisfied. - - The compiler currently implements no method of hinting what format a library - should be linked with. When dynamically linking, the compiler will attempt to - maximize dynamic dependencies while still allowing some dependencies to be - linked in via an rlib. - - For most situations, having all libraries available as a dylib is recommended - if dynamically linking. For other situations, the compiler will emit a - warning if it is unable to determine which formats to link each library with. - -In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for -all compilation needs, and the other options are just available if more -fine-grained control is desired over the output format of a Rust crate. - -# Unsafety - -Unsafe operations are those that potentially violate the memory-safety -guarantees of Rust's static semantics. - -The following language level features cannot be used in the safe subset of -Rust: - -- Dereferencing a [raw pointer](#pointer-types). -- Reading or writing a [mutable static variable](#mutable-statics). -- Calling an unsafe function (including an intrinsic or foreign function). - -## Unsafe functions - -Unsafe functions are functions that are not safe in all contexts and/or for all -possible inputs. Such a function must be prefixed with the keyword `unsafe` and -can only be called from an `unsafe` block or another `unsafe` function. - -## Unsafe blocks - -A block of code can be prefixed with the `unsafe` keyword, to permit calling -`unsafe` functions or dereferencing raw pointers within a safe function. - -When a programmer has sufficient conviction that a sequence of potentially -unsafe operations is actually safe, they can encapsulate that sequence (taken -as a whole) within an `unsafe` block. The compiler will consider uses of such -code safe, in the surrounding context. - -Unsafe blocks are used to wrap foreign libraries, make direct use of hardware -or implement features not directly present in the language. For example, Rust -provides the language features necessary to implement memory-safe concurrency -in the language but the implementation of threads and message passing is in the -standard library. - -Rust's type system is a conservative approximation of the dynamic safety -requirements, so in some cases there is a performance cost to using safe code. -For example, a doubly-linked list is not a tree structure and can only be -represented with reference-counted pointers in safe code. By using `unsafe` -blocks to represent the reverse links as raw pointers, it can be implemented -with only boxes. - -## Behavior considered undefined - -The following is a list of behavior which is forbidden in all Rust code, -including within `unsafe` blocks and `unsafe` functions. Type checking provides -the guarantee that these issues are never caused by safe code. - -* Data races -* Dereferencing a null/dangling raw pointer -* Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values) - (uninitialized) memory -* Breaking the [pointer aliasing - rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules) - with raw pointers (a subset of the rules used by C) -* `&mut T` and `&T` follow LLVM’s scoped [noalias] model, except if the `&T` - contains an `UnsafeCell`. Unsafe code must not violate these aliasing - guarantees. -* Mutating non-mutable data (that is, data reached through a shared reference or - data owned by a `let` binding), unless that data is contained within an `UnsafeCell`. -* Invoking undefined behavior via compiler intrinsics: - * Indexing outside of the bounds of an object with `std::ptr::offset` - (`offset` intrinsic), with - the exception of one byte past the end which is permitted. - * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64` - intrinsics) on overlapping buffers -* Invalid values in primitive types, even in private fields/locals: - * Dangling/null references or boxes - * A value other than `false` (0) or `true` (1) in a `bool` - * A discriminant in an `enum` not included in the type definition - * A value in a `char` which is a surrogate or above `char::MAX` - * Non-UTF-8 byte sequences in a `str` -* Unwinding into Rust from foreign code or unwinding from Rust into foreign - code. Rust's failure system is not compatible with exception handling in - other languages. Unwinding must be caught and handled at FFI boundaries. - -[noalias]: http://llvm.org/docs/LangRef.html#noalias - -## Behavior not considered unsafe - -This is a list of behavior not considered *unsafe* in Rust terms, but that may -be undesired. - -* Deadlocks -* Leaks of memory and other resources -* Exiting without calling destructors -* Integer overflow - - Overflow is considered "unexpected" behavior and is always user-error, - unless the `wrapping` primitives are used. In non-optimized builds, the compiler - will insert debug checks that panic on overflow, but in optimized builds overflow - instead results in wrapped values. See [RFC 560] for the rationale and more details. - -[RFC 560]: https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md - -# Appendix: Influences - -Rust is not a particularly original language, with design elements coming from -a wide range of sources. Some of these are listed below (including elements -that have since been removed): - -* SML, OCaml: algebraic data types, pattern matching, type inference, - semicolon statement separation -* C++: references, RAII, smart pointers, move semantics, monomorphization, - memory model -* ML Kit, Cyclone: region based memory management -* Haskell (GHC): typeclasses, type families -* Newsqueak, Alef, Limbo: channels, concurrency -* Erlang: message passing, thread failure, ~~linked thread failure~~, - ~~lightweight concurrency~~ -* Swift: optional bindings -* Scheme: hygienic macros -* C#: attributes -* Ruby: ~~block syntax~~ -* NIL, Hermes: ~~typestate~~ -* [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and - pattern syntax - -[ffi]: book/ffi.html -[plugin]: book/compiler-plugins.html -[procedural macros]: book/procedural-macros.html +We've split up the reference into chapters. Please find it at its new +home [here](reference/index.html). diff --git a/src/doc/reference/.gitignore b/src/doc/reference/.gitignore new file mode 100644 index 0000000000000..7585238efedfc --- /dev/null +++ b/src/doc/reference/.gitignore @@ -0,0 +1 @@ +book diff --git a/src/doc/reference/src/SUMMARY.md b/src/doc/reference/src/SUMMARY.md new file mode 100644 index 0000000000000..a07e195a7184f --- /dev/null +++ b/src/doc/reference/src/SUMMARY.md @@ -0,0 +1,58 @@ +# The Rust Reference + +[Introduction](introduction.md) + +- [Notation](notation.md) + - [Unicode productions](unicode-productions.md) + - [String table productions](string-table-productions.md) + +- [Lexical structure](lexical-structure.md) + - [Input format](input-format.md) + - [Identifiers](identifiers.md) + - [Comments](comments.md) + - [Whitespace](whitespace.md) + - [Tokens](tokens.md) + - [Paths](paths.md) + +- [Macros](macros.md) + - [Macros By Example](macros-by-example.md) + - [Procedrual Macros](procedural-macros.md) + +- [Crates and source files](crates-and-source-files.md) + +- [Items and attributes](items-and-attributes.md) + - [Items](items.md) + - [Visibility and Privacy](visibility-and-privacy.md) + - [Attributes](attributes.md) + +- [Statements and expressions](statements-and-expressions.md) + - [Statements](statements.md) + - [Expressions](expressions.md) + +- [Type system](type-system.md) + - [Types](types.md) + - [Subtyping](subtyping.md) + - [Type coercions](type-coercions.md) + +- [Special traits](special-traits.md) + - [The Copy trait](the-copy-trait.md) + - [The Sized trait](the-sized-trait.md) + - [The Drop trait](the-drop-trait.md) + - [The Deref trait](the-deref-trait.md) + - [The Send trait](the-send-trait.md) + - [The Sync trait](the-sync-trait.md) + +- [Memory model](memory-model.md) + - [Memory allocation and lifetime](memory-allocation-and-lifetime.md) + - [Memory ownership](memory-ownership.md) + - [Variables](variables.md) + +- [Linkage](linkage.md) + +- [Unsafety](unsafety.md) + - [Unsafe functions](unsafe-functions.md) + - [Unsafe blocks](unsafe-blocks.md) + - [Behavior considered undefined](behavior-considered-undefined.md) + - [Behavior not considered unsafe](behavior-not-considered-unsafe.md) + +[Appendix: Influences](influences.md) diff --git a/src/doc/reference/src/attributes.md b/src/doc/reference/src/attributes.md new file mode 100644 index 0000000000000..da43e1cc057eb --- /dev/null +++ b/src/doc/reference/src/attributes.md @@ -0,0 +1,630 @@ +# Attributes + +Any item declaration may have an _attribute_ applied to it. Attributes in Rust +are modeled on Attributes in ECMA-335, with the syntax coming from ECMA-334 +(C#). An attribute is a general, free-form metadatum that is interpreted +according to name, convention, and language and compiler version. Attributes +may appear as any of: + +* A single identifier, the attribute name +* An identifier followed by the equals sign '=' and a literal, providing a + key/value pair +* An identifier followed by a parenthesized list of sub-attribute arguments + +Attributes with a bang ("!") after the hash ("#") apply to the item that the +attribute is declared within. Attributes that do not have a bang after the hash +apply to the item that follows the attribute. + +An example of attributes: + +```{.rust} +// General metadata applied to the enclosing module or crate. +#![crate_type = "lib"] + +// A function marked as a unit test +#[test] +fn test_foo() { + /* ... */ +} + +// A conditionally-compiled module +#[cfg(target_os="linux")] +mod bar { + /* ... */ +} + +// A lint attribute used to suppress a warning/error +#[allow(non_camel_case_types)] +type int8_t = i8; +``` + +> **Note:** At some point in the future, the compiler will distinguish between +> language-reserved and user-available attributes. Until then, there is +> effectively no difference between an attribute handled by a loadable syntax +> extension and the compiler. + +## Crate-only attributes + +- `crate_name` - specify the crate's crate name. +- `crate_type` - see [linkage](linkage.html). +- `feature` - see [compiler features](#compiler-features). +- `no_builtins` - disable optimizing certain code patterns to invocations of + library functions that are assumed to exist +- `no_main` - disable emitting the `main` symbol. Useful when some other + object being linked to defines `main`. +- `no_start` - disable linking to the `native` crate, which specifies the + "start" language item. +- `no_std` - disable linking to the `std` crate. +- `plugin` - load a list of named crates as compiler plugins, e.g. + `#![plugin(foo, bar)]`. Optional arguments for each plugin, + i.e. `#![plugin(foo(... args ...))]`, are provided to the plugin's + registrar function. The `plugin` feature gate is required to use + this attribute. +- `recursion_limit` - Sets the maximum depth for potentially + infinitely-recursive compile-time operations like + auto-dereference or macro expansion. The default is + `#![recursion_limit="64"]`. + +### Module-only attributes + +- `no_implicit_prelude` - disable injecting `use std::prelude::*` in this + module. +- `path` - specifies the file to load the module from. `#[path="foo.rs"] mod + bar;` is equivalent to `mod bar { /* contents of foo.rs */ }`. The path is + taken relative to the directory that the current module is in. + +## Function-only attributes + +- `main` - indicates that this function should be passed to the entry point, + rather than the function in the crate root named `main`. +- `plugin_registrar` - mark this function as the registration point for + [compiler plugins][plugin], such as loadable syntax extensions. +- `start` - indicates that this function should be used as the entry point, + overriding the "start" language item. See the "start" [language + item](#language-items) for more details. +- `test` - indicates that this function is a test function, to only be compiled + in case of `--test`. +- `should_panic` - indicates that this test function should panic, inverting the success condition. +- `cold` - The function is unlikely to be executed, so optimize it (and calls + to it) differently. +- `naked` - The function utilizes a custom ABI or custom inline ASM that requires + epilogue and prologue to be skipped. + +## Static-only attributes + +- `thread_local` - on a `static mut`, this signals that the value of this + static may change depending on the current thread. The exact consequences of + this are implementation-defined. + +## FFI attributes + +On an `extern` block, the following attributes are interpreted: + +- `link_args` - specify arguments to the linker, rather than just the library + name and type. This is feature gated and the exact behavior is + implementation-defined (due to variety of linker invocation syntax). +- `link` - indicate that a native library should be linked to for the + declarations in this block to be linked correctly. `link` supports an optional + `kind` key with three possible values: `dylib`, `static`, and `framework`. See + [external blocks](items.html#external-blocks) for more about external blocks. Two + examples: `#[link(name = "readline")]` and + `#[link(name = "CoreFoundation", kind = "framework")]`. +- `linked_from` - indicates what native library this block of FFI items is + coming from. This attribute is of the form `#[linked_from = "foo"]` where + `foo` is the name of a library in either `#[link]` or a `-l` flag. This + attribute is currently required to export symbols from a Rust dynamic library + on Windows, and it is feature gated behind the `linked_from` feature. + +On declarations inside an `extern` block, the following attributes are +interpreted: + +- `link_name` - the name of the symbol that this function or static should be + imported as. +- `linkage` - on a static, this specifies the [linkage + type](http://llvm.org/docs/LangRef.html#linkage-types). + +On `enum`s: + +- `repr` - on C-like enums, this sets the underlying type used for + representation. Takes one argument, which is the primitive + type this enum should be represented for, or `C`, which specifies that it + should be the default `enum` size of the C ABI for that platform. Note that + enum representation in C is undefined, and this may be incorrect when the C + code is compiled with certain flags. + +On `struct`s: + +- `repr` - specifies the representation to use for this struct. Takes a list + of options. The currently accepted ones are `C` and `packed`, which may be + combined. `C` will use a C ABI compatible struct layout, and `packed` will + remove any padding between fields (note that this is very fragile and may + break platforms which require aligned access). + +## Macro-related attributes + +- `macro_use` on a `mod` — macros defined in this module will be visible in the + module's parent, after this module has been included. + +- `macro_use` on an `extern crate` — load macros from this crate. An optional + list of names `#[macro_use(foo, bar)]` restricts the import to just those + macros named. The `extern crate` must appear at the crate root, not inside + `mod`, which ensures proper function of the [`$crate` macro + variable](../book/macros.html#the-variable-crate). + +- `macro_reexport` on an `extern crate` — re-export the named macros. + +- `macro_export` - export a macro for cross-crate usage. + +- `no_link` on an `extern crate` — even if we load this crate for macros, don't + link it into the output. + +See the [macros section of the +book](../book/macros.html#scoping-and-macro-importexport) for more information on +macro scope. + +## Miscellaneous attributes + +- `deprecated` - mark the item as deprecated; the full attribute is + `#[deprecated(since = "crate version", note = "...")`, where both arguments + are optional. +- `export_name` - on statics and functions, this determines the name of the + exported symbol. +- `link_section` - on statics and functions, this specifies the section of the + object file that this item's contents will be placed into. +- `no_mangle` - on any item, do not apply the standard name mangling. Set the + symbol for this item to its identifier. +- `simd` - on certain tuple structs, derive the arithmetic operators, which + lower to the target's SIMD instructions, if any; the `simd` feature gate + is necessary to use this attribute. +- `unsafe_destructor_blind_to_params` - on `Drop::drop` method, asserts that the + destructor code (and all potential specializations of that code) will + never attempt to read from nor write to any references with lifetimes + that come in via generic parameters. This is a constraint we cannot + currently express via the type system, and therefore we rely on the + programmer to assert that it holds. Adding this to a Drop impl causes + the associated destructor to be considered "uninteresting" by the + Drop-Check rule, and thus it can help sidestep data ordering + constraints that would otherwise be introduced by the Drop-Check + rule. Such sidestepping of the constraints, if done incorrectly, can + lead to undefined behavior (in the form of reading or writing to data + outside of its dynamic extent), and thus this attribute has the word + "unsafe" in its name. To use this, the + `unsafe_destructor_blind_to_params` feature gate must be enabled. +- `doc` - Doc comments such as `/// foo` are equivalent to `#[doc = "foo"]`. +- `rustc_on_unimplemented` - Write a custom note to be shown along with the error + when the trait is found to be unimplemented on a type. + You may use format arguments like `{T}`, `{A}` to correspond to the + types at the point of use corresponding to the type parameters of the + trait of the same name. `{Self}` will be replaced with the type that is supposed + to implement the trait but doesn't. To use this, the `on_unimplemented` feature gate + must be enabled. +- `must_use` - on structs and enums, will warn if a value of this type isn't used or + assigned to a variable. You may also include an optional message by using + `#[must_use = "message"]` which will be given alongside the warning. + +### Conditional compilation + +Sometimes one wants to have different compiler outputs from the same code, +depending on build target, such as targeted operating system, or to enable +release builds. + +Configuration options are boolean (on or off) and are named either with a +single identifier (e.g. `foo`) or an identifier and a string (e.g. `foo = "bar"`; +the quotes are required and spaces around the `=` are unimportant). Note that +similarly-named options, such as `foo`, `foo="bar"` and `foo="baz"` may each be set +or unset independently. + +Configuration options are either provided by the compiler or passed in on the +command line using `--cfg` (e.g. `rustc main.rs --cfg foo --cfg 'bar="baz"'`). +Rust code then checks for their presence using the `#[cfg(...)]` attribute: + +``` +// The function is only included in the build when compiling for OSX +#[cfg(target_os = "macos")] +fn macos_only() { + // ... +} + +// This function is only included when either foo or bar is defined +#[cfg(any(foo, bar))] +fn needs_foo_or_bar() { + // ... +} + +// This function is only included when compiling for a unixish OS with a 32-bit +// architecture +#[cfg(all(unix, target_pointer_width = "32"))] +fn on_32bit_unix() { + // ... +} + +// This function is only included when foo is not defined +#[cfg(not(foo))] +fn needs_not_foo() { + // ... +} +``` + +This illustrates some conditional compilation can be achieved using the +`#[cfg(...)]` attribute. `any`, `all` and `not` can be used to assemble +arbitrarily complex configurations through nesting. + +The following configurations must be defined by the implementation: + +* `target_arch = "..."` - Target CPU architecture, such as `"x86"`, + `"x86_64"` `"mips"`, `"powerpc"`, `"powerpc64"`, `"arm"`, or + `"aarch64"`. This value is closely related to the first element of + the platform target triple, though it is not identical. +* `target_os = "..."` - Operating system of the target, examples + include `"windows"`, `"macos"`, `"ios"`, `"linux"`, `"android"`, + `"freebsd"`, `"dragonfly"`, `"bitrig"` , `"openbsd"` or + `"netbsd"`. This value is closely related to the second and third + element of the platform target triple, though it is not identical. +* `target_family = "..."` - Operating system family of the target, e. g. + `"unix"` or `"windows"`. The value of this configuration option is defined + as a configuration itself, like `unix` or `windows`. +* `unix` - See `target_family`. +* `windows` - See `target_family`. +* `target_env = ".."` - Further disambiguates the target platform with + information about the ABI/libc. Presently this value is either + `"gnu"`, `"msvc"`, `"musl"`, or the empty string. For historical + reasons this value has only been defined as non-empty when needed + for disambiguation. Thus on many GNU platforms this value will be + empty. This value is closely related to the fourth element of the + platform target triple, though it is not identical. For example, + embedded ABIs such as `gnueabihf` will simply define `target_env` as + `"gnu"`. +* `target_endian = "..."` - Endianness of the target CPU, either `"little"` or + `"big"`. +* `target_pointer_width = "..."` - Target pointer width in bits. This is set + to `"32"` for targets with 32-bit pointers, and likewise set to `"64"` for + 64-bit pointers. +* `target_has_atomic = "..."` - Set of integer sizes on which the target can perform + atomic operations. Values are `"8"`, `"16"`, `"32"`, `"64"` and `"ptr"`. +* `target_vendor = "..."` - Vendor of the target, for example `apple`, `pc`, or + simply `"unknown"`. +* `test` - Enabled when compiling the test harness (using the `--test` flag). +* `debug_assertions` - Enabled by default when compiling without optimizations. + This can be used to enable extra debugging code in development but not in + production. For example, it controls the behavior of the standard library's + `debug_assert!` macro. + +You can also set another attribute based on a `cfg` variable with `cfg_attr`: + +```rust,ignore +#[cfg_attr(a, b)] +``` + +This is the same as `#[b]` if `a` is set by `cfg`, and nothing otherwise. + +Lastly, configuration options can be used in expressions by invoking the `cfg!` +macro: `cfg!(a)` evaluates to `true` if `a` is set, and `false` otherwise. + +### Lint check attributes + +A lint check names a potentially undesirable coding pattern, such as +unreachable code or omitted documentation, for the static entity to which the +attribute applies. + +For any lint check `C`: + +* `allow(C)` overrides the check for `C` so that violations will go + unreported, +* `deny(C)` signals an error after encountering a violation of `C`, +* `forbid(C)` is the same as `deny(C)`, but also forbids changing the lint + level afterwards, +* `warn(C)` warns about violations of `C` but continues compilation. + +The lint checks supported by the compiler can be found via `rustc -W help`, +along with their default settings. [Compiler +plugins](../book/compiler-plugins.html#lint-plugins) can provide additional +lint checks. + +```{.ignore} +pub mod m1 { + // Missing documentation is ignored here + #[allow(missing_docs)] + pub fn undocumented_one() -> i32 { 1 } + + // Missing documentation signals a warning here + #[warn(missing_docs)] + pub fn undocumented_too() -> i32 { 2 } + + // Missing documentation signals an error here + #[deny(missing_docs)] + pub fn undocumented_end() -> i32 { 3 } +} +``` + +This example shows how one can use `allow` and `warn` to toggle a particular +check on and off: + +```{.ignore} +#[warn(missing_docs)] +pub mod m2{ + #[allow(missing_docs)] + pub mod nested { + // Missing documentation is ignored here + pub fn undocumented_one() -> i32 { 1 } + + // Missing documentation signals a warning here, + // despite the allow above. + #[warn(missing_docs)] + pub fn undocumented_two() -> i32 { 2 } + } + + // Missing documentation signals a warning here + pub fn undocumented_too() -> i32 { 3 } +} +``` + +This example shows how one can use `forbid` to disallow uses of `allow` for +that lint check: + +```{.ignore} +#[forbid(missing_docs)] +pub mod m3 { + // Attempting to toggle warning signals an error here + #[allow(missing_docs)] + /// Returns 2. + pub fn undocumented_too() -> i32 { 2 } +} +``` + +### Language items + +Some primitive Rust operations are defined in Rust code, rather than being +implemented directly in C or assembly language. The definitions of these +operations have to be easy for the compiler to find. The `lang` attribute +makes it possible to declare these operations. For example, the `str` module +in the Rust standard library defines the string equality function: + +```{.ignore} +#[lang = "str_eq"] +pub fn eq_slice(a: &str, b: &str) -> bool { + // details elided +} +``` + +The name `str_eq` has a special meaning to the Rust compiler, and the presence +of this definition means that it will use this definition when generating calls +to the string equality function. + +The set of language items is currently considered unstable. A complete +list of the built-in language items will be added in the future. + +### Inline attributes + +The inline attribute suggests that the compiler should place a copy of +the function or static in the caller, rather than generating code to +call the function or access the static where it is defined. + +The compiler automatically inlines functions based on internal heuristics. +Incorrectly inlining functions can actually make the program slower, so it +should be used with care. + +`#[inline]` and `#[inline(always)]` always cause the function to be serialized +into the crate metadata to allow cross-crate inlining. + +There are three different types of inline attributes: + +* `#[inline]` hints the compiler to perform an inline expansion. +* `#[inline(always)]` asks the compiler to always perform an inline expansion. +* `#[inline(never)]` asks the compiler to never perform an inline expansion. + +### `derive` + +The `derive` attribute allows certain traits to be automatically implemented +for data structures. For example, the following will create an `impl` for the +`PartialEq` and `Clone` traits for `Foo`, the type parameter `T` will be given +the `PartialEq` or `Clone` constraints for the appropriate `impl`: + +``` +#[derive(PartialEq, Clone)] +struct Foo { + a: i32, + b: T, +} +``` + +The generated `impl` for `PartialEq` is equivalent to + +``` +# struct Foo { a: i32, b: T } +impl PartialEq for Foo { + fn eq(&self, other: &Foo) -> bool { + self.a == other.a && self.b == other.b + } + + fn ne(&self, other: &Foo) -> bool { + self.a != other.a || self.b != other.b + } +} +``` + +You can implement `derive` for your own type through [procedural +macros](procedural-macros.html). + +### Compiler Features + +Certain aspects of Rust may be implemented in the compiler, but they're not +necessarily ready for every-day use. These features are often of "prototype +quality" or "almost production ready", but may not be stable enough to be +considered a full-fledged language feature. + +For this reason, Rust recognizes a special crate-level attribute of the form: + +```{.ignore} +#![feature(feature1, feature2, feature3)] +``` + +This directive informs the compiler that the feature list: `feature1`, +`feature2`, and `feature3` should all be enabled. This is only recognized at a +crate-level, not at a module-level. Without this directive, all features are +considered off, and using the features will result in a compiler error. + +The currently implemented features of the reference compiler are: + +* `advanced_slice_patterns` - See the [match + expressions](expressions.html#match-expressions) + section for discussion; the exact semantics of +slice patterns are subject to change, so some types are still unstable. + +* `slice_patterns` - OK, actually, slice patterns are just scary and + completely unstable. + +* `asm` - The `asm!