diff --git a/src/librustc_error_codes/error_codes.rs b/src/librustc_error_codes/error_codes.rs index 428cecf13a3b0..9fc375cc7b044 100644 --- a/src/librustc_error_codes/error_codes.rs +++ b/src/librustc_error_codes/error_codes.rs @@ -318,6 +318,7 @@ E0590: include_str!("./error_codes/E0590.md"), E0591: include_str!("./error_codes/E0591.md"), E0592: include_str!("./error_codes/E0592.md"), E0593: include_str!("./error_codes/E0593.md"), +E0594: include_str!("./error_codes/E0594.md"), E0595: include_str!("./error_codes/E0595.md"), E0596: include_str!("./error_codes/E0596.md"), E0597: include_str!("./error_codes/E0597.md"), @@ -566,7 +567,6 @@ E0744: include_str!("./error_codes/E0744.md"), // E0563, // cannot determine a type for this `impl Trait` removed in 6383de15 // E0564, // only named lifetimes are allowed in `impl Trait`, // but `{}` was found in the type `{}` - E0594, // cannot assign to {} // E0598, // lifetime of {} is too short to guarantee its contents can be... // E0611, // merged into E0616 // E0612, // merged into E0609 diff --git a/src/librustc_error_codes/error_codes/E0594.md b/src/librustc_error_codes/error_codes/E0594.md new file mode 100644 index 0000000000000..ad8eb631e63b1 --- /dev/null +++ b/src/librustc_error_codes/error_codes/E0594.md @@ -0,0 +1,23 @@ +A non-mutable value was assigned a value. + +Erroneous code example: + +```compile_fail,E0594 +struct SolarSystem { + earth: i32, +} + +let ss = SolarSystem { earth: 3 }; +ss.earth = 2; // error! +``` + +To fix this error, declare `ss` as mutable by using the `mut` keyword: + +``` +struct SolarSystem { + earth: i32, +} + +let mut ss = SolarSystem { earth: 3 }; // declaring `ss` as mutable +ss.earth = 2; // ok! +``` diff --git a/src/librustc_mir/error_codes.rs b/src/librustc_mir/error_codes.rs deleted file mode 100644 index 5e9bf4be75ead..0000000000000 --- a/src/librustc_mir/error_codes.rs +++ /dev/null @@ -1,2577 +0,0 @@ -syntax::register_diagnostics! { - - -E0001: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error suggests that the expression arm corresponding to the noted pattern -will never be reached as for all possible values of the expression being -matched, one of the preceding patterns will match. - -This means that perhaps some of the preceding patterns are too general, this -one is too specific or the ordering is incorrect. - -For example, the following `match` block has too many arms: - -``` -match Some(0) { - Some(bar) => {/* ... */} - x => {/* ... */} // This handles the `None` case - _ => {/* ... */} // All possible cases have already been handled -} -``` - -`match` blocks have their patterns matched in order, so, for example, putting -a wildcard arm above a more specific arm will make the latter arm irrelevant. - -Ensure the ordering of the match arm is correct and remove any superfluous -arms. -"##, - -E0002: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error indicates that an empty match expression is invalid because the type -it is matching on is non-empty (there exist values of this type). In safe code -it is impossible to create an instance of an empty type, so empty match -expressions are almost never desired. This error is typically fixed by adding -one or more cases to the match expression. - -An example of an empty type is `enum Empty { }`. So, the following will work: - -``` -enum Empty {} - -fn foo(x: Empty) { - match x { - // empty - } -} -``` - -However, this won't: - -```compile_fail -fn foo(x: Option) { - match x { - // empty - } -} -``` -"##, - -E0004: r##" -This error indicates that the compiler cannot guarantee a matching pattern for -one or more possible inputs to a match expression. Guaranteed matches are -required in order to assign values to match expressions, or alternatively, -determine the flow of execution. - -Erroneous code example: - -```compile_fail,E0004 -enum Terminator { - HastaLaVistaBaby, - TalkToMyHand, -} - -let x = Terminator::HastaLaVistaBaby; - -match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered - Terminator::TalkToMyHand => {} -} -``` - -If you encounter this error you must alter your patterns so that every possible -value of the input type is matched. For types with a small number of variants -(like enums) you should probably cover all cases explicitly. Alternatively, the -underscore `_` wildcard pattern can be added after all other patterns to match -"anything else". Example: - -``` -enum Terminator { - HastaLaVistaBaby, - TalkToMyHand, -} - -let x = Terminator::HastaLaVistaBaby; - -match x { - Terminator::TalkToMyHand => {} - Terminator::HastaLaVistaBaby => {} -} - -// or: - -match x { - Terminator::TalkToMyHand => {} - _ => {} -} -``` -"##, - -E0005: r##" -Patterns used to bind