` macro provides a means for inline assembly. This is often + useful, but the exact syntax for this feature along with its + semantics are likely to change, so this macro usage must be opted + into. + +* `associated_consts` - Allows constants to be defined in `impl` and `trait` + blocks, so that they can be associated with a type or + trait in a similar manner to methods and associated + types. + +* `box_patterns` - Allows `box` patterns, the exact semantics of which + is subject to change. + +* `box_syntax` - Allows use of `box` expressions, the exact semantics of which + is subject to change. + +* `cfg_target_vendor` - Allows conditional compilation using the `target_vendor` + matcher which is subject to change. + +* `cfg_target_has_atomic` - Allows conditional compilation using the `target_has_atomic` + matcher which is subject to change. + +* `concat_idents` - Allows use of the `concat_idents` macro, which is in many + ways insufficient for concatenating identifiers, and may be + removed entirely for something more wholesome. + +* `custom_attribute` - Allows the usage of attributes unknown to the compiler + so that new attributes can be added in a backwards compatible + manner (RFC 572). + +* `custom_derive` - Allows the use of `#[derive(Foo,Bar)]` as sugar for + `#[derive_Foo] #[derive_Bar]`, which can be user-defined syntax + extensions. + +* `inclusive_range_syntax` - Allows use of the `a...b` and `...b` syntax for inclusive ranges. + +* `inclusive_range` - Allows use of the types that represent desugared inclusive ranges. + +* `intrinsics` - Allows use of the "rust-intrinsics" ABI. Compiler intrinsics + are inherently unstable and no promise about them is made. + +* `lang_items` - Allows use of the `#[lang]` attribute. Like `intrinsics`, + lang items are inherently unstable and no promise about them + is made. + +* `link_args` - This attribute is used to specify custom flags to the linker, + but usage is strongly discouraged. The compiler's usage of the + system linker is not guaranteed to continue in the future, and + if the system linker is not used then specifying custom flags + doesn't have much meaning. + +* `link_llvm_intrinsics` - Allows linking to LLVM intrinsics via + `#[link_name="llvm.*"]`. + +* `linkage` - Allows use of the `linkage` attribute, which is not portable. + +* `log_syntax` - Allows use of the `log_syntax` macro attribute, which is a + nasty hack that will certainly be removed. + +* `main` - Allows use of the `#[main]` attribute, which changes the entry point + into a Rust program. This capability is subject to change. + +* `macro_reexport` - Allows macros to be re-exported from one crate after being imported + from another. This feature was originally designed with the sole + use case of the Rust standard library in mind, and is subject to + change. + +* `non_ascii_idents` - The compiler supports the use of non-ascii identifiers, + but the implementation is a little rough around the + edges, so this can be seen as an experimental feature + for now until the specification of identifiers is fully + fleshed out. + +* `no_std` - Allows the `#![no_std]` crate attribute, which disables the implicit + `extern crate std`. This typically requires use of the unstable APIs + behind the libstd "facade", such as libcore and libcollections. It + may also cause problems when using syntax extensions, including + `#[derive]`. + +* `on_unimplemented` - Allows the `#[rustc_on_unimplemented]` attribute, which allows + trait definitions to add specialized notes to error messages + when an implementation was expected but not found. + +* `optin_builtin_traits` - Allows the definition of default and negative trait + implementations. Experimental. + +* `plugin` - Usage of [compiler plugins][plugin] for custom lints or syntax extensions. + These depend on compiler internals and are subject to change. + +* `plugin_registrar` - Indicates that a crate provides [compiler plugins][plugin]. + +* `quote` - Allows use of the `quote_*!` family of macros, which are + implemented very poorly and will likely change significantly + with a proper implementation. + +* `rustc_attrs` - Gates internal `#[rustc_*]` attributes which may be + for internal use only or have meaning added to them in the future. + +* `rustc_diagnostic_macros`- A mysterious feature, used in the implementation + of rustc, not meant for mortals. + +* `simd` - Allows use of the `#[simd]` attribute, which is overly simple and + not the SIMD interface we want to expose in the long term. + +* `simd_ffi` - Allows use of SIMD vectors in signatures for foreign functions. + The SIMD interface is subject to change. + +* `start` - Allows use of the `#[start]` attribute, which changes the entry point + into a Rust program. This capability, especially the signature for the + annotated function, is subject to change. + +* `thread_local` - The usage of the `#[thread_local]` attribute is experimental + and should be seen as unstable. This attribute is used to + declare a `static` as being unique per-thread leveraging + LLVM's implementation which works in concert with the kernel + loader and dynamic linker. This is not necessarily available + on all platforms, and usage of it is discouraged. + +* `trace_macros` - Allows use of the `trace_macros` macro, which is a nasty + hack that will certainly be removed. + +* `unboxed_closures` - Rust's new closure design, which is currently a work in + progress feature with many known bugs. + +* `allow_internal_unstable` - Allows `macro_rules!` macros to be tagged with the + `#[allow_internal_unstable]` attribute, designed + to allow `std` macros to call + `#[unstable]`/feature-gated functionality + internally without imposing on callers + (i.e. making them behave like function calls in + terms of encapsulation). + +* `default_type_parameter_fallback` - Allows type parameter defaults to + influence type inference. + +* `stmt_expr_attributes` - Allows attributes on expressions. + +* `type_ascription` - Allows type ascription expressions `expr: Type`. + +* `abi_vectorcall` - Allows the usage of the vectorcall calling convention + (e.g. `extern "vectorcall" func fn_();`) + +* `abi_sysv64` - Allows the usage of the system V AMD64 calling convention + (e.g. `extern "sysv64" func fn_();`) + +If a feature is promoted to a language feature, then all existing programs will +start to receive compilation warnings about `#![feature]` directives which enabled +the new feature (because the directive is no longer necessary). However, if a +feature is decided to be removed from the language, errors will be issued (if +there isn't a parser error first). The directive in this case is no longer +necessary, and it's likely that existing code will break if the feature isn't +removed. + +If an unknown feature is found in a directive, it results in a compiler error. +An unknown feature is one which has never been recognized by the compiler. diff --git a/src/doc/reference/src/behavior-considered-undefined.md b/src/doc/reference/src/behavior-considered-undefined.md new file mode 100644 index 0000000000000..b617ee3d78fa7 --- /dev/null +++ b/src/doc/reference/src/behavior-considered-undefined.md @@ -0,0 +1,35 @@ +## Behavior considered undefined + +The following is a list of behavior which is forbidden in all Rust code, +including within `unsafe` blocks and `unsafe` functions. Type checking provides +the guarantee that these issues are never caused by safe code. + +* Data races +* Dereferencing a null/dangling raw pointer +* Reads of [undef](http://llvm.org/docs/LangRef.html#undefined-values) + (uninitialized) memory +* Breaking the [pointer aliasing + rules](http://llvm.org/docs/LangRef.html#pointer-aliasing-rules) + with raw pointers (a subset of the rules used by C) +* `&mut T` and `&T` follow LLVM’s scoped [noalias] model, except if the `&T` + contains an `UnsafeCell`. Unsafe code must not violate these aliasing + guarantees. +* Mutating non-mutable data (that is, data reached through a shared reference or + data owned by a `let` binding), unless that data is contained within an `UnsafeCell`. +* Invoking undefined behavior via compiler intrinsics: + * Indexing outside of the bounds of an object with `std::ptr::offset` + (`offset` intrinsic), with + the exception of one byte past the end which is permitted. + * Using `std::ptr::copy_nonoverlapping_memory` (`memcpy32`/`memcpy64` + intrinsics) on overlapping buffers +* Invalid values in primitive types, even in private fields/locals: + * Dangling/null references or boxes + * A value other than `false` (0) or `true` (1) in a `bool` + * A discriminant in an `enum` not included in the type definition + * A value in a `char` which is a surrogate or above `char::MAX` + * Non-UTF-8 byte sequences in a `str` +* Unwinding into Rust from foreign code or unwinding from Rust into foreign + code. Rust's failure system is not compatible with exception handling in + other languages. Unwinding must be caught and handled at FFI boundaries. + +[noalias]: http://llvm.org/docs/LangRef.html#noalias diff --git a/src/doc/reference/src/behavior-not-considered-unsafe.md b/src/doc/reference/src/behavior-not-considered-unsafe.md new file mode 100644 index 0000000000000..e16103372f552 --- /dev/null +++ b/src/doc/reference/src/behavior-not-considered-unsafe.md @@ -0,0 +1,15 @@ +## Behavior not considered unsafe + +This is a list of behavior not considered *unsafe* in Rust terms, but that may +be undesired. + +* Deadlocks +* Leaks of memory and other resources +* Exiting without calling destructors +* Integer overflow + - Overflow is considered "unexpected" behavior and is always user-error, + unless the `wrapping` primitives are used. In non-optimized builds, the compiler + will insert debug checks that panic on overflow, but in optimized builds overflow + instead results in wrapped values. See [RFC 560] for the rationale and more details. + +[RFC 560]: https://github.com/rust-lang/rfcs/blob/master/text/0560-integer-overflow.md diff --git a/src/doc/reference/src/comments.md b/src/doc/reference/src/comments.md new file mode 100644 index 0000000000000..784e19affd9da --- /dev/null +++ b/src/doc/reference/src/comments.md @@ -0,0 +1,20 @@ +# Comments + +Comments in Rust code follow the general C++ style of line (`//`) and +block (`/* ... */`) comment forms. Nested block comments are supported. + +Line comments beginning with exactly _three_ slashes (`///`), and block +comments (`/** ... */`), are interpreted as a special syntax for `doc` +[attributes]. That is, they are equivalent to writing +`#[doc="..."]` around the body of the comment, i.e., `/// Foo` turns into +`#[doc="Foo"]`. + +Line comments beginning with `//!` and block comments `/*! ... */` are +doc comments that apply to the parent of the comment, rather than the item +that follows. That is, they are equivalent to writing `#![doc="..."]` around +the body of the comment. `//!` comments are usually used to document +modules that occupy a source file. + +Non-doc comments are interpreted as a form of whitespace. + +[attributes]: attributes.html diff --git a/src/doc/reference/src/crates-and-source-files.md b/src/doc/reference/src/crates-and-source-files.md new file mode 100644 index 0000000000000..d41a8dc964095 --- /dev/null +++ b/src/doc/reference/src/crates-and-source-files.md @@ -0,0 +1,73 @@ +# Crates and source files + +Although Rust, like any other language, can be implemented by an interpreter as +well as a compiler, the only existing implementation is a compiler, +and the language has +always been designed to be compiled. For these reasons, this section assumes a +compiler. + +Rust's semantics obey a *phase distinction* between compile-time and +run-time.[^phase-distinction] Semantic rules that have a *static +interpretation* govern the success or failure of compilation, while +semantic rules +that have a *dynamic interpretation* govern the behavior of the program at +run-time. + +The compilation model centers on artifacts called _crates_. Each compilation +processes a single crate in source form, and if successful, produces a single +crate in binary form: either an executable or some sort of +library.[^cratesourcefile] + +A _crate_ is a unit of compilation and linking, as well as versioning, +distribution and runtime loading. A crate contains a _tree_ of nested +[module] scopes. The top level of this tree is a module that is +anonymous (from the point of view of paths within the module) and any item +within a crate has a canonical [module path] denoting its location +within the crate's module tree. + +The Rust compiler is always invoked with a single source file as input, and +always produces a single output crate. The processing of that source file may +result in other source files being loaded as modules. Source files have the +extension `.rs`. + +A Rust source file describes a module, the name and location of which — +in the module tree of the current crate — are defined from outside the +source file: either by an explicit `mod_item` in a referencing source file, or +by the name of the crate itself. Every source file is a module, but not every +module needs its own source file: [module definitions][module] can be nested +within one file. + +Each source file contains a sequence of zero or more `item` definitions, and +may optionally begin with any number of [attributes] +that apply to the containing module, most of which influence the behavior of +the compiler. The anonymous crate module can have additional attributes that +apply to the crate as a whole. + +```rust,no_run +// Specify the crate name. +#![crate_name = "projx"] + +// Specify the type of output artifact. +#![crate_type = "lib"] + +// Turn on a warning. +// This can be done in any module, not just the anonymous crate module. +#![warn(non_camel_case_types)] +``` + +A crate that contains a `main` function can be compiled to an executable. If a +`main` function is present, its return type must be `()` +("[unit]") and it must take no arguments. + +[^phase-distinction]: This distinction would also exist in an interpreter. + Static checks like syntactic analysis, type checking, and lints should + happen before the program is executed regardless of when it is executed. + +[^cratesourcefile]: A crate is somewhat analogous to an *assembly* in the + ECMA-335 CLI model, a *library* in the SML/NJ Compilation Manager, a *unit* + in the Owens and Flatt module system, or a *configuration* in Mesa. + +[module]: items.html#modules +[module path]: paths.html +[attributes]: items-and-attributes.html +[unit]: types.html#tuple-types \ No newline at end of file diff --git a/src/doc/reference/src/expressions.md b/src/doc/reference/src/expressions.md new file mode 100644 index 0000000000000..c9c0496dac61c --- /dev/null +++ b/src/doc/reference/src/expressions.md @@ -0,0 +1,863 @@ +# Expressions + +An expression may have two roles: it always produces a *value*, and it may have +*effects* (otherwise known as "side effects"). An expression *evaluates to* a +value, and has effects during *evaluation*. Many expressions contain +sub-expressions (operands). The meaning of each kind of expression dictates +several things: + +* Whether or not to evaluate the sub-expressions when evaluating the expression +* The order in which to evaluate the sub-expressions +* How to combine the sub-expressions' values to obtain the value of the expression + +In this way, the structure of expressions dictates the structure of execution. +Blocks are just another kind of expression, so blocks, statements, expressions, +and blocks again can recursively nest inside each other to an arbitrary depth. + +### Lvalues, rvalues and temporaries + +Expressions are divided into two main categories: _lvalues_ and _rvalues_. +Likewise within each expression, sub-expressions may occur in _lvalue context_ +or _rvalue context_. The evaluation of an expression depends both on its own +category and the context it occurs within. + +An lvalue is an expression that represents a memory location. These expressions +are [paths](#path-expressions) (which refer to local variables, function and +method arguments, or static variables), dereferences (`*expr`), [indexing +expressions](#index-expressions) (`expr[expr]`), and [field +references](#field-expressions) (`expr.f`). All other expressions are rvalues. + +The left operand of an [assignment](#assignment-expressions) or +[compound-assignment](#compound-assignment-expressions) expression is +an lvalue context, as is the single operand of a unary +[borrow](#unary-operator-expressions). The discriminant or subject of +a [match expression](#match-expressions) may be an lvalue context, if +ref bindings are made, but is otherwise an rvalue context. All other +expression contexts are rvalue contexts. + +When an lvalue is evaluated in an _lvalue context_, it denotes a memory +location; when evaluated in an _rvalue context_, it denotes the value held _in_ +that memory location. + +#### Temporary lifetimes + +When an rvalue is used in an lvalue context, a temporary un-named +lvalue is created and used instead. The lifetime of temporary values +is typically the innermost enclosing statement; the tail expression of +a block is considered part of the statement that encloses the block. + +When a temporary rvalue is being created that is assigned into a `let` +declaration, however, the temporary is created with the lifetime of +the enclosing block instead, as using the enclosing statement (the +`let` declaration) would be a guaranteed error (since a pointer to the +temporary would be stored into a variable, but the temporary would be +freed before the variable could be used). The compiler uses simple +syntactic rules to decide which values are being assigned into a `let` +binding, and therefore deserve a longer temporary lifetime. + +Here are some examples: + +- `let x = foo(&temp())`. The expression `temp()` is an rvalue. As it + is being borrowed, a temporary is created which will be freed after + the innermost enclosing statement (the `let` declaration, in this case). +- `let x = temp().foo()`. This is the same as the previous example, + except that the value of `temp()` is being borrowed via autoref on a + method-call. Here we are assuming that `foo()` is an `&self` method + defined in some trait, say `Foo`. In other words, the expression + `temp().foo()` is equivalent to `Foo::foo(&temp())`. +- `let x = &temp()`. Here, the same temporary is being assigned into + `x`, rather than being passed as a parameter, and hence the + temporary's lifetime is considered to be the enclosing block. +- `let x = SomeStruct { foo: &temp() }`. As in the previous case, the + temporary is assigned into a struct which is then assigned into a + binding, and hence it is given the lifetime of the enclosing block. +- `let x = [ &temp() ]`. As in the previous case, the + temporary is assigned into an array which is then assigned into a + binding, and hence it is given the lifetime of the enclosing block. +- `let ref x = temp()`. In this case, the temporary is created using a ref binding, + but the result is the same: the lifetime is extended to the enclosing block. + +### Moved and copied types + +When a [local variable](variables.html) is used as an +[rvalue](expressions.html#lvalues-rvalues-and-temporaries), the variable will +be copied if its type implements `Copy`. All others are moved. + +## Literal expressions + +A _literal expression_ consists of one of the [literal](tokens.html#literals) forms +described earlier. It directly describes a number, character, string, boolean +value, or the unit value. + +```text +(); // unit type +"hello"; // string type +'5'; // character type +5; // integer type +``` + +## Path expressions + +A [path](paths.html) used as an expression context denotes either a local +variable or an item. Path expressions are +[lvalues](expressions.html#lvalues-rvalues-and-temporaries). + +## Tuple expressions + +Tuples are written by enclosing zero or more comma-separated expressions in +parentheses. They are used to create [tuple-typed](types.html#tuple-types) +values. + +```{.tuple} +(0.0, 4.5); +("a", 4usize, true); +``` + +You can disambiguate a single-element tuple from a value in parentheses with a +comma: + +``` +(0,); // single-element tuple +(0); // zero in parentheses +``` + +## Struct expressions + +There are several forms of struct expressions. A _struct expression_ +consists of the [path](paths.html) of a [struct item](items.html#structs), followed +by a brace-enclosed list of zero or more comma-separated name-value pairs, +providing the field values of a new instance of the struct. A field name can be +any identifier, and is separated from its value expression by a colon. The +location denoted by a struct field is mutable if and only if the enclosing +struct is mutable. + +A _tuple struct expression_ consists of the [path](paths.html) of a [struct +item](items.html#structs), followed by a parenthesized list of one or more +comma-separated expressions (in other words, the path of a struct item followed +by a tuple expression). The struct item must be a tuple struct item. + +A _unit-like struct expression_ consists only of the [path](paths.html) of a +[struct item](items.html#structs). + +The following are examples of struct expressions: + +``` +# struct Point { x: f64, y: f64 } +# struct NothingInMe { } +# struct TuplePoint(f64, f64); +# mod game { pub struct User<'a> { pub name: &'a str, pub age: u32, pub score: usize } } +# struct Cookie; fn some_fn(t: T) {} +Point {x: 10.0, y: 20.0}; +NothingInMe {}; +TuplePoint(10.0, 20.0); +let u = game::User {name: "Joe", age: 35, score: 100_000}; +some_fn::(Cookie); +``` + +A struct expression forms a new value of the named struct type. Note +that for a given *unit-like* struct type, this will always be the same +value. + +A struct expression can terminate with the syntax `..` followed by an +expression to denote a functional update. The expression following `..` (the +base) must have the same struct type as the new struct type being formed. +The entire expression denotes the result of constructing a new struct (with +the same type as the base expression) with the given values for the fields that +were explicitly specified and the values in the base expression for all other +fields. + +``` +# struct Point3d { x: i32, y: i32, z: i32 } +let base = Point3d {x: 1, y: 2, z: 3}; +Point3d {y: 0, z: 10, .. base}; +``` + +#### Struct field init shorthand + +When initializing a data structure (struct, enum, union) with named fields, +it is allowed to write `fieldname` as a shorthand for `fieldname: fieldname`. +This allows a compact syntax with less duplication. + +Example: + +``` +# struct Point3d { x: i32, y: i32, z: i32 } +# let x = 0; +# let y_value = 0; +# let z = 0; +Point3d { x: x, y: y_value, z: z }; +Point3d { x, y: y_value, z }; +``` + +## Block expressions + +A _block expression_ is similar to a module in terms of the declarations that +are possible. Each block conceptually introduces a new namespace scope. Use +items can bring new names into scopes and declared items are in scope for only +the block itself. + +A block will execute each statement sequentially, and then execute the +expression (if given). If the block ends in a statement, its value is `()`: + +``` +let x: () = { println!("Hello."); }; +``` + +If it ends in an expression, its value and type are that of the expression: + +``` +let x: i32 = { println!("Hello."); 5 }; + +assert_eq!(5, x); +``` + +## Method-call expressions + +A _method call_ consists of an expression followed by a single dot, an +identifier, and a parenthesized expression-list. Method calls are resolved to +methods on specific traits, either statically dispatching to a method if the +exact `self`-type of the left-hand-side is known, or dynamically dispatching if +the left-hand-side expression is an indirect [trait +object](types.html#trait-objects). + +## Field expressions + +A _field expression_ consists of an expression followed by a single dot and an +identifier, when not immediately followed by a parenthesized expression-list +(the latter is a [method call expression](#method-call-expressions)). A field +expression denotes a field of a [struct](types.html#struct-types). + +```{.ignore .field} +mystruct.myfield; +foo().x; +(Struct {a: 10, b: 20}).a; +``` + +A field access is an [lvalue](expressions.html#lvalues-rvalues-and-temporaries) +referring to the value of that field. When the type providing the field +inherits mutability, it can be [assigned](#assignment-expressions) to. + +Also, if the type of the expression to the left of the dot is a +pointer, it is automatically dereferenced as many times as necessary +to make the field access possible. In cases of ambiguity, we prefer +fewer autoderefs to more. + +## Array expressions + +An [array](types.html#array-and-slice-types) _expression_ is written by +enclosing zero or more comma-separated expressions of uniform type in square +brackets. + +In the `[expr ';' expr]` form, the expression after the `';'` must be a +constant expression that can be evaluated at compile time, such as a +[literal](tokens.html#literals) or a [static item](items.html#static-items). + +``` +[1, 2, 3, 4]; +["a", "b", "c", "d"]; +[0; 128]; // array with 128 zeros +[0u8, 0u8, 0u8, 0u8]; +``` + +## Index expressions + +[Array](types.html#array-and-slice-types)-typed expressions can be indexed by +writing a square-bracket-enclosed expression (the index) after them. When the +array is mutable, the resulting +[lvalue](expressions.html#lvalues-rvalues-and-temporaries) can be assigned to. + +Indices are zero-based, and may be of any integral type. Vector access is +bounds-checked at compile-time for constant arrays being accessed with a +constant index value. Otherwise a check will be performed at run-time that +will put the thread in a _panicked state_ if it fails. + +```{should-fail} +([1, 2, 3, 4])[0]; + +let x = (["a", "b"])[10]; // compiler error: const index-expr is out of bounds + +let n = 10; +let y = (["a", "b"])[n]; // panics + +let arr = ["a", "b"]; +arr[10]; // panics +``` + +Also, if the type of the expression to the left of the brackets is a +pointer, it is automatically dereferenced as many times as necessary +to make the indexing possible. In cases of ambiguity, we prefer fewer +autoderefs to more. + +## Range expressions + +The `..` operator will construct an object of one of the `std::ops::Range` variants. + +``` +1..2; // std::ops::Range +3..; // std::ops::RangeFrom +..4; // std::ops::RangeTo +..; // std::ops::RangeFull +``` + +The following expressions are equivalent. + +``` +let x = std::ops::Range {start: 0, end: 10}; +let y = 0..10; + +assert_eq!(x, y); +``` + +Similarly, the `...` operator will construct an object of one of the +`std::ops::RangeInclusive` variants. + +``` +# #![feature(inclusive_range_syntax)] +1...2; // std::ops::RangeInclusive +...4; // std::ops::RangeToInclusive +``` + +The following expressions are equivalent. + +``` +# #![feature(inclusive_range_syntax, inclusive_range)] +let x = std::ops::RangeInclusive::NonEmpty {start: 0, end: 10}; +let y = 0...10; + +assert_eq!(x, y); +``` + +## Unary operator expressions + +Rust defines the following unary operators. With the exception of `?`, they are +all written as prefix operators, before the expression they apply to. + +* `-` + : Negation. Signed integer types and floating-point types support negation. It + is an error to apply negation to unsigned types; for example, the compiler + rejects `-1u32`. +* `*` + : Dereference. When applied to a [pointer](types.html#pointer-types) it + denotes the pointed-to location. For pointers to mutable locations, the + resulting [lvalue](expressions.html#lvalues-rvalues-and-temporaries) can be + assigned to. On non-pointer types, it calls the `deref` method of the + `std::ops::Deref` trait, or the `deref_mut` method of the + `std::ops::DerefMut` trait (if implemented by the type and required for an + outer expression that will or could mutate the dereference), and produces + the result of dereferencing the `&` or `&mut` borrowed pointer returned + from the overload method. +* `!` + : Logical negation. On the boolean type, this flips between `true` and + `false`. On integer types, this inverts the individual bits in the + two's complement representation of the value. +* `&` and `&mut` + : Borrowing. When applied to an lvalue, these operators produce a + reference (pointer) to the lvalue. The lvalue is also placed into + a borrowed state for the duration of the reference. For a shared + borrow (`&`), this implies that the lvalue may not be mutated, but + it may be read or shared again. For a mutable borrow (`&mut`), the + lvalue may not be accessed in any way until the borrow expires. + If the `&` or `&mut` operators are applied to an rvalue, a + temporary value is created; the lifetime of this temporary value + is defined by [syntactic rules](#temporary-lifetimes). +* `?` + : Propagating errors if applied to `Err(_)` and unwrapping if + applied to `Ok(_)`. Only works on the `Result` type, + and written in postfix notation. + +## Binary operator expressions + +Binary operators expressions are given in terms of [operator +precedence](#operator-precedence). + +### Arithmetic operators + +Binary arithmetic expressions are syntactic sugar for calls to built-in traits, +defined in the `std::ops` module of the `std` library. This means that +arithmetic operators can be overridden for user-defined types. The default +meaning of the operators on standard types is given here. + +* `+` + : Addition and array/string concatenation. + Calls the `add` method on the `std::ops::Add` trait. +* `-` + : Subtraction. + Calls the `sub` method on the `std::ops::Sub` trait. +* `*` + : Multiplication. + Calls the `mul` method on the `std::ops::Mul` trait. +* `/` + : Quotient. + Calls the `div` method on the `std::ops::Div` trait. +* `%` + : Remainder. + Calls the `rem` method on the `std::ops::Rem` trait. + +### Bitwise operators + +Like the [arithmetic operators](#arithmetic-operators), bitwise operators are +syntactic sugar for calls to methods of built-in traits. This means that +bitwise operators can be overridden for user-defined types. The default +meaning of the operators on standard types is given here. Bitwise `&`, `|` and +`^` applied to boolean arguments are equivalent to logical `&&`, `||` and `!=` +evaluated in non-lazy fashion. + +* `&` + : Bitwise AND. + Calls the `bitand` method of the `std::ops::BitAnd` trait. +* `|` + : Bitwise inclusive OR. + Calls the `bitor` method of the `std::ops::BitOr` trait. +* `^` + : Bitwise exclusive OR. + Calls the `bitxor` method of the `std::ops::BitXor` trait. +* `<<` + : Left shift. + Calls the `shl` method of the `std::ops::Shl` trait. +* `>>` + : Right shift (arithmetic). + Calls the `shr` method of the `std::ops::Shr` trait. + +### Lazy boolean operators + +The operators `||` and `&&` may be applied to operands of boolean type. The +`||` operator denotes logical 'or', and the `&&` operator denotes logical +'and'. They differ from `|` and `&` in that the right-hand operand is only +evaluated when the left-hand operand does not already determine the result of +the expression. That is, `||` only evaluates its right-hand operand when the +left-hand operand evaluates to `false`, and `&&` only when it evaluates to +`true`. + +### Comparison operators + +Comparison operators are, like the [arithmetic +operators](#arithmetic-operators), and [bitwise operators](#bitwise-operators), +syntactic sugar for calls to built-in traits. This means that comparison +operators can be overridden for user-defined types. The default meaning of the +operators on standard types is given here. + +* `==` + : Equal to. + Calls the `eq` method on the `std::cmp::PartialEq` trait. +* `!=` + : Unequal to. + Calls the `ne` method on the `std::cmp::PartialEq` trait. +* `<` + : Less than. + Calls the `lt` method on the `std::cmp::PartialOrd` trait. +* `>` + : Greater than. + Calls the `gt` method on the `std::cmp::PartialOrd` trait. +* `<=` + : Less than or equal. + Calls the `le` method on the `std::cmp::PartialOrd` trait. +* `>=` + : Greater than or equal. + Calls the `ge` method on the `std::cmp::PartialOrd` trait. + +### Type cast expressions + +A type cast expression is denoted with the binary operator `as`. + +Executing an `as` expression casts the value on the left-hand side to the type +on the right-hand side. + +An example of an `as` expression: + +``` +# fn sum(values: &[f64]) -> f64 { 0.0 } +# fn len(values: &[f64]) -> i32 { 0 } + +fn average(values: &[f64]) -> f64 { + let sum: f64 = sum(values); + let size: f64 = len(values) as f64; + sum / size +} +``` + +Some of the conversions which can be done through the `as` operator +can also be done implicitly at various points in the program, such as +argument passing and assignment to a `let` binding with an explicit +type. Implicit conversions are limited to "harmless" conversions that +do not lose information and which have minimal or no risk of +surprising side-effects on the dynamic execution semantics. + +### Assignment expressions + +An _assignment expression_ consists of an +[lvalue](expressions.html#lvalues-rvalues-and-temporaries) expression followed +by an equals sign (`=`) and an +[rvalue](expressions.html#lvalues-rvalues-and-temporaries) expression. + +Evaluating an assignment expression [either copies or +moves](#moved-and-copied-types) its right-hand operand to its left-hand +operand. + +``` +# let mut x = 0; +# let y = 0; +x = y; +``` + +### Compound assignment expressions + +The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, and `>>` operators may be +composed with the `=` operator. The expression `lval OP= val` is equivalent to +`lval = lval OP val`. For example, `x = x + 1` may be written as `x += 1`. + +Any such expression always has the [`unit`](types.html#tuple-types) type. + +### Operator precedence + +The precedence of Rust binary operators is ordered as follows, going from +strong to weak: + +```{.text .precedence} +as : +* / % ++ - +<< >> +& +^ +| +== != < > <= >= +&& +|| +.. ... +<- += +``` + +Operators at the same precedence level are evaluated left-to-right. [Unary +operators](#unary-operator-expressions) have the same precedence level and are +stronger than any of the binary operators. + +## Grouped expressions + +An expression enclosed in parentheses evaluates to the result of the enclosed +expression. Parentheses can be used to explicitly specify evaluation order +within an expression. + +An example of a parenthesized expression: + +``` +let x: i32 = (2 + 3) * 4; +``` + + +## Call expressions + +A _call expression_ invokes a function, providing zero or more input variables +and an optional location to move the function's output into. If the function +eventually returns, then the expression completes. + +Some examples of call expressions: + +``` +# fn add(x: i32, y: i32) -> i32 { 0 } + +let x: i32 = add(1i32, 2i32); +let pi: Result = "3.14".parse(); +``` + +## Lambda expressions + +A _lambda expression_ (sometimes called an "anonymous function expression") +defines a function and denotes it as a value, in a single expression. A lambda +expression is a pipe-symbol-delimited (`|`) list of identifiers followed by an +expression. + +A lambda expression denotes a function that maps a list of parameters +(`ident_list`) onto the expression that follows the `ident_list`. The +identifiers in the `ident_list` are the parameters to the function. These +parameters' types need not be specified, as the compiler infers them from +context. + +Lambda expressions are most useful when passing functions as arguments to other +functions, as an abbreviation for defining and capturing a separate function. + +Significantly, lambda expressions _capture their environment_, which regular +[function definitions](items.html#functions) do not. The exact type of capture +depends on the [function type](types.html#function-types) inferred for the +lambda expression. In the simplest and least-expensive form (analogous to a +```|| { }``` expression), the lambda expression captures its environment by +reference, effectively borrowing pointers to all outer variables mentioned +inside the function. Alternately, the compiler may infer that a lambda +expression should copy or move values (depending on their type) from the +environment into the lambda expression's captured environment. A lambda can be +forced to capture its environment by moving values by prefixing it with the +`move` keyword. + +In this example, we define a function `ten_times` that takes a higher-order +function argument, and we then call it with a lambda expression as an argument, +followed by a lambda expression that moves values from its environment. + +``` +fn ten_times(f: F) where F: Fn(i32) { + for index in 0..10 { + f(index); + } +} + +ten_times(|j| println!("hello, {}", j)); + +let word = "konnichiwa".to_owned(); +ten_times(move |j| println!("{}, {}", word, j)); +``` + +## Infinite loops + +A `loop` expression denotes an infinite loop. + +A `loop` expression may optionally have a _label_. The label is written as +a lifetime preceding the loop expression, as in `'foo: loop{ }`. If a +label is present, then labeled `break` and `continue` expressions nested +within this loop may exit out of this loop or return control to its head. +See [break expressions](#break-expressions) and [continue +expressions](#continue-expressions). + +## `break` expressions + +A `break` expression has an optional _label_. If the label is absent, then +executing a `break` expression immediately terminates the innermost loop +enclosing it. It is only permitted in the body of a loop. If the label is +present, then `break 'foo` terminates the loop with label `'foo`, which need not +be the innermost label enclosing the `break` expression, but must enclose it. + +## `continue` expressions + +A `continue` expression has an optional _label_. If the label is absent, then +executing a `continue` expression immediately terminates the current iteration +of the innermost loop enclosing it, returning control to the loop *head*. In +the case of a `while` loop, the head is the conditional expression controlling +the loop. In the case of a `for` loop, the head is the call-expression +controlling the loop. If the label is present, then `continue 'foo` returns +control to the head of the loop with label `'foo`, which need not be the +innermost label enclosing the `continue` expression, but must enclose it. + +A `continue` expression is only permitted in the body of a loop. + +## `while` loops + +A `while` loop begins by evaluating the boolean loop conditional expression. +If the loop conditional expression evaluates to `true`, the loop body block +executes and control returns to the loop conditional expression. If the loop +conditional expression evaluates to `false`, the `while` expression completes. + +An example: + +``` +let mut i = 0; + +while i < 10 { + println!("hello"); + i = i + 1; +} +``` + +Like `loop` expressions, `while` loops can be controlled with `break` or +`continue`, and may optionally have a _label_. See [infinite +loops](#infinite-loops), [break expressions](#break-expressions), and +[continue expressions](#continue-expressions) for more information. + +## `for` expressions + +A `for` expression is a syntactic construct for looping over elements provided +by an implementation of `std::iter::IntoIterator`. + +An example of a `for` loop over the contents of an array: + +``` +# type Foo = i32; +# fn bar(f: &Foo) { } +# let a = 0; +# let b = 0; +# let c = 0; + +let v: &[Foo] = &[a, b, c]; + +for e in v { + bar(e); +} +``` + +An example of a for loop over a series of integers: + +``` +# fn bar(b:usize) { } +for i in 0..256 { + bar(i); +} +``` + +Like `loop` expressions, `for` loops can be controlled with `break` or +`continue`, and may optionally have a _label_. See [infinite +loops](#infinite-loops), [break expressions](#break-expressions), and +[continue expressions](#continue-expressions) for more information. + +## `if` expressions + +An `if` expression is a conditional branch in program control. The form of an +`if` expression is a condition expression, followed by a consequent block, any +number of `else if` conditions and blocks, and an optional trailing `else` +block. The condition expressions must have type `bool`. If a condition +expression evaluates to `true`, the consequent block is executed and any +subsequent `else if` or `else` block is skipped. If a condition expression +evaluates to `false`, the consequent block is skipped and any subsequent `else +if` condition is evaluated. If all `if` and `else if` conditions evaluate to +`false` then any `else` block is executed. + +## `match` expressions + +A `match` expression branches on a *pattern*. The exact form of matching that +occurs depends on the pattern. Patterns consist of some combination of +literals, destructured arrays or enum constructors, structs and tuples, +variable binding specifications, wildcards (`..`), and placeholders (`_`). A +`match` expression has a *head expression*, which is the value to compare to +the patterns. The type of the patterns must equal the type of the head +expression. + +In a pattern whose head expression has an `enum` type, a placeholder (`_`) +stands for a *single* data field, whereas a wildcard `..` stands for *all* the +fields of a particular variant. + +A `match` behaves differently depending on whether or not the head expression +is an [lvalue or an rvalue](expressions.html#lvalues-rvalues-and-temporaries). +If the head expression is an rvalue, it is first evaluated into a temporary +location, and the resulting value is sequentially compared to the patterns in +the arms until a match is found. The first arm with a matching pattern is +chosen as the branch target of the `match`, any variables bound by the pattern +are assigned to local variables in the arm's block, and control enters the +block. + +When the head expression is an lvalue, the match does not allocate a temporary +location (however, a by-value binding may copy or move from the lvalue). When +possible, it is preferable to match on lvalues, as the lifetime of these +matches inherits the lifetime of the lvalue, rather than being restricted to +the inside of the match. + +An example of a `match` expression: + +``` +let x = 1; + +match x { + 1 => println!("one"), + 2 => println!("two"), + 3 => println!("three"), + 4 => println!("four"), + 5 => println!("five"), + _ => println!("something else"), +} +``` + +Patterns that bind variables default to binding to a copy or move of the +matched value (depending on the matched value's type). This can be changed to +bind to a reference by using the `ref` keyword, or to a mutable reference using +`ref mut`. + +Subpatterns can also be bound to variables by the use of the syntax `variable @ +subpattern`. For example: + +``` +let x = 1; + +match x { + e @ 1 ... 5 => println!("got a range element {}", e), + _ => println!("anything"), +} +``` + +Patterns can also dereference pointers by using the `&`, `&mut` and `box` +symbols, as appropriate. For example, these two matches on `x: &i32` are +equivalent: + +``` +# let x = &3; +let y = match *x { 0 => "zero", _ => "some" }; +let z = match x { &0 => "zero", _ => "some" }; + +assert_eq!(y, z); +``` + +Multiple match patterns may be joined with the `|` operator. A range of values +may be specified with `...`. For example: + +``` +# let x = 2; + +let message = match x { + 0 | 1 => "not many", + 2 ... 9 => "a few", + _ => "lots" +}; +``` + +Range patterns only work on scalar types (like integers and characters; not +like arrays and structs, which have sub-components). A range pattern may not +be a sub-range of another range pattern inside the same `match`. + +Finally, match patterns can accept *pattern guards* to further refine the +criteria for matching a case. Pattern guards appear after the pattern and +consist of a bool-typed expression following the `if` keyword. A pattern guard +may refer to the variables bound within the pattern they follow. + +``` +# let maybe_digit = Some(0); +# fn process_digit(i: i32) { } +# fn process_other(i: i32) { } + +let message = match maybe_digit { + Some(x) if x < 10 => process_digit(x), + Some(x) => process_other(x), + None => panic!(), +}; +``` + +## `if let` expressions + +An `if let` expression is semantically identical to an `if` expression but in +place of a condition expression it expects a `let` statement with a refutable +pattern. If the value of the expression on the right hand side of the `let` +statement matches the pattern, the corresponding block will execute, otherwise +flow proceeds to the first `else` block that follows. + +``` +let dish = ("Ham", "Eggs"); + +// this body will be skipped because the pattern is refuted +if let ("Bacon", b) = dish { + println!("Bacon is served with {}", b); +} + +// this body will execute +if let ("Ham", b) = dish { + println!