names must be irrefutable, that is, they must guarantee -that a name will be extracted in all cases. - -Erroneous code example: - -```compile_fail,E0005 -let x = Some(1); -let Some(y) = x; -// error: refutable pattern in local binding: `None` not covered -``` - -If you encounter this error you probably need to use a `match` or `if let` to -deal with the possibility of failure. Example: - -``` -let x = Some(1); - -match x { - Some(y) => { - // do something - }, - None => {} -} - -// or: - -if let Some(y) = x { - // do something -} -``` -"##, - -E0007: r##" -This error indicates that the bindings in a match arm would require a value to -be moved into more than one location, thus violating unique ownership. Code -like the following is invalid as it requires the entire `Option` to be -moved into a variable called `op_string` while simultaneously requiring the -inner `String` to be moved into a variable called `s`. - -Erroneous code example: - -```compile_fail,E0007 -let x = Some("s".to_string()); - -match x { - op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings - None => {}, -} -``` - -See also the error E0303. -"##, - -E0009: r##" -In a pattern, all values that don't implement the `Copy` trait have to be bound -the same way. The goal here is to avoid binding simultaneously by-move and -by-ref. - -This limitation may be removed in a future version of Rust. - -Erroneous code example: - -```compile_fail,E0009 -struct X { x: (), } - -let x = Some((X { x: () }, X { x: () })); -match x { - Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the - // same pattern - None => panic!() -} -``` - -You have two solutions: - -Solution #1: Bind the pattern's values the same way. - -``` -struct X { x: (), } - -let x = Some((X { x: () }, X { x: () })); -match x { - Some((ref y, ref z)) => {}, - // or Some((y, z)) => {} - None => panic!() -} -``` - -Solution #2: Implement the `Copy` trait for the `X` structure. - -However, please keep in mind that the first solution should be preferred. - -``` -#[derive(Clone, Copy)] -struct X { x: (), } - -let x = Some((X { x: () }, X { x: () })); -match x { - Some((y, ref z)) => {}, - None => panic!() -} -``` -"##, - -E0010: r##" -The value of statics and constants must be known at compile time, and they live -for the entire lifetime of a program. Creating a boxed value allocates memory on -the heap at runtime, and therefore cannot be done at compile time. - -Erroneous code example: - -```compile_fail,E0010 -#![feature(box_syntax)] - -const CON : Box = box 0; -``` -"##, - -E0013: r##" -Static and const variables can refer to other const variables. But a const -variable cannot refer to a static variable. - -Erroneous code example: - -```compile_fail,E0013 -static X: i32 = 42; -const Y: i32 = X; -``` - -In this example, `Y` cannot refer to `X` here. To fix this, the value can be -extracted as a const and then used: - -``` -const A: i32 = 42; -static X: i32 = A; -const Y: i32 = A; -``` -"##, - -// FIXME(#57563) Change the language here when const fn stabilizes -E0015: r##" -The only functions that can be called in static or constant expressions are -`const` functions, and struct/enum constructors. `const` functions are only -available on a nightly compiler. Rust currently does not support more general -compile-time function execution. - -``` -const FOO: Option = Some(1); // enum constructor -struct Bar {x: u8} -const BAR: Bar = Bar {x: 1}; // struct constructor -``` - -See [RFC 911] for more details on the design of `const fn`s. - -[RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md -"##, - -E0017: r##" -References in statics and constants may only refer to immutable values. - -Erroneous code example: - -```compile_fail,E0017 -static X: i32 = 1; -const C: i32 = 2; - -// these three are not allowed: -const CR: &mut i32 = &mut C; -static STATIC_REF: &'static mut i32 = &mut X; -static CONST_REF: &'static mut i32 = &mut C; -``` - -Statics are shared everywhere, and if they refer to mutable data one might -violate memory safety since holding multiple mutable references to shared data -is not allowed. - -If you really want global mutable state, try using `static mut` or a global -`UnsafeCell`. -"##, - -E0019: r##" -A function call isn't allowed in the const's initialization expression -because the expression's value must be known at compile-time. - -Erroneous code example: - -```compile_fail,E0019 -#![feature(box_syntax)] - -fn main() { - struct MyOwned; - - static STATIC11: Box = box MyOwned; // error! -} -``` - -Remember: you can't use a function call inside a const's initialization -expression! However, you can totally use it anywhere else: - -``` -enum Test { - V1 -} - -impl Test { - fn func(&self) -> i32 { - 12 - } -} - -fn main() { - const FOO: Test = Test::V1; - - FOO.func(); // here is good - let x = FOO.func(); // or even here! -} -``` -"##, - -E0030: r##" -When matching against a range, the compiler verifies that the range is -non-empty. Range patterns include both end-points, so this is equivalent to -requiring the start of the range to be less than or equal to the end of the -range. - -Erroneous code example: - -```compile_fail,E0030 -match 5u32 { - // This range is ok, albeit pointless. - 1 ..