("Ham is served with {}", b); +} +``` + +## `while let` loops + +A `while let` loop is semantically identical to a `while` loop but in place of +a condition expression it expects `let` statement with a refutable pattern. If +the value of the expression on the right hand side of the `let` statement +matches the pattern, the loop body block executes and control returns to the +pattern matching statement. Otherwise, the while expression completes. + +## `return` expressions + +Return expressions are denoted with the keyword `return`. Evaluating a `return` +expression moves its argument into the designated output location for the +current function call, destroys the current function activation frame, and +transfers control to the caller frame. + +An example of a `return` expression: + +``` +fn max(a: i32, b: i32) -> i32 { + if a > b { + return a; + } + return b; +} +``` diff --git a/src/doc/reference/src/identifiers.md b/src/doc/reference/src/identifiers.md new file mode 100644 index 0000000000000..de657e3e312d5 --- /dev/null +++ b/src/doc/reference/src/identifiers.md @@ -0,0 +1,24 @@ +# Identifiers + +An identifier is any nonempty Unicode[^non_ascii_idents] string of the following form: + +Either + + * The first character has property `XID_start` + * The remaining characters have property `XID_continue` + +Or + + * The first character is `_` + * The identifier is more than one character, `_` alone is not an identifier + * The remaining characters have property `XID_continue` + +that does _not_ occur in the set of [keywords]. + +> **Note**: `XID_start` and `XID_continue` as character properties cover the +> character ranges used to form the more familiar C and Java language-family +> identifiers. + +[keywords]: ../grammar.html#keywords +[^non_ascii_idents]: Non-ASCII characters in identifiers are currently feature + gated. This is expected to improve soon. diff --git a/src/doc/reference/src/influences.md b/src/doc/reference/src/influences.md new file mode 100644 index 0000000000000..46082bfc0b006 --- /dev/null +++ b/src/doc/reference/src/influences.md @@ -0,0 +1,22 @@ +# Influences + +Rust is not a particularly original language, with design elements coming from +a wide range of sources. Some of these are listed below (including elements +that have since been removed): + +* SML, OCaml: algebraic data types, pattern matching, type inference, + semicolon statement separation +* C++: references, RAII, smart pointers, move semantics, monomorphization, + memory model +* ML Kit, Cyclone: region based memory management +* Haskell (GHC): typeclasses, type families +* Newsqueak, Alef, Limbo: channels, concurrency +* Erlang: message passing, thread failure, linked thread failure, + lightweight concurrency +* Swift: optional bindings +* Scheme: hygienic macros +* C#: attributes +* Ruby: block syntax +* NIL, Hermes: typestate +* [Unicode Annex #31](http://www.unicode.org/reports/tr31/): identifier and + pattern syntax diff --git a/src/doc/reference/src/input-format.md b/src/doc/reference/src/input-format.md new file mode 100644 index 0000000000000..0dbba4be92a05 --- /dev/null +++ b/src/doc/reference/src/input-format.md @@ -0,0 +1,10 @@ +# Input format + +Rust input is interpreted as a sequence of Unicode code points encoded in UTF-8. +Most Rust grammar rules are defined in terms of printable ASCII-range +code points, but a small number are defined in terms of Unicode properties or +explicit code point lists. [^inputformat] + +[^inputformat]: Substitute definitions for the special Unicode productions are + provided to the grammar verifier, restricted to ASCII range, when verifying the + grammar in this document. diff --git a/src/doc/reference/src/introduction.md b/src/doc/reference/src/introduction.md new file mode 100644 index 0000000000000..3a00dfa4572bf --- /dev/null +++ b/src/doc/reference/src/introduction.md @@ -0,0 +1,31 @@ +# Introduction + +This document is the primary reference for the Rust programming language. It +provides three kinds of material: + + - Chapters that informally describe each language construct and their use. + - Chapters that informally describe the memory model, concurrency model, + runtime services, linkage model and debugging facilities. + - Appendix chapters providing rationale and references to languages that + influenced the design. + +This document does not serve as an introduction to the language. Background +familiarity with the language is assumed. A separate [book] is available to +help acquire such background familiarity. + +This document also does not serve as a reference to the [standard] library +included in the language distribution. Those libraries are documented +separately by extracting documentation attributes from their source code. Many +of the features that one might expect to be language features are library +features in Rust, so what you're looking for may be there, not here. + +Finally, this document is not normative. It may include details that are +specific to `rustc` itself, and should not be taken as a specification for +the Rust language. We intend to produce such a document someday, but this +is what we have for now. + +You may also be interested in the [grammar]. + +[book]: ../book/index.html +[standard]: ../std/index.html +[grammar]: ../grammar.html diff --git a/src/doc/reference/src/items-and-attributes.md b/src/doc/reference/src/items-and-attributes.md new file mode 100644 index 0000000000000..bd5018d69cc78 --- /dev/null +++ b/src/doc/reference/src/items-and-attributes.md @@ -0,0 +1,7 @@ +# Items and attributes + +Crates contain [items], each of which may have some number of +[attributes] attached to it. + +[items]: items.html +[attributes]: attributes.html diff --git a/src/doc/reference/src/items.md b/src/doc/reference/src/items.md new file mode 100644 index 0000000000000..ba3f4195ba62d --- /dev/null +++ b/src/doc/reference/src/items.md @@ -0,0 +1,1091 @@ +# Items + +An _item_ is a component of a crate. Items are organized within a crate by a +nested set of [modules]. Every crate has a single "outermost" +anonymous module; all further items within the crate have [paths] +within the module tree of the crate. + +[modules]: #modules +[paths]: paths.html + +Items are entirely determined at compile-time, generally remain fixed during +execution, and may reside in read-only memory. + +There are several kinds of item: + +* [`extern crate` declarations](#extern-crate-declarations) +* [`use` declarations](#use-declarations) +* [modules](#modules) +* [function definitions](#functions) +* [`extern` blocks](#external-blocks) +* [type definitions](../grammar.html#type-definitions) +* [struct definitions](#structs) +* [enumeration definitions](#enumerations) +* [constant items](#constant-items) +* [static items](#static-items) +* [trait definitions](#traits) +* [implementations](#implementations) + +Some items form an implicit scope for the declaration of sub-items. In other +words, within a function or module, declarations of items can (in many cases) +be mixed with the statements, control blocks, and similar artifacts that +otherwise compose the item body. The meaning of these scoped items is the same +as if the item was declared outside the scope — it is still a static item +— except that the item's *path name* within the module namespace is +qualified by the name of the enclosing item, or is private to the enclosing +item (in the case of functions). The grammar specifies the exact locations in +which sub-item declarations may appear. + +## Type Parameters + +All items except modules, constants and statics may be *parameterized* by type. +Type parameters are given as a comma-separated list of identifiers enclosed in +angle brackets (`<...>`), after the name of the item and before its definition. +The type parameters of an item are considered "part of the name", not part of +the type of the item. A referencing [path] must (in principle) provide +type arguments as a list of comma-separated types enclosed within angle +brackets, in order to refer to the type-parameterized item. In practice, the +type-inference system can usually infer such argument types from context. There +are no general type-parametric types, only type-parametric items. That is, Rust +has no notion of type abstraction: there are no higher-ranked (or "forall") types +abstracted over other types, though higher-ranked types do exist for lifetimes. + +[path]: paths.html + +## Modules + +A module is a container for zero or more [items]. + +[items]: items.html + +A _module item_ is a module, surrounded in braces, named, and prefixed with the +keyword `mod`. A module item introduces a new, named module into the tree of +modules making up a crate. Modules can nest arbitrarily. + +An example of a module: + +```rust +mod math { + type Complex = (f64, f64); + fn sin(f: f64) -> f64 { + /* ... */ +# panic!(); + } + fn cos(f: f64) -> f64 { + /* ... */ +# panic!(); + } + fn tan(f: f64) -> f64 { + /* ... */ +# panic!(); + } +} +``` + +Modules and types share the same namespace. Declaring a named type with +the same name as a module in scope is forbidden: that is, a type definition, +trait, struct, enumeration, or type parameter can't shadow the name of a module +in scope, or vice versa. + +A module without a body is loaded from an external file, by default with the +same name as the module, plus the `.rs` extension. When a nested submodule is +loaded from an external file, it is loaded from a subdirectory path that +mirrors the module hierarchy. + +```rust,ignore +// Load the `vec` module from `vec.rs` +mod vec; + +mod thread { + // Load the `local_data` module from `thread/local_data.rs` + // or `thread/local_data/mod.rs`. + mod local_data; +} +``` + +The directories and files used for loading external file modules can be +influenced with the `path` attribute. + +```rust,ignore +#[path = "thread_files"] +mod thread { + // Load the `local_data` module from `thread_files/tls.rs` + #[path = "tls.rs"] + mod local_data; +} +``` + +### Extern crate declarations + +An _`extern crate` declaration_ specifies a dependency on an external crate. +The external crate is then bound into the declaring scope as the `ident` +provided in the `extern_crate_decl`. + +The external crate is resolved to a specific `soname` at compile time, and a +runtime linkage requirement to that `soname` is passed to the linker for +loading at runtime. The `soname` is resolved at compile time by scanning the +compiler's library path and matching the optional `crateid` provided against +the `crateid` attributes that were declared on the external crate when it was +compiled. If no `crateid` is provided, a default `name` attribute is assumed, +equal to the `ident` given in the `extern_crate_decl`. + +Three examples of `extern crate` declarations: + +```rust,ignore +extern crate pcre; + +extern crate std; // equivalent to: extern crate std as std; + +extern crate std as ruststd; // linking to 'std' under another name +``` + +When naming Rust crates, hyphens are disallowed. However, Cargo packages may +make use of them. In such case, when `Cargo.toml` doesn't specify a crate name, +Cargo will transparently replace `-` with `_` (Refer to [RFC 940] for more +details). + +Here is an example: + +```rust,ignore +// Importing the Cargo package hello-world +extern crate hello_world; // hyphen replaced with an underscore +``` + +[RFC 940]: https://github.com/rust-lang/rfcs/blob/master/text/0940-hyphens-considered-harmful.md + +### Use declarations + +A _use declaration_ creates one or more local name bindings synonymous with +some other [path]. Usually a `use` declaration is used to shorten the +path required to refer to a module item. These declarations may appear in +[modules] and [blocks], usually at the top. + +[path]: paths.html +[modules]: #modules +[blocks]: ../grammar.html#block-expressions + +> **Note**: Unlike in many languages, +> `use` declarations in Rust do *not* declare linkage dependency with external crates. +> Rather, [`extern crate` declarations](#extern-crate-declarations) declare linkage dependencies. + +Use declarations support a number of convenient shortcuts: + +* Rebinding the target name as a new local name, using the syntax `use p::q::r as x;` +* Simultaneously binding a list of paths differing only in their final element, + using the glob-like brace syntax `use a::b::{c,d,e,f};` +* Binding all paths matching a given prefix, using the asterisk wildcard syntax + `use a::b::*;` +* Simultaneously binding a list of paths differing only in their final element + and their immediate parent module, using the `self` keyword, such as + `use a::b::{self, c, d};` + +An example of `use` declarations: + +```rust +use std::option::Option::{Some, None}; +use std::collections::hash_map::{self, HashMap}; + +fn foo(_: T){} +fn bar(map1: HashMap, map2: hash_map::HashMap){} + +fn main() { + // Equivalent to 'foo(vec![std::option::Option::Some(1.0f64), + // std::option::Option::None]);' + foo(vec![Some(1.0f64), None]); + + // Both `hash_map` and `HashMap` are in scope. + let map1 = HashMap::new(); + let map2 = hash_map::HashMap::new(); + bar(map1, map2); +} +``` + +Like items, `use` declarations are private to the containing module, by +default. Also like items, a `use` declaration can be public, if qualified by +the `pub` keyword. Such a `use` declaration serves to _re-export_ a name. A +public `use` declaration can therefore _redirect_ some public name to a +different target definition: even a definition with a private canonical path, +inside a different module. If a sequence of such redirections form a cycle or +cannot be resolved unambiguously, they represent a compile-time error. + +An example of re-exporting: + +```rust +# fn main() { } +mod quux { + pub use quux::foo::{bar, baz}; + + pub mod foo { + pub fn bar() { } + pub fn baz() { } + } +} +``` + +In this example, the module `quux` re-exports two public names defined in +`foo`. + +Also note that the paths contained in `use` items are relative to the crate +root. So, in the previous example, the `use` refers to `quux::foo::{bar, +baz}`, and not simply to `foo::{bar, baz}`. This also means that top-level +module declarations should be at the crate root if direct usage of the declared +modules within `use` items is desired. It is also possible to use `self` and +`super` at the beginning of a `use` item to refer to the current and direct +parent modules respectively. All rules regarding accessing declared modules in +`use` declarations apply to both module declarations and `extern crate` +declarations. + +An example of what will and will not work for `use` items: + +```rust +# #![allow(unused_imports)] +use foo::baz::foobaz; // good: foo is at the root of the crate + +mod foo { + + mod example { + pub mod iter {} + } + + use foo::example::iter; // good: foo is at crate root +// use example::iter; // bad: example is not at the crate root + use self::baz::foobaz; // good: self refers to module 'foo' + use foo::bar::foobar; // good: foo is at crate root + + pub mod bar { + pub fn foobar() { } + } + + pub mod baz { + use super::bar::foobar; // good: super refers to module 'foo' + pub fn foobaz() { } + } +} + +fn main() {} +``` + +## Functions + +A _function item_ defines a sequence of [statements] and a +final [expression], along with a name and a set of +parameters. Other than a name, all these are optional. +Functions are declared with the keyword `fn`. Functions may declare a +set of *input* [*variables*][variables] as parameters, through which the caller +passes arguments into the function, and the *output* [*type*][type] +of the value the function will return to its caller on completion. + +[statements]: statements.html +[expression]: expressions.html +[variables]: variables.html +[type]: types.html + +A function may also be copied into a first-class *value*, in which case the +value has the corresponding [*function type*][function type], and can be used +otherwise exactly as a function item (with a minor additional cost of calling +the function indirectly). + +[function type]: types.html#function-types + +Every control path in a function logically ends with a `return` expression or a +diverging expression. If the outermost block of a function has a +value-producing expression in its final-expression position, that expression is +interpreted as an implicit `return` expression applied to the final-expression. + +An example of a function: + +```rust +fn add(x: i32, y: i32) -> i32 { + x + y +} +``` + +As with `let` bindings, function arguments are irrefutable patterns, so any +pattern that is valid in a let binding is also valid as an argument. + +```rust +fn first((value, _): (i32, i32)) -> i32 { value } +``` + + +### Generic functions + +A _generic function_ allows one or more _parameterized types_ to appear in its +signature. Each type parameter must be explicitly declared in an +angle-bracket-enclosed and comma-separated list, following the function name. + +```rust,ignore +// foo is generic over A and B + +fn foo(x: A, y: B) { +``` + +Inside the function signature and body, the name of the type parameter can be +used as a type name. [Trait](#traits) bounds can be specified for type parameters +to allow methods with that trait to be called on values of that type. This is +specified using the `where` syntax: + +```rust,ignore +fn foo(x: T) where T: Debug { +``` + +When a generic function is referenced, its type is instantiated based on the +context of the reference. For example, calling the `foo` function here: + +```rust +use std::fmt::Debug; + +fn foo(x: &[T]) where T: Debug { + // details elided + # () +} + +foo(&[1, 2]); +``` + +will instantiate type parameter `T` with `i32`. + +The type parameters can also be explicitly supplied in a trailing +[path] component after the function name. This might be necessary if +there is not sufficient context to determine the type parameters. For example, +`mem::size_of::() == 4`. + +[path]: paths.html + +### Diverging functions + +A special kind of function can be declared with a `!` character where the +output type would normally be. For example: + +```rust +fn my_err(s: &str) -> ! { + println!("{}", s); + panic!(); +} +``` + +We call such functions "diverging" because they never return a value to the +caller. Every control path in a diverging function must end with a `panic!()` or +a call to another diverging function on every control path. The `!` annotation +does *not* denote a type. + +It might be necessary to declare a diverging function because as mentioned +previously, the typechecker checks that every control path in a function ends +with a [`return`] or diverging expression. So, if `my_err` +were declared without the `!` annotation, the following code would not +typecheck: + +[`return`]: expressions.html#return-expressions + +```rust +# fn my_err(s: &str) -> ! { panic!() } + +fn f(i: i32) -> i32 { + if i == 42 { + return 42; + } + else { + my_err("Bad number!"); + } +} +``` + +This will not compile without the `!` annotation on `my_err`, since the `else` +branch of the conditional in `f` does not return an `i32`, as required by the +signature of `f`. Adding the `!` annotation to `my_err` informs the +typechecker that, should control ever enter `my_err`, no further type judgments +about `f` need to hold, since control will never resume in any context that +relies on those judgments. Thus the return type on `f` only needs to reflect +the `if` branch of the conditional. + +### Extern functions + +Extern functions are part of Rust's foreign function interface, providing the +opposite functionality to [external blocks](#external-blocks). Whereas +external blocks allow Rust code to call foreign code, extern functions with +bodies defined in Rust code _can be called by foreign code_. They are defined +in the same way as any other Rust function, except that they have the `extern` +modifier. + +```rust +// Declares an extern fn, the ABI defaults to "C" +extern fn new_i32() -> i32 { 0 } + +// Declares an extern fn with "stdcall" ABI +extern "stdcall" fn new_i32_stdcall() -> i32 { 0 } +``` + +Unlike normal functions, extern fns have type `extern "ABI" fn()`. This is the +same type as the functions declared in an extern block. + +```rust +# extern fn new_i32() -> i32 { 0 } +let fptr: extern "C" fn() -> i32 = new_i32; +``` + +Extern functions may be called directly from Rust code as Rust uses large, +contiguous stack segments like C. + +## Type aliases + +A _type alias_ defines a new name for an existing [type]. Type +aliases are declared with the keyword `type`. Every value has a single, +specific type, but may implement several different traits, or be compatible with +several different type constraints. + +[type]: types.html + +For example, the following defines the type `Point` as a synonym for the type +`(u8, u8)`, the type of pairs of unsigned 8 bit integers: + +```rust +type Point = (u8, u8); +let p: Point = (41, 68); +``` + +Currently a type alias to an enum type cannot be used to qualify the +constructors: + +```rust +enum E { A } +type F = E; +let _: F = E::A; // OK +// let _: F = F::A; // Doesn't work +``` + +## Structs + +A _struct_ is a nominal [struct type] defined with the +keyword `struct`. + +An example of a `struct` item and its use: + +```rust +struct Point {x: i32, y: i32} +let p = Point {x: 10, y: 11}; +let px: i32 = p.x; +``` + +A _tuple struct_ is a nominal [tuple type], also defined with +the keyword `struct`. For example: + +[struct type]: types.html#struct-types +[tuple type]: types.html#tuple-types + +```rust +struct Point(i32, i32); +let p = Point(10, 11); +let px: i32 = match p { Point(x, _) => x }; +``` + +A _unit-like struct_ is a struct without any fields, defined by leaving off +the list of fields entirely. Such a struct implicitly defines a constant of +its type with the same name. For example: + +```rust +struct Cookie; +let c = [Cookie, Cookie {}, Cookie, Cookie {}]; +``` + +is equivalent to + +```rust +struct Cookie {} +const Cookie: Cookie = Cookie {}; +let c = [Cookie, Cookie {}, Cookie, Cookie {}]; +``` + +The precise memory layout of a struct is not specified. One can specify a +particular layout using the [`repr` attribute]. + +[`repr` attribute]: attributes.html#ffi-attributes + +## Enumerations + +An _enumeration_ is a simultaneous definition of a nominal [enumerated +type] as well as a set of *constructors*, that can be used +to create or pattern-match values of the corresponding enumerated type. + +[enumerated type]: types.html#enumerated-types + +Enumerations are declared with the keyword `enum`. + +An example of an `enum` item and its use: + +```rust +enum Animal { + Dog, + Cat, +} + +let mut a: Animal = Animal::Dog; +a = Animal::Cat; +``` + +Enumeration constructors can have either named or unnamed fields: + +```rust +enum Animal { + Dog (String, f64), + Cat { name: String, weight: f64 }, +} + +let mut a: Animal = Animal::Dog("Cocoa".to_string(), 37.2); +a = Animal::Cat { name: "Spotty".to_string(), weight: 2.7 }; +``` + +In this example, `Cat` is a _struct-like enum variant_, +whereas `Dog` is simply called an enum variant. + +Each enum value has a _discriminant_ which is an integer associated to it. You +can specify it explicitly: + +```rust +enum Foo { + Bar = 123, +} +``` + +The right hand side of the specification is interpreted as an `isize` value, +but the compiler is allowed to use a smaller type in the actual memory layout. +The [`repr` attribute] can be added in order to change +the type of the right hand side and specify the memory layout. + +[`repr` attribute]: attributes.html#ffi-attributes + +If a discriminant isn't specified, they start at zero, and add one for each +variant, in order. + +You can cast an enum to get its discriminant: + +```rust +# enum Foo { Bar = 123 } +let x = Foo::Bar as u32; // x is now 123u32 +``` + +This only works as long as none of the variants have data attached. If +it were `Bar(i32)`, this is disallowed. + +## Constant items + +A *constant item* is a named _constant value_ which is not associated with a +specific memory location in the program. Constants are essentially inlined +wherever they are used, meaning that they are copied directly into the relevant +context when used. References to the same constant are not necessarily +guaranteed to refer to the same memory address. + +Constant values must not have destructors, and otherwise permit most forms of +data. Constants may refer to the address of other constants, in which case the +address will have elided lifetimes where applicable, otherwise – in most cases – +defaulting to the `static` lifetime. (See below on [static lifetime elision].) +The compiler is, however, still at liberty to translate the constant many times, +so the address referred to may not be stable. + +[static lifetime elision]: #static-lifetime-elision + +Constants must be explicitly typed. The type may be `bool`, `char`, a number, or +a type derived from those primitive types. The derived types are references with +the `static` lifetime, fixed-size arrays, tuples, enum variants, and structs. + +```rust +const BIT1: u32 = 1 << 0; +const BIT2: u32 = 1 << 1; + +const BITS: [u32; 2] = [BIT1, BIT2]; +const STRING: &'static str = "bitstring"; + +struct BitsNStrings<'a> { + mybits: [u32; 2], + mystring: &'a str, +} + +const BITS_N_STRINGS: BitsNStrings<'static> = BitsNStrings { + mybits: BITS, + mystring: STRING, +}; +``` + +## Static items + +A *static item* is similar to a *constant*, except that it represents a precise +memory location in the program. A static is never "inlined" at the usage site, +and all references to it refer to the same memory location. Static items have +the `static` lifetime, which outlives all other lifetimes in a Rust program. +Static items may be placed in read-only memory if they do not contain any +interior mutability. + +Statics may contain interior mutability through the `UnsafeCell` language item. +All access to a static is safe, but there are a number of restrictions on +statics: + +* Statics may not contain any destructors. +* The types of static values must ascribe to `Sync` to allow thread-safe access. +* Statics may not refer to other statics by value, only by reference. +* Constants cannot refer to statics. + +Constants should in general be preferred over statics, unless large amounts of +data are being stored, or single-address and mutability properties are required. + +### Mutable statics + +If a static item is declared with the `mut` keyword, then it is allowed to +be modified by the program. One of Rust's goals is to make concurrency bugs +hard to run into, and this is obviously a very large source of race conditions +or other bugs. For this reason, an `unsafe` block is required when either +reading or writing a mutable static variable. Care should be taken to ensure +that modifications to a mutable static are safe with respect to other threads +running in the same process. + +Mutable statics are still very useful, however. They can be used with C +libraries and can also be bound from C libraries (in an `extern` block). + +```rust +# fn atomic_add(_: &mut u32, _: u32) -> u32 { 2 } + +static mut LEVELS: u32 = 0; + +// This violates the idea of no shared state, and this doesn't internally +// protect against races, so this function is `unsafe` +unsafe fn bump_levels_unsafe1() -> u32 { + let ret = LEVELS; + LEVELS += 1; + return ret; +} + +// Assuming that we have an atomic_add function which returns the old value, +// this function is "safe" but the meaning of the return value may not be what +// callers expect, so it's still marked as `unsafe` +unsafe fn bump_levels_unsafe2() -> u32 { + return atomic_add(&mut LEVELS, 1); +} +``` + +Mutable statics have the same restrictions as normal statics, except that the +type of the value is not required to ascribe to `Sync`. + +#### `'static` lifetime elision + +[Unstable] Both constant and static declarations of reference types have +*implicit* `'static` lifetimes unless an explicit lifetime is specified. As +such, the constant declarations involving `'static` above may be written +without the lifetimes. Returning to our previous example: + +```rust +# #![feature(static_in_const)] +const BIT1: u32 = 1 << 0; +const BIT2: u32 = 1 << 1; + +const BITS: [u32; 2] = [BIT1, BIT2]; +const STRING: &str = "bitstring"; + +struct BitsNStrings<'a> { + mybits: [u32; 2], + mystring: &'a str, +} + +const BITS_N_STRINGS: BitsNStrings = BitsNStrings { + mybits: BITS, + mystring: STRING, +}; +``` + +Note that if the `static` or `const` items include function or closure +references, which themselves include references, the compiler will first try the +standard elision rules ([see discussion in the nomicon][elision-nomicon]). If it +is unable to resolve the lifetimes by its usual rules, it will default to using +the `'static` lifetime. By way of example: + +[elision-nomicon]: ../nomicon/lifetime-elision.html + +```rust,ignore +// Resolved as `fn<'a>(&'a str) -> &'a str`. +const RESOLVED_SINGLE: fn(&str) -> &str = .. + +// Resolved as `Fn<'a, 'b, 'c>(&'a Foo, &'b Bar, &'c Baz) -> usize`. +const RESOLVED_MULTIPLE: Fn(&Foo, &Bar, &Baz) -> usize = .. + +// There is insufficient information to bound the return reference lifetime +// relative to the argument lifetimes, so the signature is resolved as +// `Fn(&'static Foo, &'static Bar) -> &'static Baz`. +const RESOLVED_STATIC: Fn(&Foo, &Bar) -> &Baz = .. +``` + +### Traits + +A _trait_ describes an abstract interface that types can +implement. This interface consists of associated items, which come in +three varieties: + +- functions +- constants +- types + +Associated functions whose first parameter is named `self` are called +methods and may be invoked using `.