= 1 => {} - // This range is empty, and the compiler can tell. - 1000 ..= 5 => {} -} -``` -"##, - -E0133: r##" -Unsafe code was used outside of an unsafe function or block. - -Erroneous code example: - -```compile_fail,E0133 -unsafe fn f() { return; } // This is the unsafe code - -fn main() { - f(); // error: call to unsafe function requires unsafe function or block -} -``` - -Using unsafe functionality is potentially dangerous and disallowed by safety -checks. Examples: - -* Dereferencing raw pointers -* Calling functions via FFI -* Calling functions marked unsafe - -These safety checks can be relaxed for a section of the code by wrapping the -unsafe instructions with an `unsafe` block. For instance: - -``` -unsafe fn f() { return; } - -fn main() { - unsafe { f(); } // ok! -} -``` - -See also https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html -"##, - -E0158: r##" -An associated const has been referenced in a pattern. - -Erroneous code example: - -```compile_fail,E0158 -enum EFoo { A, B, C, D } - -trait Foo { - const X: EFoo; -} - -fn test(arg: EFoo) { - match arg { - A::X => { // error! - println!("A::X"); - } - } -} -``` - -`const` and `static` mean different things. A `const` is a compile-time -constant, an alias for a literal value. This property means you can match it -directly within a pattern. - -The `static` keyword, on the other hand, guarantees a fixed location in memory. -This does not always mean that the value is constant. For example, a global -mutex can be declared `static` as well. - -If you want to match against a `static`, consider using a guard instead: - -``` -static FORTY_TWO: i32 = 42; - -match Some(42) { - Some(x) if x == FORTY_TWO => {} - _ => {} -} -``` -"##, - -E0161: r##" -A value was moved. However, its size was not known at compile time, and only -values of a known size can be moved. - -Erroneous code example: - -```compile_fail,E0161 -#![feature(box_syntax)] - -fn main() { - let array: &[isize] = &[1, 2, 3]; - let _x: Box<[isize]> = box *array; - // error: cannot move a value of type [isize]: the size of [isize] cannot - // be statically determined -} -``` - -In Rust, you can only move a value when its size is known at compile time. - -To work around this restriction, consider "hiding" the value behind a reference: -either `&x` or `&mut x`. Since a reference has a fixed size, this lets you move -it around as usual. Example: - -``` -#![feature(box_syntax)] - -fn main() { - let array: &[isize] = &[1, 2, 3]; - let _x: Box<&[isize]> = box array; // ok! -} -``` -"##, - -E0162: r##" -#### Note: this error code is no longer emitted by the compiler. - -An if-let pattern attempts to match the pattern, and enters the body if the -match was successful. If the match is irrefutable (when it cannot fail to -match), use a regular `let`-binding instead. For instance: - -``` -struct Irrefutable(i32); -let irr = Irrefutable(0); - -// This fails to compile because the match is irrefutable. -if let Irrefutable(x) = irr { - // This body will always be executed. - // ... -} -``` - -Try this instead: - -``` -struct Irrefutable(i32); -let irr = Irrefutable(0); - -let Irrefutable(x) = irr; -println!("{}", x); -``` -"##, - -E0165: r##" -#### Note: this error code is no longer emitted by the compiler. - -A while-let pattern attempts to match the pattern, and enters the body if the -match was successful. If the match is irrefutable (when it cannot fail to -match), use a regular `let`-binding inside a `loop` instead. For instance: - -```no_run -struct Irrefutable(i32); -let irr = Irrefutable(0); - -// This fails to compile because the match is irrefutable. -while let Irrefutable(x) = irr { - // ... -} -``` - -Try this instead: - -```no_run -struct Irrefutable(i32); -let irr = Irrefutable(0); - -loop { - let Irrefutable(x) = irr; - // ... -} -``` -"##, - -E0170: r##" -Enum variants are qualified by default. For example, given this type: - -``` -enum Method { - GET, - POST, -} -``` - -You would match it using: - -``` -enum Method { - GET, - POST, -} - -let m = Method::GET; - -match m { - Method::GET => {}, - Method::POST => {}, -} -``` - -If you don't qualify the names, the code will bind new variables named "GET" and -"POST" instead. This behavior is likely not what you want, so `rustc` warns when -that happens. - -Qualified names are good practice, and most code works well with them. But if -you prefer them unqualified, you can import the variants into scope: - -``` -use Method::*; -enum Method { GET, POST } -# fn main() {} -``` - -If you want others to be able to import variants from your module directly, use -`pub use`: - -``` -pub use Method::*; -pub enum Method { GET, POST } -# fn main() {} -``` -"##, - - -E0297: r##" -#### Note: this error code is no longer emitted by the compiler. - -Patterns used to bind names must be irrefutable. That is, they must guarantee -that a name will be extracted in all cases. Instead of pattern matching the -loop variable, consider using a `match` or `if let` inside the loop body. For -instance: - -```compile_fail,E0005 -let xs : Vec> = vec![Some(1), None]; - -// This fails because `None` is not covered. -for Some(x) in xs { - // ... -} -``` - -Match inside the loop instead: - -``` -let xs : Vec> = vec![Some(1), None]; - -for item in xs { - match item { - Some(x) => {}, - None => {}, - } -} -``` - -Or use `if let`: - -``` -let xs : Vec> = vec![Some(1), None]; - -for item in xs { - if let Some(x) = item { - // ... - } -} -``` -"##, - -E0301: r##" -#### Note: this error code is no longer emitted by the compiler. - -Mutable borrows are not allowed in pattern guards, because matching cannot have -side effects. Side effects could alter the matched object or the environment -on which the match depends in such a way, that the match would not be -exhaustive. For instance, the following would not match any arm if mutable -borrows were allowed: - -```compile_fail,E0596 -match Some(()) { - None => { }, - option if option.take().is_none() => { - /* impossible, option is `Some` */ - }, - Some(_) => { } // When the previous match failed, the option became `None`. -} -``` -"##, - -E0302: r##" -#### Note: this error code is no longer emitted by the compiler. - -Assignments are not allowed in pattern guards, because matching cannot have -side effects. Side effects could alter the matched object or the environment -on which the match depends in such a way, that the match would not be -exhaustive. For instance, the following would not match any arm if assignments -were allowed: - -```compile_fail,E0594 -match Some(()) { - None => { }, - option if { option = None; false } => { }, - Some(_) => { } // When the previous match failed, the option became `None`. -} -``` -"##, - -E0303: r##" -In certain cases it is possible for sub-bindings to violate memory safety. -Updates to the borrow checker in a future version of Rust may remove this -restriction, but for now patterns must be rewritten without sub-bindings. - -Before: - -```compile_fail,E0303 -match Some("hi".to_string()) { - ref op_string_ref @ Some(s) => {}, - None => {}, -} -``` - -After: - -``` -match Some("hi".to_string()) { - Some(ref s) => { - let op_string_ref = &Some(s); - // ... - }, - None => {}, -} -``` - -The `op_string_ref` binding has type `&Option<&String>` in both cases. - -See also https://github.com/rust-lang/rust/issues/14587 -"##, - -E0373: r##" -This error occurs when an attempt is made to use data captured by a closure, -when that data may no longer exist. It's most commonly seen when attempting to -return a closure: - -```compile_fail,E0373 -fn foo() -> Box u32> { - let x = 0u32; - Box::new(|y| x + y) -} -``` - -Notice that `x` is stack-allocated by `foo()`. By default, Rust captures -closed-over data by reference. This means that once `foo()` returns, `x` no -longer exists. An attempt to access `x` within the closure would thus be -unsafe. - -Another situation where this might be encountered is when spawning threads: - -```compile_fail,E0373 -fn foo() { - let x = 0u32; - let y = 1u32; - - let thr = std::thread::spawn(|| { - x + y - }); -} -``` - -Since our new thread runs in parallel, the stack frame containing `x` and `y` -may well have disappeared by the time we try to use them. Even if we call -`thr.join()` within foo (which blocks until `thr` has completed, ensuring the -stack frame won't disappear), we will not succeed: the compiler cannot prove -that this behaviour is safe, and so won't let us do it. - -The solution to this problem is usually to switch to using a `move` closure. -This approach moves (or copies, where possible) data into the closure, rather -than taking references to it. For example: - -``` -fn foo() -> Box u32> { - let x = 0u32; - Box::new(move |y| x + y) -} -``` - -Now that the closure has its own copy of the data, there's no need to worry -about safety. -"##, - -E0381: r##" -It is not allowed to use or capture an uninitialized variable. - -Erroneous code example: - -```compile_fail,E0381 -fn main() { - let x: i32; - let y = x; // error, use of possibly-uninitialized variable -} -``` - -To fix this, ensure that any declared variables are initialized before being -used. Example: - -``` -fn main() { - let x: i32 = 0; - let y = x; // ok! -} -``` -"##, - -E0382: r##" -This error occurs when an attempt is made to use a variable after its contents -have been moved elsewhere. - -Erroneous code example: - -```compile_fail,E0382 -struct MyStruct { s: u32 } - -fn main() { - let mut x = MyStruct{ s: 5u32 }; - let y = x; - x.s = 6; - println!