` notation (e.g., `x.foo()`). + +All traits define an implicit type parameter `Self` that refers to +"the type that is implementing this interface". Traits may also +contain additional type parameters. These type parameters (including +`Self`) may be constrained by other traits and so forth as usual. + +Trait bounds on `Self` are considered "supertraits". These are +required to be acyclic. Supertraits are somewhat different from other +constraints in that they affect what methods are available in the +vtable when the trait is used as a [trait object]. + +Traits are implemented for specific types through separate +[implementations]. + +Consider the following trait: + +```rust +# type Surface = i32; +# type BoundingBox = i32; +trait Shape { + fn draw(&self, Surface); + fn bounding_box(&self) -> BoundingBox; +} +``` + +This defines a trait with two methods. All values that have +[implementations] of this trait in scope can have their +`draw` and `bounding_box` methods called, using `value.bounding_box()` +[syntax]. + +[trait object]: types.html#trait-objects +[implementations]: #implementations +[syntax]: expressions.html#method-call-expressions + +Traits can include default implementations of methods, as in: + +```rust +trait Foo { + fn bar(&self); + fn baz(&self) { println!("We called baz."); } +} +``` + +Here the `baz` method has a default implementation, so types that implement +`Foo` need only implement `bar`. It is also possible for implementing types +to override a method that has a default implementation. + +Type parameters can be specified for a trait to make it generic. These appear +after the trait name, using the same syntax used in [generic +functions](#generic-functions). + +```rust +trait Seq { + fn len(&self) -> u32; + fn elt_at(&self, n: u32) -> T; + fn iter(&self, F) where F: Fn(T); +} +``` + +It is also possible to define associated types for a trait. Consider the +following example of a `Container` trait. Notice how the type is available +for use in the method signatures: + +```rust +trait Container { + type E; + fn empty() -> Self; + fn insert(&mut self, Self::E); +} +``` + +In order for a type to implement this trait, it must not only provide +implementations for every method, but it must specify the type `E`. Here's +an implementation of `Container` for the standard library type `Vec`: + +```rust +# trait Container { +# type E; +# fn empty() -> Self; +# fn insert(&mut self, Self::E); +# } +impl Container for Vec { + type E = T; + fn empty() -> Vec { Vec::new() } + fn insert(&mut self, x: T) { self.push(x); } +} +``` + +Generic functions may use traits as _bounds_ on their type parameters. This +will have two effects: + +- Only types that have the trait may instantiate the parameter. +- Within the generic function, the methods of the trait can be + called on values that have the parameter's type. + +For example: + +```rust +# type Surface = i32; +# trait Shape { fn draw(&self, Surface); } +fn draw_twice(surface: Surface, sh: T) { + sh.draw(surface); + sh.draw(surface); +} +``` + +Traits also define a [trait object] with the same +name as the trait. Values of this type are created by coercing from a +pointer of some specific type to a pointer of trait type. For example, +`&T` could be coerced to `&Shape` if `T: Shape` holds (and similarly +for `Box`). This coercion can either be implicit or +[explicit]. Here is an example of an explicit +coercion: + +[trait object]: types.html#trait-objects +[explicit]: expressions.html#type-cast-expressions + +```rust +trait Shape { } +impl Shape for i32 { } +let mycircle = 0i32; +let myshape: Box = Box::new(mycircle) as Box; +``` + +The resulting value is a box containing the value that was cast, along with +information that identifies the methods of the implementation that was used. +Values with a trait type can have [methods called] on +them, for any method in the trait, and can be used to instantiate type +parameters that are bounded by the trait. + +[methods called]: expressions.html#method-call-expressions + +Trait methods may be static, which means that they lack a `self` argument. +This means that they can only be called with function call syntax (`f(x)`) and +not method call syntax (`obj.f()`). The way to refer to the name of a static +method is to qualify it with the trait name, treating the trait name like a +module. For example: + +```rust +trait Num { + fn from_i32(n: i32) -> Self; +} +impl Num for f64 { + fn from_i32(n: i32) -> f64 { n as f64 } +} +let x: f64 = Num::from_i32(42); +``` + +Traits may inherit from other traits. Consider the following example: + +```rust +trait Shape { fn area(&self) -> f64; } +trait Circle : Shape { fn radius(&self) -> f64; } +``` + +The syntax `Circle : Shape` means that types that implement `Circle` must also +have an implementation for `Shape`. Multiple supertraits are separated by `+`, +`trait Circle : Shape + PartialEq { }`. In an implementation of `Circle` for a +given type `T`, methods can refer to `Shape` methods, since the typechecker +checks that any type with an implementation of `Circle` also has an +implementation of `Shape`: + +```rust +struct Foo; + +trait Shape { fn area(&self) -> f64; } +trait Circle : Shape { fn radius(&self) -> f64; } +impl Shape for Foo { + fn area(&self) -> f64 { + 0.0 + } +} +impl Circle for Foo { + fn radius(&self) -> f64 { + println!("calling area: {}", self.area()); + + 0.0 + } +} + +let c = Foo; +c.radius(); +``` + +In type-parameterized functions, methods of the supertrait may be called on +values of subtrait-bound type parameters. Referring to the previous example of +`trait Circle : Shape`: + +```rust +# trait Shape { fn area(&self) -> f64; } +# trait Circle : Shape { fn radius(&self) -> f64; } +fn radius_times_area(c: T) -> f64 { + // `c` is both a Circle and a Shape + c.radius() * c.area() +} +``` + +Likewise, supertrait methods may also be called on trait objects. + +```rust,ignore +# trait Shape { fn area(&self) -> f64; } +# trait Circle : Shape { fn radius(&self) -> f64; } +# impl Shape for i32 { fn area(&self) -> f64 { 0.0 } } +# impl Circle for i32 { fn radius(&self) -> f64 { 0.0 } } +# let mycircle = 0i32; +let mycircle = Box::new(mycircle) as Box; +let nonsense = mycircle.radius() * mycircle.area(); +``` + +### Implementations + +An _implementation_ is an item that implements a [trait](#traits) for a +specific type. + +Implementations are defined with the keyword `impl`. + +```rust +# #[derive(Copy, Clone)] +# struct Point {x: f64, y: f64}; +# type Surface = i32; +# struct BoundingBox {x: f64, y: f64, width: f64, height: f64}; +# trait Shape { fn draw(&self, Surface); fn bounding_box(&self) -> BoundingBox; } +# fn do_draw_circle(s: Surface, c: Circle) { } +struct Circle { + radius: f64, + center: Point, +} + +impl Copy for Circle {} + +impl Clone for Circle { + fn clone(&self) -> Circle { *self } +} + +impl Shape for Circle { + fn draw(&self, s: Surface) { do_draw_circle(s, *self); } + fn bounding_box(&self) -> BoundingBox { + let r = self.radius; + BoundingBox { + x: self.center.x - r, + y: self.center.y - r, + width: 2.0 * r, + height: 2.0 * r, + } + } +} +``` + +It is possible to define an implementation without referring to a trait. The +methods in such an implementation can only be used as direct calls on the values +of the type that the implementation targets. In such an implementation, the +trait type and `for` after `impl` are omitted. Such implementations are limited +to nominal types (enums, structs, trait objects), and the implementation must +appear in the same crate as the `self` type: + +```rust +struct Point {x: i32, y: i32} + +impl Point { + fn log(&self) { + println!("Point is at ({}, {})", self.x, self.y); + } +} + +let my_point = Point {x: 10, y:11}; +my_point.log(); +``` + +When a trait _is_ specified in an `impl`, all methods declared as part of the +trait must be implemented, with matching types and type parameter counts. + +An implementation can take type parameters, which can be different from the +type parameters taken by the trait it implements. Implementation parameters +are written after the `impl` keyword. + +```rust +# trait Seq { fn dummy(&self, _: T) { } } +impl Seq for Vec { + /* ... */ +} +impl Seq for u32 { + /* Treat the integer as a sequence of bits */ +} +``` + +### External blocks + +External blocks form the basis for Rust's foreign function interface. +Declarations in an external block describe symbols in external, non-Rust +libraries. + +Functions within external blocks are declared in the same way as other Rust +functions, with the exception that they may not have a body and are instead +terminated by a semicolon. + +Functions within external blocks may be called by Rust code, just like +functions defined in Rust. The Rust compiler automatically translates between +the Rust ABI and the foreign ABI. + +Functions within external blocks may be variadic by specifying `...` after one +or more named arguments in the argument list: + +```rust,ignore +extern { + fn foo(x: i32, ...); +} +``` + +A number of [attributes] control the behavior of external blocks. + +[attributes]: attributes.html#ffi-attributes + +By default external blocks assume that the library they are calling uses the +standard C ABI on the specific platform. Other ABIs may be specified using an +`abi` string, as shown here: + +```rust,ignore +// Interface to the Windows API +extern "stdcall" { } +``` + +There are three ABI strings which are cross-platform, and which all compilers +are guaranteed to support: + +* `extern "Rust"` -- The default ABI when you write a normal `fn foo()` in any + Rust code. +* `extern "C"` -- This is the same as `extern fn foo()`; whatever the default + your C compiler supports. +* `extern "system"` -- Usually the same as `extern "C"`, except on Win32, in + which case it's `"stdcall"`, or what you should use to link to the Windows API + itself + +There are also some platform-specific ABI strings: + +* `extern "cdecl"` -- The default for x86\_32 C code. +* `extern "stdcall"` -- The default for the Win32 API on x86\_32. +* `extern "win64"` -- The default for C code on x86\_64 Windows. +* `extern "sysv64"` -- The default for C code on non-Windows x86\_64. +* `extern "aapcs"` -- The default for ARM. +* `extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's + `__fastcall` and GCC and clang's `__attribute__((fastcall))` +* `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's + `__vectorcall` and clang's `__attribute__((vectorcall))` + +Finally, there are some rustc-specific ABI strings: + +* `extern "rust-intrinsic"` -- The ABI of rustc intrinsics. +* `extern "rust-call"` -- The ABI of the Fn::call trait functions. +* `extern "platform-intrinsic"` -- Specific platform intrinsics -- like, for + example, `sqrt` -- have this ABI. You should never have to deal with it. + +The `link` attribute allows the name of the library to be specified. When +specified the compiler will attempt to link against the native library of the +specified name. + +```rust,ignore +#[link(name = "crypto")] +extern { } +``` + +The type of a function declared in an extern block is `extern "abi" fn(A1, ..., +An) -> R`, where `A1...An` are the declared types of its arguments and `R` is +the declared return type. + +It is valid to add the `link` attribute on an empty extern block. You can use +this to satisfy the linking requirements of extern blocks elsewhere in your code +(including upstream crates) instead of adding the attribute to each extern block. diff --git a/src/doc/reference/src/lexical-structure.md b/src/doc/reference/src/lexical-structure.md new file mode 100644 index 0000000000000..5e1388e0d5a2b --- /dev/null +++ b/src/doc/reference/src/lexical-structure.md @@ -0,0 +1 @@ +# Lexical structure diff --git a/src/doc/reference/src/linkage.md b/src/doc/reference/src/linkage.md new file mode 100644 index 0000000000000..4755e4be8b685 --- /dev/null +++ b/src/doc/reference/src/linkage.md @@ -0,0 +1,127 @@ +# Linkage + +The Rust compiler supports various methods to link crates together both +statically and dynamically. This section will explore the various methods to +link Rust crates together, and more information about native libraries can be +found in the [FFI section of the book][ffi]. + +[ffi]: ../book/ffi.html + +In one session of compilation, the compiler can generate multiple artifacts +through the usage of either command line flags or the `crate_type` attribute. +If one or more command line flags are specified, all `crate_type` attributes will +be ignored in favor of only building the artifacts specified by command line. + +* `--crate-type=bin`, `#[crate_type = "bin"]` - A runnable executable will be + produced. This requires that there is a `main` function in the crate which + will be run when the program begins executing. This will link in all Rust and + native dependencies, producing a distributable binary. + +* `--crate-type=lib`, `#[crate_type = "lib"]` - A Rust library will be produced. + This is an ambiguous concept as to what exactly is produced because a library + can manifest itself in several forms. The purpose of this generic `lib` option + is to generate the "compiler recommended" style of library. The output library + will always be usable by rustc, but the actual type of library may change from + time-to-time. The remaining output types are all different flavors of + libraries, and the `lib` type can be seen as an alias for one of them (but the + actual one is compiler-defined). + +* `--crate-type=dylib`, `#[crate_type = "dylib"]` - A dynamic Rust library will + be produced. This is different from the `lib` output type in that this forces + dynamic library generation. The resulting dynamic library can be used as a + dependency for other libraries and/or executables. This output type will + create `*.so` files on linux, `*.dylib` files on osx, and `*.dll` files on + windows. + +* `--crate-type=staticlib`, `#[crate_type = "staticlib"]` - A static system + library will be produced. This is different from other library outputs in that + the Rust compiler will never attempt to link to `staticlib` outputs. The + purpose of this output type is to create a static library containing all of + the local crate's code along with all upstream dependencies. The static + library is actually a `*.a` archive on linux and osx and a `*.lib` file on + windows. This format is recommended for use in situations such as linking + Rust code into an existing non-Rust application because it will not have + dynamic dependencies on other Rust code. + +* `--crate-type=cdylib`, `#[crate_type = "cdylib"]` - A dynamic system + library will be produced. This is used when compiling Rust code as + a dynamic library to be loaded from another language. This output type will + create `*.so` files on Linux, `*.dylib` files on OSX, and `*.dll` files on + Windows. + +* `--crate-type=rlib`, `#[crate_type = "rlib"]` - A "Rust library" file will be + produced. This is used as an intermediate artifact and can be thought of as a + "static Rust library". These `rlib` files, unlike `staticlib` files, are + interpreted by the Rust compiler in future linkage. This essentially means + that `rustc` will look for metadata in `rlib` files like it looks for metadata + in dynamic libraries. This form of output is used to produce statically linked + executables as well as `staticlib` outputs. + +* `--crate-type=proc-macro`, `#[crate_type = "proc-macro"]` - The output + produced is not specified, but if a `-L` path is provided to it then the + compiler will recognize the output artifacts as a macro and it can be loaded + for a program. If a crate is compiled with the `proc-macro` crate type it + will forbid exporting any items in the crate other than those functions + tagged `#[proc_macro_derive]` and those functions must also be placed at the + crate root. Finally, the compiler will automatically set the + `cfg(proc_macro)` annotation whenever any crate type of a compilation is the + `proc-macro` crate type. + +Note that these outputs are stackable in the sense that if multiple are +specified, then the compiler will produce each form of output at once without +having to recompile. However, this only applies for outputs specified by the +same method. If only `crate_type` attributes are specified, then they will all +be built, but if one or more `--crate-type` command line flags are specified, +then only those outputs will be built. + +With all these different kinds of outputs, if crate A depends on crate B, then +the compiler could find B in various different forms throughout the system. The +only forms looked for by the compiler, however, are the `rlib` format and the +dynamic library format. With these two options for a dependent library, the +compiler must at some point make a choice between these two formats. With this +in mind, the compiler follows these rules when determining what format of +dependencies will be used: + +1. If a static library is being produced, all upstream dependencies are + required to be available in `rlib` formats. This requirement stems from the + reason that a dynamic library cannot be converted into a static format. + + Note that it is impossible to link in native dynamic dependencies to a static + library, and in this case warnings will be printed about all unlinked native + dynamic dependencies. + +2. If an `rlib` file is being produced, then there are no restrictions on what + format the upstream dependencies are available in. It is simply required that + all upstream dependencies be available for reading metadata from. + + The reason for this is that `rlib` files do not contain any of their upstream + dependencies. It wouldn't be very efficient for all `rlib` files to contain a + copy of `libstd.rlib`! + +3. If an executable is being produced and the `-C prefer-dynamic` flag is not + specified, then dependencies are first attempted to be found in the `rlib` + format. If some dependencies are not available in an rlib format, then + dynamic linking is attempted (see below). + +4. If a dynamic library or an executable that is being dynamically linked is + being produced, then the compiler will attempt to reconcile the available + dependencies in either the rlib or dylib format to create a final product. + + A major goal of the compiler is to ensure that a library never appears more + than once in any artifact. For example, if dynamic libraries B and C were + each statically linked to library A, then a crate could not link to B and C + together because there would be two copies of A. The compiler allows mixing + the rlib and dylib formats, but this restriction must be satisfied. + + The compiler currently implements no method of hinting what format a library + should be linked with. When dynamically linking, the compiler will attempt to + maximize dynamic dependencies while still allowing some dependencies to be + linked in via an rlib. + + For most situations, having all libraries available as a dylib is recommended + if dynamically linking. For other situations, the compiler will emit a + warning if it is unable to determine which formats to link each library with. + +In general, `--crate-type=bin` or `--crate-type=lib` should be sufficient for +all compilation needs, and the other options are just available if more +fine-grained control is desired over the output format of a Rust crate. diff --git a/src/doc/reference/src/macros-by-example.md b/src/doc/reference/src/macros-by-example.md new file mode 100644 index 0000000000000..a007b232e4c97 --- /dev/null +++ b/src/doc/reference/src/macros-by-example.md @@ -0,0 +1,88 @@ +# Macros By Example + +`macro_rules` allows users to define syntax extension in a declarative way. We +call such extensions "macros by example" or simply "macros". + +Currently, macros can expand to expressions, statements, items, or patterns. + +(A `sep_token` is any token other than `*` and `+`. A `non_special_token` is +any token other than a delimiter or `$`.) + +The macro expander looks up macro invocations by name, and tries each macro +rule in turn. It transcribes the first successful match. Matching and +transcription are closely related to each other, and we will describe them +together. + +The macro expander matches and transcribes every token that does not begin with +a `$` literally, including delimiters. For parsing reasons, delimiters must be +balanced, but they are otherwise not special. + +In the matcher, `$` _name_ `:` _designator_ matches the nonterminal in the Rust +syntax named by _designator_. Valid designators are: + +* `item`: an [item] +* `block`: a [block] +* `stmt`: a [statement] +* `pat`: a [pattern] +* `expr`: an [expression] +* `ty`: a [type] +* `ident`: an [identifier] +* `path`: a [path] +* `tt`: a token tree (a single [token] by matching `()`, `[]`, or `{}`) +* `meta`: the contents of an [attribute] + +[item]: items.html +[block]: expressions.html#block-expressions +[statement]: statements.html +[pattern]: expressions.html#match-expressions +[expression]: expressions.html +[type]: types.html +[identifier]: identifiers.html +[path]: paths.html +[token]: tokens.html +[attribute]: attributes.html + +In the transcriber, the +designator is already known, and so only the name of a matched nonterminal comes +after the dollar sign. + +In both the matcher and transcriber, the Kleene star-like operator indicates +repetition. The Kleene star operator consists of `$` and parentheses, optionally +followed by a separator token, followed by `*` or `+`. `*` means zero or more +repetitions, `+` means at least one repetition. The parentheses are not matched or +transcribed. On the matcher side, a name is bound to _all_ of the names it +matches, in a structure that mimics the structure of the repetition encountered +on a successful match. The job of the transcriber is to sort that structure +out. + +The rules for transcription of these repetitions are called "Macro By Example". +Essentially, one "layer" of repetition is discharged at a time, and all of them +must be discharged by the time a name is transcribed. Therefore, `( $( $i:ident +),* ) => ( $i )` is an invalid macro, but `( $( $i:ident ),* ) => ( $( $i:ident +),* )` is acceptable (if trivial). + +When Macro By Example encounters a repetition, it examines all of the `$` +_name_ s that occur in its body. At the "current layer", they all must repeat +the same number of times, so ` ( $( $i:ident ),* ; $( $j:ident ),* ) => ( $( +($i,$j) ),* )` is valid if given the argument `(a,b,c ; d,e,f)`, but not +`(a,b,c ; d,e)`. The repetition walks through the choices at that layer in +lockstep, so the former input transcribes to `(a,d), (b,e), (c,f)`. + +Nested repetitions are allowed. + +### Parsing limitations + +The parser used by the macro system is reasonably powerful, but the parsing of +Rust syntax is restricted in two ways: + +1. Macro definitions are required to include suitable separators after parsing + expressions and other bits of the Rust grammar. This implies that + a macro definition like `$i:expr [ , ]` is not legal, because `[` could be part + of an expression. A macro definition like `$i:expr,` or `$i:expr;` would be legal, + however, because `,` and `;` are legal separators. See [RFC 550] for more information. +2. The parser must have eliminated all ambiguity by the time it reaches a `$` + _name_ `:` _designator_. This requirement most often affects name-designator + pairs when they occur at the beginning of, or immediately after, a `$(...)*`; + requiring a distinctive token in front can solve the problem. + +[RFC 550]: https://github.com/rust-lang/rfcs/blob/master/text/0550-macro-future-proofing.md diff --git a/src/doc/reference/src/macros.md b/src/doc/reference/src/macros.md new file mode 100644 index 0000000000000..9ec5f2d6945e5 --- /dev/null +++ b/src/doc/reference/src/macros.md @@ -0,0 +1,17 @@ +# Macros + +A number of minor features of Rust are not central enough to have their own +syntax, and yet are not implementable as functions. Instead, they are given +names, and invoked through a consistent syntax: `some_extension!(...)`. + +Users of `rustc` can define new macros in two ways: + +* [Macros] define new syntax in a higher-level, + declarative way. +* [Procedural Macros] can be used to implement custom derive. + +And one unstable way: [compiler plugins]. + +[Macros]: ../book/macros.html +[Procedural Macros]: ../book/procedural-macros.html +[compiler plugins]: ../book/compiler-plugins.html diff --git a/src/doc/reference/src/memory-allocation-and-lifetime.md b/src/doc/reference/src/memory-allocation-and-lifetime.md new file mode 100644 index 0000000000000..24addb1dd39d4 --- /dev/null +++ b/src/doc/reference/src/memory-allocation-and-lifetime.md @@ -0,0 +1,13 @@ +# Memory allocation and lifetime + +The _items_ of a program are those functions, modules and types that have their +value calculated at compile-time and stored uniquely in the memory image of the +rust process. Items are neither dynamically allocated nor freed. + +The _heap_ is a general term that describes boxes. The lifetime of an +allocation in the heap depends on the lifetime of the box values pointing to +it. Since box values may themselves be passed in and out of frames, or stored +in the heap, heap allocations may outlive the frame they are allocated within. +An allocation in the heap is guaranteed to reside at a single location in the +heap for the whole lifetime of the allocation - it will never be relocated as +a result of moving a box value. diff --git a/src/doc/reference/src/memory-model.md b/src/doc/reference/src/memory-model.md new file mode 100644 index 0000000000000..aa57ae6ae9bea --- /dev/null +++ b/src/doc/reference/src/memory-model.md @@ -0,0 +1,10 @@ +# Memory model + +A Rust program's memory consists of a static set of *items* and a *heap*. +Immutable portions of the heap may be safely shared between threads, mutable +portions may not be safely shared, but several mechanisms for effectively-safe +sharing of mutable values, built on unsafe code but enforcing a safe locking +discipline, exist in the standard library. + +Allocations in the stack consist of *variables*, and allocations in the heap +consist of *boxes*. diff --git a/src/doc/reference/src/memory-ownership.md b/src/doc/reference/src/memory-ownership.md new file mode 100644 index 0000000000000..aed07ef2961a5 --- /dev/null +++ b/src/doc/reference/src/memory-ownership.md @@ -0,0 +1,4 @@ +## Memory ownership + +When a stack frame is exited, its local allocations are all released, and its +references to boxes are dropped. diff --git a/src/doc/reference/src/notation.md b/src/doc/reference/src/notation.md new file mode 100644 index 0000000000000..642bff440ad93 --- /dev/null +++ b/src/doc/reference/src/notation.md @@ -0,0 +1 @@ +# Notation diff --git a/src/doc/reference/src/paths.md b/src/doc/reference/src/paths.md new file mode 100644 index 0000000000000..e9fd07e5664b9 --- /dev/null +++ b/src/doc/reference/src/paths.md @@ -0,0 +1,105 @@ +# Paths + +A _path_ is a sequence of one or more path components _logically_ separated by +a namespace qualifier (`::`). If a path consists of only one component, it may +refer to either an [item] or a [variable] in a local control +scope. If a path has multiple components, it refers to an item. + +[item]: items.html +[variable]: variables.html + +Every item has a _canonical path_ within its crate, but the path naming an item +is only meaningful within a given crate. There is no global namespace across +crates; an item's canonical path merely identifies it within the crate. + +Two examples of simple paths consisting of only identifier components: + +```{.ignore} +x; +x::y::z; +``` + +Path components are usually [identifiers], but they may +also include angle-bracket-enclosed lists of type arguments. In +[expression] context, the type argument list is given +after a `::` namespace qualifier in order to disambiguate it from a +relational expression involving the less-than symbol (`<`). In type +expression context, the final namespace qualifier is omitted. + +[identifiers]: identifiers.html +[expression]: expressions.html + +Two examples of paths with type arguments: + +```rust +# struct HashMap(K,V); +# fn f() { +# fn id(t: T) -> T { t } +type T = HashMap; // Type arguments used in a type expression +let x = id::(10); // Type arguments used in a call expression +# } +``` + +Paths can be denoted with various leading qualifiers to change the meaning of +how it is resolved: + +* Paths starting with `::` are considered to be global paths where the + components of the path start being resolved from the crate root. Each + identifier in the path must resolve to an item. + +```rust +mod a { + pub fn foo() {} +} +mod b { + pub fn foo() { + ::a::foo(); // call a's foo function + } +} +# fn main() {} +``` + +* Paths starting with the keyword `super` begin resolution relative to the + parent module. Each further identifier must resolve to an item. + +```rust +mod a { + pub fn foo() {} +} +mod b { + pub fn foo() { + super::a::foo(); // call a's foo function + } +} +# fn main() {} +``` + +* Paths starting with the keyword `self` begin resolution relative to the + current module. Each further identifier must resolve to an item. + +```rust +fn foo() {} +fn bar() { + self::foo(); +} +# fn main() {} +``` + +Additionally keyword `super` may be repeated several times after the first +`super` or `self` to refer to ancestor modules. + +```rust +mod a { + fn foo() {} + + mod b { + mod c { + fn foo() { + super::super::foo(); // call a's foo function + self::super::super::foo(); // call a's foo function + } + } + } +} +# fn main() {} +``` diff --git a/src/doc/reference/src/procedural-macros.md b/src/doc/reference/src/procedural-macros.md new file mode 100644 index 0000000000000..b1fd35653d9e7 --- /dev/null +++ b/src/doc/reference/src/procedural-macros.md @@ -0,0 +1,23 @@ +## Procedural Macros + +"Procedural macros" are the second way to implement a macro. For now, the only +thing they can be used for is to implement derive on your own types. See +[the book][procedural macros] for a tutorial. + +[procedural macros]: ../book/procedural-macros.html + +Procedural macros involve a few different parts of the language and its +standard libraries. First is the `proc_macro` crate, included with Rust, +that defines an interface for building a procedural macro. The +`#[proc_macro_derive(Foo)]` attribute is used to mark the deriving +function. This function must have the type signature: + +```rust,ignore +use proc_macro::TokenStream; + +#[proc_macro_derive(Hello)] +pub fn hello_world(input: TokenStream) -> TokenStream +``` + +Finally, procedural macros must be in their own crate, with the `proc-macro` +crate type. diff --git a/src/doc/reference/src/special-traits.md b/src/doc/reference/src/special-traits.md new file mode 100644 index 0000000000000..ae3eebe392d1d --- /dev/null +++ b/src/doc/reference/src/special-traits.md @@ -0,0 +1,3 @@ +# Special traits + +Several traits define special evaluation behavior. diff --git a/src/doc/reference/src/statements-and-expressions.md b/src/doc/reference/src/statements-and-expressions.md new file mode 100644 index 0000000000000..bb59108f17f32 --- /dev/null +++ b/src/doc/reference/src/statements-and-expressions.md @@ -0,0 +1,11 @@ +# Statements and expressions + +Rust is _primarily_ an expression language. This means that most forms of +value-producing or effect-causing evaluation are directed by the uniform syntax +category of _expressions_. Each kind of expression can typically _nest_ within +each other kind of expression, and rules for evaluation of expressions involve +specifying both the value produced by the expression and the order in which its +sub-expressions are themselves evaluated. + +In contrast, statements in Rust serve _mostly_ to contain and explicitly +sequence expression evaluation. diff --git a/src/doc/reference/src/statements.md b/src/doc/reference/src/statements.md new file mode 100644 index 0000000000000..ebb7d8bfa7cd0 --- /dev/null +++ b/src/doc/reference/src/statements.md @@ -0,0 +1,42 @@ +# Statements + +A _statement_ is a component of a block, which is in turn a component of an +outer [expression](expressions.html) or [function](items.html#functions). + +Rust has two kinds of statement: [declaration +statements](#declaration-statements) and [expression +statements](#expression-statements). + +## Declaration statements + +A _declaration statement_ is one that introduces one or more *names* into the +enclosing statement block. The declared names may denote new variables or new +items. + +### Item declarations + +An _item declaration statement_ has a syntactic form identical to an +[item](items.html) declaration within a module. Declaring an item — a +function, enumeration, struct, type, static, trait, implementation or module +— locally within a statement block is simply a way of restricting its +scope to a narrow region containing all of its uses; it is otherwise identical +in meaning to declaring the item outside the statement block. + +> **Note**: there is no implicit capture of the function's dynamic environment when +> declaring a function-local item. + +### `let` statements + +A _`let` statement_ introduces a new set of variables, given by a pattern. The +pattern may be followed by a type annotation, and/or an initializer expression. +When no type annotation is given, the compiler will infer the type, or signal +an error if insufficient type information is available for definite inference. +Any variables introduced by a variable declaration are visible from the point of +declaration until the end of the enclosing block scope. + +## Expression statements + +An _expression statement_ is one that evaluates an [expression](expressions.html) +and ignores its result. The type of an expression statement `e;` is always +`()`, regardless of the type of `e`. As a rule, an expression statement's +purpose is to trigger the effects of evaluating its expression. diff --git a/src/doc/reference/src/string-table-productions.md b/src/doc/reference/src/string-table-productions.md new file mode 100644 index 0000000000000..4e9742e3bbb80 --- /dev/null +++ b/src/doc/reference/src/string-table-productions.md @@ -0,0 +1,18 @@ +# String table productions + +Some rules in the grammar — notably [unary +operators], [binary operators], and [keywords][keywords] — are +given in a simplified form: as a listing of a table of unquoted, printable +whitespace-separated strings. These cases form a subset of the rules regarding +the [token][tokens] rule, and are assumed to be the result of a +lexical-analysis phase feeding the parser, driven by a DFA, operating over the +disjunction of all such string table entries. + +When such a string enclosed in double-quotes (`"`) occurs inside the grammar, +it is an implicit reference to a single member of such a string table +production. See [tokens] for more information. + +[binary operators]: expressions.html#binary-operator-expressions +[keywords]: ../grammar.html#keywords +[tokens]: tokens.html +[unary operators]: expressions.html#unary-operator-expressions \ No newline at end of file diff --git a/src/doc/reference/src/subtyping.md b/src/doc/reference/src/subtyping.md new file mode 100644 index 0000000000000..a43b041a69338 --- /dev/null +++ b/src/doc/reference/src/subtyping.md @@ -0,0 +1,19 @@ +# Subtyping + +Subtyping is implicit and can occur at any stage in type checking or +inference. Subtyping in Rust is very restricted and occurs only due to +variance with respect to lifetimes and between types with higher ranked +lifetimes. If we were to erase lifetimes from types, then the only subtyping +would be due to type equality. + +Consider the following example: string literals always have `'static` +lifetime. Nevertheless, we can assign `s` to `t`: + +``` +fn bar<'a>() { + let s: &'static str = "hi"; + let t: &'a str = s; +} +``` +Since `'static` "lives longer" than `'a`, `&'static str` is a subtype of +`&'a str`. diff --git a/src/doc/reference/src/the-copy-trait.md b/src/doc/reference/src/the-copy-trait.md new file mode 100644 index 0000000000000..d593165e48d58 --- /dev/null +++ b/src/doc/reference/src/the-copy-trait.md @@ -0,0 +1,4 @@ +# The `Copy` trait + +The `Copy` trait changes the semantics of a type implementing it. Values whose +type implements `Copy` are copied rather than moved upon assignment. diff --git a/src/doc/reference/src/the-deref-trait.md b/src/doc/reference/src/the-deref-trait.md new file mode 100644 index 0000000000000..a4d84ab83ea67 --- /dev/null +++ b/src/doc/reference/src/the-deref-trait.md @@ -0,0 +1,7 @@ +# The `Deref` trait + +The `Deref` trait allows a type to implicitly implement all the methods +of the type `U`. When attempting to resolve a method call, the compiler will search +the top-level type for the implementation of the called method. If no such method is +found, `.deref()` is called and the compiler continues to search for the method +implementation in the returned type `U`. diff --git a/src/doc/reference/src/the-drop-trait.md b/src/doc/reference/src/the-drop-trait.md new file mode 100644 index 0000000000000..42bf6eb0f2014 --- /dev/null +++ b/src/doc/reference/src/the-drop-trait.md @@ -0,0 +1,4 @@ +# The `Drop` trait + +The `Drop` trait provides a destructor, to be run whenever a value of this type +is to be destroyed. diff --git a/src/doc/reference/src/the-send-trait.md b/src/doc/reference/src/the-send-trait.md new file mode 100644 index 0000000000000..9ec669289a567 --- /dev/null +++ b/src/doc/reference/src/the-send-trait.md @@ -0,0 +1,4 @@ +# The `Send` trait + +The `Send` trait indicates that a value of this type is safe to send from one +thread to another. diff --git a/src/doc/reference/src/the-sized-trait.md b/src/doc/reference/src/the-sized-trait.md new file mode 100644 index 0000000000000..a2aa17c95f295 --- /dev/null +++ b/src/doc/reference/src/the-sized-trait.md @@ -0,0 +1,3 @@ +# The `Sized` trait + +The `Sized` trait indicates that the size of this type is known at compile-time. diff --git a/src/doc/reference/src/the-sync-trait.md b/src/doc/reference/src/the-sync-trait.md new file mode 100644 index 0000000000000..fd9365134b292 --- /dev/null +++ b/src/doc/reference/src/the-sync-trait.md @@ -0,0 +1,4 @@ +# The `Sync` trait + +The `Sync` trait indicates that a value of this type is safe to share between +multiple threads. diff --git a/src/doc/reference/src/theme/book.css b/src/doc/reference/src/theme/book.css new file mode 100644 index 0000000000000..ee92e2f8710a9 --- /dev/null +++ b/src/doc/reference/src/theme/book.css @@ -0,0 +1,798 @@ +html, +body { + font-family: "Open Sans", sans-serif; + color: #333; +} +.left { + float: left; +} +.right { + float: right; +} +.hidden { + display: none; +} +h2, +h3 { + margin-top: 2.5em; +} +h4, +h5 { + margin-top: 2em; +} +.header + .header h3, +.header + .header h4, +.header + .header h5 { + margin-top: 1em; +} +table { + margin: 0 auto; + border-collapse: collapse; +} +table td { + padding: 3px 20px; + border: 1px solid; +} +table thead td { + font-weight: 700; +} +.sidebar { + position: absolute; + left: 0; + top: 0; + bottom: 0; + width: 300px; + overflow-y: auto; + padding: 10px 10px; + font-size: 0.875em; + -webkit-box-sizing: border-box; + -moz-box-sizing: border-box; + box-sizing: border-box; + -webkit-overflow-scrolling: touch; + -webkit-transition: left 0.5s; + -moz-transition: left 0.5s; + -o-transition: left 0.5s; + -ms-transition: left 0.5s; + transition: left 0.5s; +} +@media only screen and (max-width: 1060px) { + .sidebar { + left: -300px; + } +} +.sidebar code { + line-height: 2em; +} +.sidebar-hidden .sidebar { + left: -300px; +} +.sidebar-visible .sidebar { + left: 0; +} +.chapter { + list-style: none outside none; + padding-left: 0; + line-height: 1.9em; +} +.chapter li a { + padding: 5px 0; + text-decoration: none; +} +.chapter li a:hover { + text-decoration: none; +} +.chapter .spacer { + width: 100%; + height: 3px; + margin: 10px 0px; +} +.section { + list-style: none outside none; + padding-left: 20px; + line-height: 2.5em; +} +.section li { + -o-text-overflow: ellipsis; + text-overflow: ellipsis; + overflow: hidden; + white-space: nowrap; +} +.page-wrapper { + position: absolute; + overflow-y: auto; + left: 315px; + right: 0; + top: 0; + bottom: 0; + -webkit-box-sizing: border-box; + -moz-box-sizing: border-box; + box-sizing: border-box; + -webkit-overflow-scrolling: touch; + min-height: 100%; + -webkit-transition: left 0.5s; + -moz-transition: left 0.5s; + -o-transition: left 0.5s; + -ms-transition: left 0.5s; + transition: left 0.5s; +} +@media only screen and (max-width: 1060px) { + .page-wrapper { + left: 15px; + padding-right: 15px; + } +} +.sidebar-hidden .page-wrapper { + left: 15px; +} +.sidebar-visible .page-wrapper { + left: 315px; +} +.page { + position: absolute; + top: 0; + right: 0; + left: 0; + bottom: 0; + padding-right: 15px; + overflow-y: auto; +} +.content { + margin-left: auto; + margin-right: auto; + max-width: 750px; + padding-bottom: 50px; +} +.content a { + text-decoration: none; +} +.content a:hover { + text-decoration: underline; +} +.content img { + max-width: 100%; +} +.menu-bar { + position: relative; + height: 50px; +} +.menu-bar i { + position: relative; + margin: 0 10px; + z-index: 10; + line-height: 50px; + -webkit-transition: color 0.5s; + -moz-transition: color 0.5s; + -o-transition: color 0.5s; + -ms-transition: color 0.5s; + transition: color 0.5s; +} +.menu-bar i:hover { + cursor: pointer; +} +.menu-bar .left-buttons { + float: left; +} +.menu-bar .right-buttons { + float: right; +} +.menu-title { + display: inline-block; + font-weight: 200; + font-size: 20px; + line-height: 50px; + position: absolute; + top: 0; + left: 0; + right: 0; + bottom: 0; + text-align: center; + margin: 0; + opacity: 0; + -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)"; + filter: alpha(opacity=0); + -webkit-transition: opacity 0.5s ease-in-out; + -moz-transition: opacity 0.5s ease-in-out; + -o-transition: opacity 0.5s ease-in-out; + -ms-transition: opacity 0.5s ease-in-out; + transition: opacity 0.5s ease-in-out; +} +.menu-bar:hover .menu-title { + opacity: 1; + -ms-filter: none; + filter: none; +} +.nav-chapters { + font-size: 2.5em; + text-align: center; + text-decoration: none; + position: absolute; + top: 50px /* Height of menu-bar */; + bottom: 0; + margin: 0; + max-width: 150px; + min-width: 90px; + display: -webkit-box; + display: -moz-box; + display: -webkit-flex; + display: -ms-flexbox; + display: box; + display: flex; + -webkit-box-pack: center; + -moz-box-pack: center; + -o-box-pack: center; + -ms-flex-pack: center; + -webkit-justify-content: center; + justify-content: center; + -ms-flex-line-pack: center; + -webkit-align-content: center; + align-content: center; + -webkit-box-orient: vertical; + -moz-box-orient: vertical; + -o-box-orient: vertical; + -webkit-flex-direction: column; + -ms-flex-direction: column; + flex-direction: column; + -webkit-transition: color 0.5s; + -moz-transition: color 0.5s; + -o-transition: color 0.5s; + -ms-transition: color 0.5s; + transition: color 0.5s; +} +.mobile-nav-chapters { + display: none; +} +.nav-chapters:hover { + text-decoration: none; +} +.previous { + left: 0; +} +.next { + right: 15px; +} +.theme-popup { + position: relative; + left: 10px; + z-index: 1000; + -webkit-border-radius: 4px; + border-radius: 4px; + font-size: 0.7em; +} +.theme-popup .theme { + margin: 0; + padding: 2px 10px; + line-height: 25px; + white-space: nowrap; +} +.theme-popup .theme:hover:first-child { + border-top-left-radius: inherit; + border-top-right-radius: inherit; +} +.theme-popup .theme:hover:last-child { + border-bottom-left-radius: inherit; + border-bottom-right-radius: inherit; +} + +@media only screen and (max-width: 1250px) { + .nav-chapters { + display: none; + } + .mobile-nav-chapters { + font-size: 2.5em; + text-align: center; + text-decoration: none; + max-width: 150px; + min-width: 90px; + -webkit-box-pack: center; + -moz-box-pack: center; + -o-box-pack: center; + -ms-flex-pack: center; + -webkit-justify-content: center; + justify-content: center; + -ms-flex-line-pack: center; + -webkit-align-content: center; + align-content: center; + position: relative; + display: inline-block; + margin-bottom: 50px; + -webkit-border-radius: 5px; + border-radius: 5px; + } + .next { + float: right; + } + .previous { + float: left; + } +} +.light { + color: #333; + background-color: #fff; +/* Inline code */ +} +.light .content .header:link, +.light .content .header:visited { + color: #333; + pointer: cursor; +} +.light .content .header:link:hover, +.light .content .header:visited:hover { + text-decoration: none; +} +.light .sidebar { + background-color: #fafafa; + color: #364149; +} +.light .chapter li { + color: #aaa; +} +.light .chapter li a { + color: #364149; +} +.light .chapter li .active, +.light .chapter li a:hover { +/* Animate color change */ + color: #008cff; +} +.light .chapter .spacer { + background-color: #f4f4f4; +} +.light .menu-bar, +.light .menu-bar:visited, +.light .nav-chapters, +.light .nav-chapters:visited, +.light .mobile-nav-chapters, +.light .mobile-nav-chapters:visited { + color: #ccc; +} +.light .menu-bar i:hover, +.light .nav-chapters:hover, +.light .mobile-nav-chapters i:hover { + color: #333; +} +.light .mobile-nav-chapters i:hover { + color: #364149; +} +.light .mobile-nav-chapters { + background-color: #fafafa; +} +.light .content a:link, +.light a:visited { + color: #4183c4; +} +.light .theme-popup { + color: #333; + background: #fafafa; + border: 1px solid #ccc; +} +.light .theme-popup .theme:hover { + background-color: #e6e6e6; +} +.light .theme-popup .default { + color: #ccc; +} +.light blockquote { + margin: 20px 0; + padding: 0 20px; + color: #333; + background-color: #f2f7f9; + border-top: 0.1em solid #e1edf1; + border-bottom: 0.1em solid #e1edf1; +} +.light table td { + border-color: #f2f2f2; +} +.light table tbody tr:nth-child(2n) { + background: #f7f7f7; +} +.light table thead { + background: #ccc; +} +.light table thead td { + border: none; +} +.light table thead tr { + border: 1px #ccc solid; +} +.light :not(pre) > .hljs { + display: inline-block; + vertical-align: middle; + padding: 0.1em 0.3em; + -webkit-border-radius: 3px; + border-radius: 3px; +} +.light pre { + position: relative; +} +.light pre > .buttons { + position: absolute; + right: 5px; + top: 5px; + color: #364149; + cursor: pointer; +} +.light pre > .buttons :hover { + color: #008cff; +} +.light pre > .buttons i { + margin-left: 8px; +} +.light pre > .result { + margin-top: 10px; +} +.coal { + color: #98a3ad; + background-color: #141617; +/* Inline code */ +} +.coal .content .header:link, +.coal .content .header:visited { + color: #98a3ad; + pointer: cursor; +} +.coal .content .header:link:hover, +.coal .content .header:visited:hover { + text-decoration: none; +} +.coal .sidebar { + background-color: #292c2f; + color: #a1adb8; +} +.coal .chapter li { + color: #505254; +} +.coal .chapter li a { + color: #a1adb8; +} +.coal .chapter li .active, +.coal .chapter li a:hover { +/* Animate color change */ + color: #3473ad; +} +.coal .chapter .spacer { + background-color: #393939; +} +.coal .menu-bar, +.coal .menu-bar:visited, +.coal .nav-chapters, +.coal .nav-chapters:visited, +.coal .mobile-nav-chapters, +.coal .mobile-nav-chapters:visited { + color: #43484d; +} +.coal .menu-bar i:hover, +.coal .nav-chapters:hover, +.coal .mobile-nav-chapters i:hover { + color: #b3c0cc; +} +.coal .mobile-nav-chapters i:hover { + color: #a1adb8; +} +.coal .mobile-nav-chapters { + background-color: #292c2f; +} +.coal .content a:link, +.coal a:visited { + color: #2b79a2; +} +.coal .theme-popup { + color: #98a3ad; + background: #141617; + border: 1px solid #43484d; +} +.coal .theme-popup .theme:hover { + background-color: #1f2124; +} +.coal .theme-popup .default { + color: #43484d; +} +.coal blockquote { + margin: 20px 0; + padding: 0 20px; + color: #98a3ad; + background-color: #242637; + border-top: 0.1em solid #2c2f44; + border-bottom: 0.1em solid #2c2f44; +} +.coal table td { + border-color: #1f2223; +} +.coal table tbody tr:nth-child(2n) { + background: #1b1d1e; +} +.coal table thead { + background: #3f4649; +} +.coal table thead td { + border: none; +} +.coal table thead tr { + border: 1px #3f4649 solid; +} +.coal :not(pre) > .hljs { + display: inline-block; + vertical-align: middle; + padding: 0.1em 0.3em; + -webkit-border-radius: 3px; + border-radius: 3px; +} +.coal pre { + position: relative; +} +.coal pre > .buttons { + position: absolute; + right: 5px; + top: 5px; + color: #a1adb8; + cursor: pointer; +} +.coal pre > .buttons :hover { + color: #3473ad; +} +.coal pre > .buttons i { + margin-left: 8px; +} +.coal pre > .result { + margin-top: 10px; +} +.navy { + color: #bcbdd0; + background-color: #161923; +/* Inline code */ +} +.navy .content .header:link, +.navy .content .header:visited { + color: #bcbdd0; + pointer: cursor; +} +.navy .content .header:link:hover, +.navy .content .header:visited:hover { + text-decoration: none; +} +.navy .sidebar { + background-color: #282d3f; + color: #c8c9db; +} +.navy .chapter li { + color: #505274; +} +.navy .chapter li a { + color: #c8c9db; +} +.navy .chapter li .active, +.navy .chapter li a:hover { +/* Animate color change */ + color: #2b79a2; +} +.navy .chapter .spacer { + background-color: #2d334f; +} +.navy .menu-bar, +.navy .menu-bar:visited, +.navy .nav-chapters, +.navy .nav-chapters:visited, +.navy .mobile-nav-chapters, +.navy .mobile-nav-chapters:visited { + color: #737480; +} +.navy .menu-bar i:hover, +.navy .nav-chapters:hover, +.navy .mobile-nav-chapters i:hover { + color: #b7b9cc; +} +.navy .mobile-nav-chapters i:hover { + color: #c8c9db; +} +.navy .mobile-nav-chapters { + background-color: #282d3f; +} +.navy .content a:link, +.navy a:visited { + color: #2b79a2; +} +.navy .theme-popup { + color: #bcbdd0; + background: #161923; + border: 1px solid #737480; +} +.navy .theme-popup .theme:hover { + background-color: #282e40; +} +.navy .theme-popup .default { + color: #737480; +} +.navy blockquote { + margin: 20px 0; + padding: 0 20px; + color: #bcbdd0; + background-color: #262933; + border-top: 0.1em solid #2f333f; + border-bottom: 0.1em solid #2f333f; +} +.navy table td { + border-color: #1f2331; +} +.navy table tbody tr:nth-child(2n) { + background: #1b1f2b; +} +.navy table thead { + background: #39415b; +} +.navy table thead td { + border: none; +} +.navy table thead tr { + border: 1px #39415b solid; +} +.navy :not(pre) > .hljs { + display: inline-block; + vertical-align: middle; + padding: 0.1em 0.3em; + -webkit-border-radius: 3px; + border-radius: 3px; +} +.navy pre { + position: relative; +} +.navy pre > .buttons { + position: absolute; + right: 5px; + top: 5px; + color: #c8c9db; + cursor: pointer; +} +.navy pre > .buttons :hover { + color: #2b79a2; +} +.navy pre > .buttons i { + margin-left: 8px; +} +.navy pre > .result { + margin-top: 10px; +} +.rust { + color: #262625; + background-color: #e1e1db; +/* Inline code */ +} +.rust .content .header:link, +.rust .content .header:visited { + color: #262625; + pointer: cursor; +} +.rust .content .header:link:hover, +.rust .content .header:visited:hover { + text-decoration: none; +} +.rust .sidebar { + background-color: #3b2e2a; + color: #c8c9db; +} +.rust .chapter li { + color: #505254; +} +.rust .chapter li a { + color: #c8c9db; +} +.rust .chapter li .active, +.rust .chapter li a:hover { +/* Animate color change */ + color: #e69f67; +} +.rust .chapter .spacer { + background-color: #45373a; +} +.rust .menu-bar, +.rust .menu-bar:visited, +.rust .nav-chapters, +.rust .nav-chapters:visited, +.rust .mobile-nav-chapters, +.rust .mobile-nav-chapters:visited { + color: #737480; +} +.rust .menu-bar i:hover, +.rust .nav-chapters:hover, +.rust .mobile-nav-chapters i:hover { + color: #262625; +} +.rust .mobile-nav-chapters i:hover { + color: #c8c9db; +} +.rust .mobile-nav-chapters { + background-color: #3b2e2a; +} +.rust .content a:link, +.rust a:visited { + color: #2b79a2; +} +.rust .theme-popup { + color: #262625; + background: #e1e1db; + border: 1px solid #b38f6b; +} +.rust .theme-popup .theme:hover { + background-color: #99908a; +} +.rust .theme-popup .default { + color: #737480; +} +.rust blockquote { + margin: 20px 0; + padding: 0 20px; + color: #262625; + background-color: #c1c1bb; + border-top: 0.1em solid #b8b8b1; + border-bottom: 0.1em solid #b8b8b1; +} +.rust table td { + border-color: #d7d7cf; +} +.rust table tbody tr:nth-child(2n) { + background: #dbdbd4; +} +.rust table thead { + background: #b3a497; +} +.rust table thead td { + border: none; +} +.rust table thead tr { + border: 1px #b3a497 solid; +} +.rust :not(pre) > .hljs { + display: inline-block; + vertical-align: middle; + padding: 0.1em 0.3em; + -webkit-border-radius: 3px; + border-radius: 3px; +} +.rust pre { + position: relative; +} +.rust pre > .buttons { + position: absolute; + right: 5px; + top: 5px; + color: #c8c9db; + cursor: pointer; +} +.rust pre > .buttons :hover { + color: #e69f67; +} +.rust pre > .buttons i { + margin-left: 8px; +} +.rust pre > .result { + margin-top: 10px; +} + +@media print { + #sidebar { + display: none; + } + #page-wrapper { + left: 0; + overflow-y: initial; + } + #content { + max-width: none; + margin: 0; + padding: 0; + } + #menu-bar { + display: none; + } + .page { + overflow-y: initial; + } + .nav-chapters { + display: none; + } + .mobile-nav-chapters { + display: none; + } +} + +div.footnote-definition p { + display: inline; +} diff --git a/src/doc/reference/src/tokens.md b/src/doc/reference/src/tokens.md new file mode 100644 index 0000000000000..ca6cde8bd2855 --- /dev/null +++ b/src/doc/reference/src/tokens.md @@ -0,0 +1,326 @@ +# Tokens + +Tokens are primitive productions in the grammar defined by regular +(non-recursive) languages. "Simple" tokens are given in [string table +production] form, and occur in the rest of the +grammar as double-quoted strings. Other tokens have exact rules given. + +[string table production]: string-table-productions.html + +## Literals + +A literal is an expression consisting of a single token, rather than a sequence +of tokens, that immediately and directly denotes the value it evaluates to, +rather than referring to it by name or some other evaluation rule. A literal is +a form of constant expression, so is evaluated (primarily) at compile time. + +### Examples + +#### Characters and strings + +| | Example | `#` sets | Characters | Escapes | +|----------------------------------------------|-----------------|------------|-------------|---------------------| +| [Character](#character-literals) | `'H'` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) | +| [String](#string-literals) | `"hello"` | `N/A` | All Unicode | [Quote](#quote-escapes) & [Byte](#byte-escapes) & [Unicode](#unicode-escapes) | +| [Raw](#raw-string-literals) | `r#"hello"#` | `0...` | All Unicode | `N/A` | +| [Byte](#byte-literals) | `b'H'` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) | +| [Byte string](#byte-string-literals) | `b"hello"` | `N/A` | All ASCII | [Quote](#quote-escapes) & [Byte](#byte-escapes) | +| [Raw byte string](#raw-byte-string-literals) | `br#"hello"#` | `0...` | All ASCII | `N/A` | + +#### Byte escapes + +| | Name | +|---|------| +| `\x7F` | 8-bit character code (exactly 2 digits) | +| `\n` | Newline | +| `\r` | Carriage return | +| `\t` | Tab | +| `\\` | Backslash | +| `\0` | Null | + +#### Unicode escapes + +| | Name | +|---|------| +| `\u{7FFF}` | 24-bit Unicode character code (up to 6 digits) | + +#### Quote escapes + +| | Name | +|---|------| +| `\'` | Single quote | +| `\"` | Double quote | + +#### Numbers + +| [Number literals](#number-literals)`*` | Example | Exponentiation | Suffixes | +|----------------------------------------|---------|----------------|----------| +| Decimal integer | `98_222` | `N/A` | Integer suffixes | +| Hex integer | `0xff` | `N/A` | Integer suffixes | +| Octal integer | `0o77` | `N/A` | Integer suffixes | +| Binary integer | `0b1111_0000` | `N/A` | Integer suffixes | +| Floating-point | `123.0E+77` | `Optional` | Floating-point suffixes | + +`*` All number literals allow `_` as a visual separator: `1_234.0E+18f64` + +#### Suffixes + +| Integer | Floating-point | +|---------|----------------| +| `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, `isize`, `usize` | `f32`, `f64` | + +### Character and string literals + +#### Character literals + +A _character literal_ is a single Unicode character enclosed within two +`U+0027` (single-quote) characters, with the exception of `U+0027` itself, +which must be _escaped_ by a preceding `U+005C` character (`\`). + +#### String literals + +A _string literal_ is a sequence of any Unicode characters enclosed within two +`U+0022` (double-quote) characters, with the exception of `U+0022` itself, +which must be _escaped_ by a preceding `U+005C` character (`\`). + +Line-break characters are allowed in string literals. Normally they represent +themselves (i.e. no translation), but as a special exception, when an unescaped +`U+005C` character (`\`) occurs immediately before the newline (`U+000A`), the +`U+005C` character, the newline, and all whitespace at the beginning of the +next line are ignored. Thus `a` and `b` are equal: + +```rust +let a = "foobar"; +let b = "foo\ + bar"; + +assert_eq!(a,b); +``` + +#### Character escapes + +Some additional _escapes_ are available in either character or non-raw string +literals. An escape starts with a `U+005C` (`\`) and continues with one of the +following forms: + +* An _8-bit code point escape_ starts with `U+0078` (`x`) and is + followed by exactly two _hex digits_. It denotes the Unicode code point + equal to the provided hex value. +* A _24-bit code point escape_ starts with `U+0075` (`u`) and is followed + by up to six _hex digits_ surrounded by braces `U+007B` (`{`) and `U+007D` + (`}`). It denotes the Unicode code point equal to the provided hex value. +* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072` + (`r`), or `U+0074` (`t`), denoting the Unicode values `U+000A` (LF), + `U+000D` (CR) or `U+0009` (HT) respectively. +* The _null escape_ is the character `U+0030` (`0`) and denotes the Unicode + value `U+0000` (NUL). +* The _backslash escape_ is the character `U+005C` (`\`) which must be + escaped in order to denote *itself*. + +#### Raw string literals + +Raw string literals do not process any escapes. They start with the character +`U+0072` (`r`), followed by zero or more of the character `U+0023` (`#`) and a +`U+0022` (double-quote) character. The _raw string body_ can contain any sequence +of Unicode characters and is terminated only by another `U+0022` (double-quote) +character, followed by the same number of `U+0023` (`#`) characters that preceded +the opening `U+0022` (double-quote) character. + +All Unicode characters contained in the raw string body represent themselves, +the characters `U+0022` (double-quote) (except when followed by at least as +many `U+0023` (`#`) characters as were used to start the raw string literal) or +`U+005C` (`\`) do not have any special meaning. + +Examples for string literals: + +``` +"foo"; r"foo"; // foo +"\"foo\""; r#""foo""#; // "foo" + +"foo #\"# bar"; +r##"foo #"# bar"##; // foo #"# bar + +"\x52"; "R"; r"R"; // R +"\\x52"; r"\x52"; // \x52 +``` + +### Byte and byte string literals + +#### Byte literals + +A _byte literal_ is a single ASCII character (in the `U+0000` to `U+007F` +range) or a single _escape_ preceded by the characters `U+0062` (`b`) and +`U+0027` (single-quote), and followed by the character `U+0027`. If the character +`U+0027` is present within the literal, it must be _escaped_ by a preceding +`U+005C` (`\`) character. It is equivalent to a `u8` unsigned 8-bit integer +_number literal_. + +#### Byte string literals + +A non-raw _byte string literal_ is a sequence of ASCII characters and _escapes_, +preceded by the characters `U+0062` (`b`) and `U+0022` (double-quote), and +followed by the character `U+0022`. If the character `U+0022` is present within +the literal, it must be _escaped_ by a preceding `U+005C` (`\`) character. +Alternatively, a byte string literal can be a _raw byte string literal_, defined +below. A byte string literal of length `n` is equivalent to a `&'static [u8; n]` borrowed fixed-sized array +of unsigned 8-bit integers. + +Some additional _escapes_ are available in either byte or non-raw byte string +literals. An escape starts with a `U+005C` (`\`) and continues with one of the +following forms: + +* A _byte escape_ escape starts with `U+0078` (`x`) and is + followed by exactly two _hex digits_. It denotes the byte + equal to the provided hex value. +* A _whitespace escape_ is one of the characters `U+006E` (`n`), `U+0072` + (`r`), or `U+0074` (`t`), denoting the bytes values `0x0A` (ASCII LF), + `0x0D` (ASCII CR) or `0x09` (ASCII HT) respectively. +* The _null escape_ is the character `U+0030` (`0`) and denotes the byte + value `0x00` (ASCII NUL). +* The _backslash escape_ is the character `U+005C` (`\`) which must be + escaped in order to denote its ASCII encoding `0x5C`. + +#### Raw byte string literals + +Raw byte string literals do not process any escapes. They start with the +character `U+0062` (`b`), followed by `U+0072` (`r`), followed by zero or more +of the character `U+0023` (`#`), and a `U+0022` (double-quote) character. The +_raw string body_ can contain any sequence of ASCII characters and is terminated +only by another `U+0022` (double-quote) character, followed by the same number of +`U+0023` (`#`) characters that preceded the opening `U+0022` (double-quote) +character. A raw byte string literal can not contain any non-ASCII byte. + +All characters contained in the raw string body represent their ASCII encoding, +the characters `U+0022` (double-quote) (except when followed by at least as +many `U+0023` (`#`) characters as were used to start the raw string literal) or +`U+005C` (`\`) do not have any special meaning. + +Examples for byte string literals: + +``` +b"foo"; br"foo"; // foo +b"\"foo\""; br#""foo""#; // "foo" + +b"foo #\"# bar"; +br##"foo #"# bar"##; // foo #"# bar + +b"\x52"; b"R"; br"R"; // R +b"\\x52"; br"\x52"; // \x52 +``` + +### Number literals + +A _number literal_ is either an _integer literal_ or a _floating-point +literal_. The grammar for recognizing the two kinds of literals is mixed. + +#### Integer literals + +An _integer literal_ has one of four forms: + +* A _decimal literal_ starts with a *decimal digit* and continues with any + mixture of *decimal digits* and _underscores_. +* A _hex literal_ starts with the character sequence `U+0030` `U+0078` + (`0x`) and continues as any mixture of hex digits and underscores. +* An _octal literal_ starts with the character sequence `U+0030` `U+006F` + (`0o`) and continues as any mixture of octal digits and underscores. +* A _binary literal_ starts with the character sequence `U+0030` `U+0062` + (`0b`) and continues as any mixture of binary digits and underscores. + +Like any literal, an integer literal may be followed (immediately, +without any spaces) by an _integer suffix_, which forcibly sets the +type of the literal. The integer suffix must be the name of one of the +integral types: `u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`, `i64`, +`isize`, or `usize`. + +The type of an _unsuffixed_ integer literal is determined by type inference: + +* If an integer type can be _uniquely_ determined from the surrounding + program context, the unsuffixed integer literal has that type. + +* If the program context under-constrains the type, it defaults to the + signed 32-bit integer `i32`. + +* If the program context over-constrains the type, it is considered a + static type error. + +Examples of integer literals of various forms: + +``` +123i32; // type i32 +123u32; // type u32 +123_u32; // type u32 +0xff_u8; // type u8 +0o70_i16; // type i16 +0b1111_1111_1001_0000_i32; // type i32 +0usize; // type usize +``` + +Note that the Rust syntax considers `-1i8` as an application of the [unary minus +operator] to an integer literal `1i8`, rather than +a single integer literal. + +[unary minus operator]: expressions.html#unary-operator-expressions + +#### Floating-point literals + +A _floating-point literal_ has one of two forms: + +* A _decimal literal_ followed by a period character `U+002E` (`.`). This is + optionally followed by another decimal literal, with an optional _exponent_. +* A single _decimal literal_ followed by an _exponent_. + +Like integer literals, a floating-point literal may be followed by a +suffix, so long as the pre-suffix part does not end with `U+002E` (`.`). +The suffix forcibly sets the type of the literal. There are two valid +_floating-point suffixes_, `f32` and `f64` (the 32-bit and 64-bit floating point +types), which explicitly determine the type of the literal. + +The type of an _unsuffixed_ floating-point literal is determined by +type inference: + +* If a floating-point type can be _uniquely_ determined from the + surrounding program context, the unsuffixed floating-point literal + has that type. + +* If the program context under-constrains the type, it defaults to `f64`. + +* If the program context over-constrains the type, it is considered a + static type error. + +Examples of floating-point literals of various forms: + +``` +123.0f64; // type f64 +0.1f64; // type f64 +0.1f32; // type f32 +12E+99_f64; // type f64 +let x: f64 = 2.; // type f64 +``` + +This last example is different because it is not possible to use the suffix +syntax with a floating point literal ending in a period. `2.f64` would attempt +to call a method named `f64` on `2`. + +The representation semantics of floating-point numbers are described in +["Machine Types"]. + +["Machine Types"]: types.html#machine-types + +### Boolean literals + +The two values of the boolean type are written `true` and `false`. + +## Symbols + +Symbols are a general class of printable [tokens] that play structural +roles in a variety of grammar productions. They are a +set of remaining miscellaneous printable tokens that do not +otherwise appear as [unary operators], [binary +operators], or [keywords]. +They are catalogued in [the Symbols section][symbols] of the Grammar document. + +[unary operators]: expressions.html#unary-operator-expressions +[binary operators]: expressions.html#binary-operator-expressions +[tokens]: #tokens +[symbols]: ../grammar.html#symbols +[keywords]: ../grammar.html#keywords \ No newline at end of file diff --git a/src/doc/reference/src/type-coercions.md b/src/doc/reference/src/type-coercions.md new file mode 100644 index 0000000000000..6301e5e83d748 --- /dev/null +++ b/src/doc/reference/src/type-coercions.md @@ -0,0 +1,145 @@ +# Type coercions + +Coercions are defined in [RFC 401]. A coercion is implicit and has no syntax. + +[RFC 401]: https://github.com/rust-lang/rfcs/blob/master/text/0401-coercions.md + +## Coercion sites + +A coercion can only occur at certain coercion sites in a program; these are +typically places where the desired type is explicit or can be derived by +propagation from explicit types (without type inference). Possible coercion +sites are: + +* `let` statements where an explicit type is given. + + For example, `42` is coerced to have type `i8` in the following: + + ```rust + let _: i8 = 42; + ``` + +* `static` and `const` statements (similar to `let` statements). + +* Arguments for function calls + + The value being coerced is the actual parameter, and it is coerced to + the type of the formal parameter. + + For example, `42` is coerced to have type `i8` in the following: + + ```rust + fn bar(_: i8) { } + + fn main() { + bar(42); + } + ``` + +* Instantiations of struct or variant fields + + For example, `42` is coerced to have type `i8` in the following: + + ```rust + struct Foo { x: i8 } + + fn main() { + Foo { x: 42 }; + } + ``` + +* Function results, either the final line of a block if it is not + semicolon-terminated or any expression in a `return` statement + + For example, `42` is coerced to have type `i8` in the following: + + ```rust + fn foo() -> i8 { + 42 + } + ``` + +If the expression in one of these coercion sites is a coercion-propagating +expression, then the relevant sub-expressions in that expression are also +coercion sites. Propagation recurses from these new coercion sites. +Propagating expressions and their relevant sub-expressions are: + +* Array literals, where the array has type `[U; n]`. Each sub-expression in +the array literal is a coercion site for coercion to type `U`. + +* Array literals with repeating syntax, where the array has type `[U; n]`. The +repeated sub-expression is a coercion site for coercion to type `U`. + +* Tuples, where a tuple is a coercion site to type `(U_0, U_1, ..., U_n)`. +Each sub-expression is a coercion site to the respective type, e.g. the +zeroth sub-expression is a coercion site to type `U_0`. + +* Parenthesized sub-expressions (`(e)`): if the expression has type `U`, then +the sub-expression is a coercion site to `U`. + +* Blocks: if a block has type `U`, then the last expression in the block (if +it is not semicolon-terminated) is a coercion site to `U`. This includes +blocks which are part of control flow statements, such as `if`/`else`, if +the block has a known type. + +## Coercion types + +Coercion is allowed between the following types: + +* `T` to `U` if `T` is a subtype of `U` (*reflexive case*) + +* `T_1` to `T_3` where `T_1` coerces to `T_2` and `T_2` coerces to `T_3` +(*transitive case*) + + Note that this is not fully supported yet + +* `&mut T` to `&T` + +* `*mut T` to `*const T` + +* `&T` to `*const T` + +* `&mut T` to `*mut T` + +* `&T` to `&U` if `T` implements `Deref`. For example: + + ```rust + use std::ops::Deref; + + struct CharContainer { + value: char, + } + + impl Deref for CharContainer { + type Target = char; + + fn deref<'a>(&'a self) -> &'a char { + &self.value + } + } + + fn foo(arg: &char) {} + + fn main() { + let x = &mut CharContainer { value: 'y' }; + foo(x); //&mut CharContainer is coerced to &char. + } + ``` + +* `&mut T` to `&mut U` if `T` implements `DerefMut`. + +* TyCtor(`T`) to TyCtor(coerce_inner(`T`)), where TyCtor(`T`) is one of + - `&T` + - `&mut T` + - `*const T` + - `*mut T` + - `Box` + + and where + - coerce_inner(`[T, ..n]`) = `[T]` + - coerce_inner(`T`) = `U` where `T` is a concrete type which implements the + trait `U`. + + In the future, coerce_inner will be recursively extended to tuples and + structs. In addition, coercions from sub-traits to super-traits will be + added. See [RFC 401] for more details. diff --git a/src/doc/reference/src/type-system.md b/src/doc/reference/src/type-system.md new file mode 100644 index 0000000000000..bed7f128e5704 --- /dev/null +++ b/src/doc/reference/src/type-system.md @@ -0,0 +1 @@ +# Type system diff --git a/src/doc/reference/src/types.md b/src/doc/reference/src/types.md new file mode 100644 index 0000000000000..2ddcba177e35d --- /dev/null +++ b/src/doc/reference/src/types.md @@ -0,0 +1,398 @@ +# Types + +Every variable, item and value in a Rust program has a type. The _type_ of a +*value* defines the interpretation of the memory holding it. + +Built-in types and type-constructors are tightly integrated into the language, +in nontrivial ways that are not possible to emulate in user-defined types. +User-defined types have limited capabilities. + +## Primitive types + +The primitive types are the following: + +* The boolean type `bool` with values `true` and `false`. +* The machine types (integer and floating-point). +* The machine-dependent integer types. +* Arrays +* Tuples +* Slices +* Function pointers + +### Machine types + +The machine types are the following: + +* The unsigned word types `u8`, `u16`, `u32` and `u64`, with values drawn from + the integer intervals [0, 2^8 - 1], [0, 2^16 - 1], [0, 2^32 - 1] and + [0, 2^64 - 1] respectively. + +* The signed two's complement word types `i8`, `i16`, `i32` and `i64`, with + values drawn from the integer intervals [-(2^(7)), 2^7 - 1], + [-(2^(15)), 2^15 - 1], [-(2^(31)), 2^31 - 1], [-(2^(63)), 2^63 - 1] + respectively. + +* The IEEE 754-2008 `binary32` and `binary64` floating-point types: `f32` and + `f64`, respectively. + +### Machine-dependent integer types + +The `usize` type is an unsigned integer type with the same number of bits as the +platform's pointer type. It can represent every memory address in the process. + +The `isize` type is a signed integer type with the same number of bits as the +platform's pointer type. The theoretical upper bound on object and array size +is the maximum `isize` value. This ensures that `isize` can be used to calculate +differences between pointers into an object or array and can address every byte +within an object along with one byte past the end. + +## Textual types + +The types `char` and `str` hold textual data. + +A value of type `char` is a [Unicode scalar value]( +http://www.unicode.org/glossary/#unicode_scalar_value) (i.e. a code point that +is not a surrogate), represented as a 32-bit unsigned word in the 0x0000 to +0xD7FF or 0xE000 to 0x10FFFF range. A `[char]` array is effectively an UCS-4 / +UTF-32 string. + +A value of type `str` is a Unicode string, represented as an array of 8-bit +unsigned bytes holding a sequence of UTF-8 code points. Since `str` is of +unknown size, it is not a _first-class_ type, but can only be instantiated +through a pointer type, such as `&str`. + +## Tuple types + +A tuple *type* is a heterogeneous product of other types, called the *elements* +of the tuple. It has no nominal name and is instead structurally typed. + +Tuple types and values are denoted by listing the types or values of their +elements, respectively, in a parenthesized, comma-separated list. + +Because tuple elements don't have a name, they can only be accessed by +pattern-matching or by using `N` directly as a field to access the +`N`th element. + +An example of a tuple type and its use: + +``` +type Pair<'a> = (i32, &'a str); +let p: Pair<'static> = (10, "ten"); +let (a, b) = p; + +assert_eq!(a, 10); +assert_eq!(b, "ten"); +assert_eq!(p.0, 10); +assert_eq!(p.1, "ten"); +``` + +For historical reasons and convenience, the tuple type with no elements (`()`) +is often called ‘unit’ or ‘the unit type’. + +## Array, and Slice types + +Rust has two different types for a list of items: + +* `[T; N]`, an 'array' +* `&[T]`, a 'slice' + +An array has a fixed size, and can be allocated on either the stack or the +heap. + +A slice is a 'view' into an array. It doesn't own the data it points +to, it borrows it. + +Examples: + +```{rust} +// A stack-allocated array +let array: [i32; 3] = [1, 2, 3]; + +// A heap-allocated array +let vector: Vec = vec![1, 2, 3]; + +// A slice into an array +let slice: &[i32] = &vector[..]; +``` + +As you can see, the `vec!` macro allows you to create a `Vec` easily. The +`vec!` macro is also part of the standard library, rather than the language. + +All in-bounds elements of arrays and slices are always initialized, and access +to an array or slice is always bounds-checked. + +## Struct types + +A `struct` *type* is a heterogeneous product of other types, called the +*fields* of the type.[^structtype] + +[^structtype]: `struct` types are analogous to `struct` types in C, + the *record* types of the ML family, + or the *struct* types of the Lisp family. + +New instances of a `struct` can be constructed with a [struct +expression](expressions.html#struct-expressions). + +The memory layout of a `struct` is undefined by default to allow for compiler +optimizations like field reordering, but it can be fixed with the +`#[repr(...)]` attribute. In either case, fields may be given in any order in +a corresponding struct *expression*; the resulting `struct` value will always +have the same memory layout. + +The fields of a `struct` may be qualified by [visibility +modifiers](visibility-and-privacy.html), to allow access to data in a +struct outside a module. + +A _tuple struct_ type is just like a struct type, except that the fields are +anonymous. + +A _unit-like struct_ type is like a struct type, except that it has no +fields. The one value constructed by the associated [struct +expression](expressions.html#struct-expressions) is the only value that inhabits such a +type. + +## Enumerated types + +An *enumerated type* is a nominal, heterogeneous disjoint union type, denoted +by the name of an [`enum` item](items.html#enumerations). [^enumtype] + +[^enumtype]: The `enum` type is analogous to a `data` constructor declaration in + ML, or a *pick ADT* in Limbo. + +An [`enum` item](items.html#enumerations) declares both the type and a number of *variant +constructors*, each of which is independently named and takes an optional tuple +of arguments. + +New instances of an `enum` can be constructed by calling one of the variant +constructors, in a [call expression](expressions.html#call-expressions). + +Any `enum` value consumes as much memory as the largest variant constructor for +its corresponding `enum` type. + +Enum types cannot be denoted *structurally* as types, but must be denoted by +named reference to an [`enum` item](items.html#enumerations). + +## Recursive types + +Nominal types — [enumerations](#enumerated-types) and +[structs](#struct-types) — may be recursive. That is, each `enum` +constructor or `struct` field may refer, directly or indirectly, to the +enclosing `enum` or `struct` type itself. Such recursion has restrictions: + +* Recursive types must include a nominal type in the recursion + (not mere [type definitions](../grammar.html#type-definitions), + or other structural types such as [arrays](#array-and-slice-types) or [tuples](#tuple-types)). +* A recursive `enum` item must have at least one non-recursive constructor + (in order to give the recursion a basis case). +* The size of a recursive type must be finite; + in other words the recursive fields of the type must be [pointer types](#pointer-types). +* Recursive type definitions can cross module boundaries, but not module *visibility* boundaries, + or crate boundaries (in order to simplify the module system and type checker). + +An example of a *recursive* type and its use: + +``` +enum List { + Nil, + Cons(T, Box>) +} + +let a: List = List::Cons(7, Box::new(List::Cons(13, Box::new(List::Nil)))); +``` + +## Pointer types + +All pointers in Rust are explicit first-class values. They can be copied, +stored into data structs, and returned from functions. There are two +varieties of pointer in Rust: + +* References (`&`) + : These point to memory _owned by some other value_. + A reference type is written `&type`, + or `&'a type` when you need to specify an explicit lifetime. + Copying a reference is a "shallow" operation: + it involves only copying the pointer itself. + Releasing a reference has no effect on the value it points to, + but a reference of a temporary value will keep it alive during the scope + of the reference itself. + +* Raw pointers (`*`) + : Raw pointers are pointers without safety or liveness guarantees. + Raw pointers are written as `*const T` or `*mut T`, + for example `*const i32` means a raw pointer to a 32-bit integer. + Copying or dropping a raw pointer has no effect on the lifecycle of any + other value. Dereferencing a raw pointer or converting it to any other + pointer type is an [`unsafe` operation](unsafe-functions.html). + Raw pointers are generally discouraged in Rust code; + they exist to support interoperability with foreign code, + and writing performance-critical or low-level functions. + +The standard library contains additional 'smart pointer' types beyond references +and raw pointers. + +## Function types + +The function type constructor `fn` forms new function types. A function type +consists of a possibly-empty set of function-type modifiers (such as `unsafe` +or `extern`), a sequence of input types and an output type. + +An example of a `fn` type: + +``` +fn add(x: i32, y: i32) -> i32 { + x + y +} + +let mut x = add(5,7); + +type Binop = fn(i32, i32) -> i32; +let bo: Binop = add; +x = bo(5,7); +``` + +### Function types for specific items + +Internal to the compiler, there are also function types that are specific to a particular +function item. In the following snippet, for example, the internal types of the functions +`foo` and `bar` are different, despite the fact that they have the same signature: + +``` +fn foo() { } +fn bar() { } +``` + +The types of `foo` and `bar` can both be implicitly coerced to the fn +pointer type `fn()`. There is currently no syntax for unique fn types, +though the compiler will emit a type like `fn() {foo}` in error +messages to indicate "the unique fn type for the function `foo`". + +## Closure types + +A [lambda expression](expressions.html#lambda-expressions) produces a closure +value with a unique, anonymous type that cannot be written out. + +Depending on the requirements of the closure, its type implements one or +more of the closure traits: + +* `FnOnce` + : The closure can be called once. A closure called as `FnOnce` + can move out values from its environment. + +* `FnMut` + : The closure can be called multiple times as mutable. A closure called as + `FnMut` can mutate values from its environment. `FnMut` inherits from + `FnOnce` (i.e. anything implementing `FnMut` also implements `FnOnce`). + +* `Fn` + : The closure can be called multiple times through a shared reference. + A closure called as `Fn` can neither move out from nor mutate values + from its environment. `Fn` inherits from `FnMut`, which itself + inherits from `FnOnce`. + + +## Trait objects + +In Rust, a type like `&SomeTrait` or `Box` is called a _trait object_. +Each instance of a trait object includes: + + - a pointer to an instance of a type `T` that implements `SomeTrait` + - a _virtual method table_, often just called a _vtable_, which contains, for + each method of `SomeTrait` that `T` implements, a pointer to `T`'s + implementation (i.e. a function pointer). + +The purpose of trait objects is to permit "late binding" of methods. Calling a +method on a trait object results in virtual dispatch at runtime: that is, a +function pointer is loaded from the trait object vtable and invoked indirectly. +The actual implementation for each vtable entry can vary on an object-by-object +basis. + +Note that for a trait object to be instantiated, the trait must be +_object-safe_. Object safety rules are defined in [RFC 255]. + +[RFC 255]: https://github.com/rust-lang/rfcs/blob/master/text/0255-object-safety.md + +Given a pointer-typed expression `E` of type `&T` or `Box`, where `T` +implements trait `R`, casting `E` to the corresponding pointer type `&R` or +`Box` results in a value of the _trait object_ `R`. This result is +represented as a pair of pointers: the vtable pointer for the `T` +implementation of `R`, and the pointer value of `E`. + +An example of a trait object: + +``` +trait Printable { + fn stringify(&self) -> String; +} + +impl Printable for i32 { + fn stringify(&self) -> String { self.to_string() } +} + +fn print(a: Box) { + println!("{}", a.stringify()); +} + +fn main() { + print(Box::new(10) as Box); +} +``` + +In this example, the trait `Printable` occurs as a trait object in both the +type signature of `print`, and the cast expression in `main`. + +### Type parameters + +Within the body of an item that has type parameter declarations, the names of +its type parameters are types: + +```ignore +fn to_vec(xs: &[A]) -> Vec { + if xs.is_empty() { + return vec![]; + } + let first: A = xs[0].clone(); + let mut rest: Vec = to_vec(&xs[1..]); + rest.insert(0, first); + rest +} +``` + +Here, `first` has type `A`, referring to `to_vec`'s `A` type parameter; and `rest` +has type `Vec`, a vector with element type `A`. + +## Self types + +The special type `Self` has a meaning within traits and impls. In a trait definition, it refers +to an implicit type parameter representing the "implementing" type. In an impl, +it is an alias for the implementing type. For example, in: + +``` +pub trait From { + fn from(T) -> Self; +} + +impl From for String { + fn from(x: i32) -> Self { + x.to_string() + } +} +``` + +The notation `Self` in the impl refers to the implementing type: `String`. In another +example: + +``` +trait Printable { + fn make_string(&self) -> String; +} + +impl Printable for String { + fn make_string(&self) -> String { + (*self).clone() + } +} +``` + +The notation `&self` is a shorthand for `self: &Self`. In this case, +in the impl, `Self` refers to the value of type `String` that is the +receiver for a call to the method `make_string`. diff --git a/src/doc/reference/src/unicode-productions.md b/src/doc/reference/src/unicode-productions.md new file mode 100644 index 0000000000000..f9d6d1d59732d --- /dev/null +++ b/src/doc/reference/src/unicode-productions.md @@ -0,0 +1,9 @@ +# Unicode productions + +A few productions in Rust's grammar permit Unicode code points outside the +ASCII range. We define these productions in terms of character properties +specified in the Unicode standard, rather than in terms of ASCII-range code +points. The grammar has a [Special Unicode Productions][unicodeproductions] +section that lists these productions. + +[unicodeproductions]: ../grammar.html#special-unicode-productions diff --git a/src/doc/reference/src/unsafe-blocks.md b/src/doc/reference/src/unsafe-blocks.md new file mode 100644 index 0000000000000..754278445d51e --- /dev/null +++ b/src/doc/reference/src/unsafe-blocks.md @@ -0,0 +1,22 @@ +# Unsafe blocks + +A block of code can be prefixed with the `unsafe` keyword, to permit calling +`unsafe` functions or dereferencing raw pointers within a safe function. + +When a programmer has sufficient conviction that a sequence of potentially +unsafe operations is actually safe, they can encapsulate that sequence (taken +as a whole) within an `unsafe` block. The compiler will consider uses of such +code safe, in the surrounding context. + +Unsafe blocks are used to wrap foreign libraries, make direct use of hardware +or implement features not directly present in the language. For example, Rust +provides the language features necessary to implement memory-safe concurrency +in the language but the implementation of threads and message passing is in the +standard library. + +Rust's type system is a conservative approximation of the dynamic safety +requirements, so in some cases there is a performance cost to using safe code. +For example, a doubly-linked list is not a tree structure and can only be +represented with reference-counted pointers in safe code. By using `unsafe` +blocks to represent the reverse links as raw pointers, it can be implemented +with only boxes. diff --git a/src/doc/reference/src/unsafe-functions.md b/src/doc/reference/src/unsafe-functions.md new file mode 100644 index 0000000000000..7a5064c08f41a --- /dev/null +++ b/src/doc/reference/src/unsafe-functions.md @@ -0,0 +1,5 @@ +# Unsafe functions + +Unsafe functions are functions that are not safe in all contexts and/or for all +possible inputs. Such a function must be prefixed with the keyword `unsafe` and +can only be called from an `unsafe` block or another `unsafe` function. diff --git a/src/doc/reference/src/unsafety.md b/src/doc/reference/src/unsafety.md new file mode 100644 index 0000000000000..abb7a9eec5848 --- /dev/null +++ b/src/doc/reference/src/unsafety.md @@ -0,0 +1,11 @@ +# Unsafety + +Unsafe operations are those that potentially violate the memory-safety +guarantees of Rust's static semantics. + +The following language level features cannot be used in the safe subset of +Rust: + +- Dereferencing a [raw pointer](types.html#pointer-types). +- Reading or writing a [mutable static variable](items.html#mutable-statics). +- Calling an unsafe function (including an intrinsic or foreign function). diff --git a/src/doc/reference/src/variables.md b/src/doc/reference/src/variables.md new file mode 100644 index 0000000000000..ce3d226d0238b --- /dev/null +++ b/src/doc/reference/src/variables.md @@ -0,0 +1,31 @@ +# Variables + +A _variable_ is a component of a stack frame, either a named function parameter, +an anonymous [temporary](expressions.html#lvalues-rvalues-and-temporaries), or a named local +variable. + +A _local variable_ (or *stack-local* allocation) holds a value directly, +allocated within the stack's memory. The value is a part of the stack frame. + +Local variables are immutable unless declared otherwise like: `let mut x = ...`. + +Function parameters are immutable unless declared with `mut`. The `mut` keyword +applies only to the following parameter (so `|mut x, y|` and `fn f(mut x: +Box, y: Box)` declare one mutable variable `x` and one immutable +variable `y`). + +Methods that take either `self` or `Box` can optionally place them in a +mutable variable by prefixing them with `mut` (similar to regular arguments): + +``` +trait Changer: Sized { + fn change(mut self) {} + fn modify(mut self: Box) {} +} +``` + +Local variables are not initialized when allocated; the entire frame worth of +local variables are allocated at once, on frame-entry, in an uninitialized +state. Subsequent statements within a function may or may not initialize the +local variables. Local variables can be used only after they have been +initialized; this is enforced by the compiler. diff --git a/src/doc/reference/src/visibility-and-privacy.md b/src/doc/reference/src/visibility-and-privacy.md new file mode 100644 index 0000000000000..50d3e7507d0ed --- /dev/null +++ b/src/doc/reference/src/visibility-and-privacy.md @@ -0,0 +1,160 @@ +# Visibility and Privacy + +These two terms are often used interchangeably, and what they are attempting to +convey is the answer to the question "Can this item be used at this location?" + +Rust's name resolution operates on a global hierarchy of namespaces. Each level +in the hierarchy can be thought of as some item. The items are one of those +mentioned above, but also include external crates. Declaring or defining a new +module can be thought of as inserting a new tree into the hierarchy at the +location of the definition. + +To control whether interfaces can be used across modules, Rust checks each use +of an item to see whether it should be allowed or not. This is where privacy +warnings are generated, or otherwise "you used a private item of another module +and weren't allowed to." + +By default, everything in Rust is *private*, with two exceptions: Associated +items in a `pub` Trait are public by default; Enum variants +in a `pub` enum are also public by default. When an item is declared as `pub`, +it can be thought of as being accessible to the outside world. For example: + +``` +# fn main() {} +// Declare a private struct +struct Foo; + +// Declare a public struct with a private field +pub struct Bar { + field: i32, +} + +// Declare a public enum with two public variants +pub enum State { + PubliclyAccessibleState, + PubliclyAccessibleState2, +} +``` + +With the notion of an item being either public or private, Rust allows item +accesses in two cases: + +1. If an item is public, then it can be used externally through any of its + public ancestors. +2. If an item is private, it may be accessed by the current module and its + descendants. + +These two cases are surprisingly powerful for creating module hierarchies +exposing public APIs while hiding internal implementation details. To help +explain, here's a few use cases and what they would entail: + +* A library developer needs to expose functionality to crates which link + against their library. As a consequence of the first case, this means that + anything which is usable externally must be `pub` from the root down to the + destination item. Any private item in the chain will disallow external + accesses. + +* A crate needs a global available "helper module" to itself, but it doesn't + want to expose the helper module as a public API. To accomplish this, the + root of the crate's hierarchy would have a private module which then + internally has a "public API". Because the entire crate is a descendant of + the root, then the entire local crate can access this private module through + the second case. + +* When writing unit tests for a module, it's often a common idiom to have an + immediate child of the module to-be-tested named `mod test`. This module + could access any items of the parent module through the second case, meaning + that internal implementation details could also be seamlessly tested from the + child module. + +In the second case, it mentions that a private item "can be accessed" by the +current module and its descendants, but the exact meaning of accessing an item +depends on what the item is. Accessing a module, for example, would mean +looking inside of it (to import more items). On the other hand, accessing a +function would mean that it is invoked. Additionally, path expressions and +import statements are considered to access an item in the sense that the +import/expression is only valid if the destination is in the current visibility +scope. + +Here's an example of a program which exemplifies the three cases outlined +above: + +``` +// This module is private, meaning that no external crate can access this +// module. Because it is private at the root of this current crate, however, any +// module in the crate may access any publicly visible item in this module. +mod crate_helper_module { + + // This function can be used by anything in the current crate + pub fn crate_helper() {} + + // This function *cannot* be used by anything else in the crate. It is not + // publicly visible outside of the `crate_helper_module`, so only this + // current module and its descendants may access it. + fn implementation_detail() {} +} + +// This function is "public to the root" meaning that it's available to external +// crates linking against this one. +pub fn public_api() {} + +// Similarly to 'public_api', this module is public so external crates may look +// inside of it. +pub mod submodule { + use crate_helper_module; + + pub fn my_method() { + // Any item in the local crate may invoke the helper module's public + // interface through a combination of the two rules above. + crate_helper_module::crate_helper(); + } + + // This function is hidden to any module which is not a descendant of + // `submodule` + fn my_implementation() {} + + #[cfg(test)] + mod test { + + #[test] + fn test_my_implementation() { + // Because this module is a descendant of `submodule`, it's allowed + // to access private items inside of `submodule` without a privacy + // violation. + super::my_implementation(); + } + } +} + +# fn main() {} +``` + +For a Rust program to pass the privacy checking pass, all paths must be valid +accesses given the two rules above. This includes all use statements, +expressions, types, etc. + +## Re-exporting and Visibility + +Rust allows publicly re-exporting items through a `pub use` directive. Because +this is a public directive, this allows the item to be used in the current +module through the rules above. It essentially allows public access into the +re-exported item. For example, this program is valid: + +``` +pub use self::implementation::api; + +mod implementation { + pub mod api { + pub fn f() {} + } +} + +# fn main() {} +``` + +This means that any external crate referencing `implementation::api::f` would +receive a privacy violation, while the path `api::f` would be allowed. + +When re-exporting a private item, it can be thought of as allowing the "privacy +chain" being short-circuited through the reexport instead of passing through +the namespace hierarchy as it normally would. diff --git a/src/doc/reference/src/whitespace.md b/src/doc/reference/src/whitespace.md new file mode 100644 index 0000000000000..2fd162bcb2da8 --- /dev/null +++ b/src/doc/reference/src/whitespace.md @@ -0,0 +1,22 @@ +# Whitespace + +Whitespace is any non-empty string containing only characters that have the +`Pattern_White_Space` Unicode property, namely: + +- `U+0009` (horizontal tab, `'\t'`) +- `U+000A` (line feed, `'\n'`) +- `U+000B` (vertical tab) +- `U+000C` (form feed) +- `U+000D` (carriage return, `'\r'`) +- `U+0020` (space, `' '`) +- `U+0085` (next line) +- `U+200E` (left-to-right mark) +- `U+200F` (right-to-left mark) +- `U+2028` (line separator) +- `U+2029` (paragraph separator) + +Rust is a "free-form" language, meaning that all forms of whitespace serve only +to separate _tokens_ in the grammar, and have no semantic significance. + +A Rust program has identical meaning if each whitespace element is replaced +with any other legal whitespace element, such as a single space character. diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs index 31a0cc6884184..12410c08f399b 100644 --- a/src/libcore/intrinsics.rs +++ b/src/libcore/intrinsics.rs @@ -687,7 +687,7 @@ extern "rust-intrinsic" { /// The [nomicon](../../nomicon/transmutes.html) has additional /// documentation. /// - /// [ub]: ../../reference.html#behavior-considered-undefined + /// [ub]: ../../reference/behavior-considered-undefined.html /// /// # Examples /// diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs index ede22ccddc62f..1e9eaaf5f3223 100644 --- a/src/libcore/marker.rs +++ b/src/libcore/marker.rs @@ -36,7 +36,7 @@ use hash::Hasher; /// /// [`Rc`]: ../../std/rc/struct.Rc.html /// [arc]: ../../std/sync/struct.Arc.html -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html #[stable(feature = "rust1", since = "1.0.0")] #[lang = "send"] #[rustc_on_unimplemented = "`{Self}` cannot be sent between threads safely"] @@ -338,7 +338,7 @@ pub trait Copy : Clone { /// [mutex]: ../../std/sync/struct.Mutex.html /// [rwlock]: ../../std/sync/struct.RwLock.html /// [unsafecell]: ../cell/struct.UnsafeCell.html -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html /// [transmute]: ../../std/mem/fn.transmute.html #[stable(feature = "rust1", since = "1.0.0")] #[lang = "sync"] diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs index 209107ef92ceb..f4ce4697d7cf4 100644 --- a/src/libcore/mem.rs +++ b/src/libcore/mem.rs @@ -167,7 +167,7 @@ pub use intrinsics::transmute; /// [FFI]: ../../book/ffi.html /// [box]: ../../std/boxed/struct.Box.html /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html #[inline] #[stable(feature = "rust1", since = "1.0.0")] pub fn forget(t: T) { @@ -318,7 +318,7 @@ pub fn align_of_val(val: &T) -> usize { /// /// [uninit]: fn.uninitialized.html /// [FFI]: ../../book/ffi.html -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html /// /// # Examples /// @@ -417,7 +417,7 @@ pub unsafe fn zeroed() -> T { /// [`Vec`]: ../../std/vec/struct.Vec.html /// [`vec!`]: ../../std/macro.vec.html /// [`Clone`]: ../../std/clone/trait.Clone.html -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html /// [write]: ../ptr/fn.write.html /// [copy]: ../intrinsics/fn.copy.html /// [copy_no]: ../intrinsics/fn.copy_nonoverlapping.html @@ -626,7 +626,7 @@ pub fn drop(_x: T) { } /// same size. This function triggers [undefined behavior][ub] if `U` is larger than /// `T`. /// -/// [ub]: ../../reference.html#behavior-considered-undefined +/// [ub]: ../../reference/behavior-considered-undefined.html /// [size_of]: fn.size_of.html /// /// # Examples diff --git a/src/libstd/f32.rs b/src/libstd/f32.rs index 7a676c041ad89..544f4f9ddbed3 100644 --- a/src/libstd/f32.rs +++ b/src/libstd/f32.rs @@ -264,7 +264,7 @@ impl f32 { /// /// assert!(abs_difference <= f32::EPSILON); /// ``` - /// [floating-point]: ../reference.html#machine-types + /// [floating-point]: ../reference/types.html#machine-types #[unstable(feature = "float_extras", reason = "signature is undecided", issue = "27752")] #[rustc_deprecated(since = "1.11.0", diff --git a/src/libstd/f64.rs b/src/libstd/f64.rs index 67a1c302483d2..dd4bc253bed4b 100644 --- a/src/libstd/f64.rs +++ b/src/libstd/f64.rs @@ -206,7 +206,7 @@ impl f64 { /// /// assert!(abs_difference < 1e-10); /// ``` - /// [floating-point]: ../reference.html#machine-types + /// [floating-point]: ../reference/types.html#machine-types #[unstable(feature = "float_extras", reason = "signature is undecided", issue = "27752")] #[rustc_deprecated(since = "1.11.0", diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs index d79a9a202d9e4..a1f092621cb44 100644 --- a/src/libstd/macros.rs +++ b/src/libstd/macros.rs @@ -441,7 +441,7 @@ pub mod builtin { /// leads to less duplicated code. /// /// The syntax given to this macro is the same syntax as [the `cfg` - /// attribute](../reference.html#conditional-compilation). + /// attribute](../book/conditional-compilation.html). /// /// # Examples ///