("{}", x.s); -} -``` - -Since `MyStruct` is a type that is not marked `Copy`, the data gets moved out -of `x` when we set `y`. This is fundamental to Rust's ownership system: outside -of workarounds like `Rc`, a value cannot be owned by more than one variable. - -Sometimes we don't need to move the value. Using a reference, we can let another -function borrow the value without changing its ownership. In the example below, -we don't actually have to move our string to `calculate_length`, we can give it -a reference to it with `&` instead. - -``` -fn main() { - let s1 = String::from("hello"); - - let len = calculate_length(&s1); - - println!("The length of '{}' is {}.", s1, len); -} - -fn calculate_length(s: &String) -> usize { - s.len() -} -``` - -A mutable reference can be created with `&mut`. - -Sometimes we don't want a reference, but a duplicate. All types marked `Clone` -can be duplicated by calling `.clone()`. Subsequent changes to a clone do not -affect the original variable. - -Most types in the standard library are marked `Clone`. The example below -demonstrates using `clone()` on a string. `s1` is first set to "many", and then -copied to `s2`. Then the first character of `s1` is removed, without affecting -`s2`. "any many" is printed to the console. - -``` -fn main() { - let mut s1 = String::from("many"); - let s2 = s1.clone(); - s1.remove(0); - println!("{} {}", s1, s2); -} -``` - -If we control the definition of a type, we can implement `Clone` on it ourselves -with `#[derive(Clone)]`. - -Some types have no ownership semantics at all and are trivial to duplicate. An -example is `i32` and the other number types. We don't have to call `.clone()` to -clone them, because they are marked `Copy` in addition to `Clone`. Implicit -cloning is more convenient in this case. We can mark our own types `Copy` if -all their members also are marked `Copy`. - -In the example below, we implement a `Point` type. Because it only stores two -integers, we opt-out of ownership semantics with `Copy`. Then we can -`let p2 = p1` without `p1` being moved. - -``` -#[derive(Copy, Clone)] -struct Point { x: i32, y: i32 } - -fn main() { - let mut p1 = Point{ x: -1, y: 2 }; - let p2 = p1; - p1.x = 1; - println!("p1: {}, {}", p1.x, p1.y); - println!("p2: {}, {}", p2.x, p2.y); -} -``` - -Alternatively, if we don't control the struct's definition, or mutable shared -ownership is truly required, we can use `Rc` and `RefCell`: - -``` -use std::cell::RefCell; -use std::rc::Rc; - -struct MyStruct { s: u32 } - -fn main() { - let mut x = Rc::new(RefCell::new(MyStruct{ s: 5u32 })); - let y = x.clone(); - x.borrow_mut().s = 6; - println!("{}", x.borrow().s); -} -``` - -With this approach, x and y share ownership of the data via the `Rc` (reference -count type). `RefCell` essentially performs runtime borrow checking: ensuring -that at most one writer or multiple readers can access the data at any one time. - -If you wish to learn more about ownership in Rust, start with the chapter in the -Book: - -https://doc.rust-lang.org/book/ch04-00-understanding-ownership.html -"##, - -E0383: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error occurs when an attempt is made to partially reinitialize a -structure that is currently uninitialized. - -For example, this can happen when a drop has taken place: - -```compile_fail -struct Foo { - a: u32, -} -impl Drop for Foo { - fn drop(&mut self) { /* ... */ } -} - -let mut x = Foo { a: 1 }; -drop(x); // `x` is now uninitialized -x.a = 2; // error, partial reinitialization of uninitialized structure `t` -``` - -This error can be fixed by fully reinitializing the structure in question: - -``` -struct Foo { - a: u32, -} -impl Drop for Foo { - fn drop(&mut self) { /* ... */ } -} - -let mut x = Foo { a: 1 }; -drop(x); -x = Foo { a: 2 }; -``` -"##, - -E0384: r##" -This error occurs when an attempt is made to reassign an immutable variable. - -Erroneous code example: - -```compile_fail,E0384 -fn main() { - let x = 3; - x = 5; // error, reassignment of immutable variable -} -``` - -By default, variables in Rust are immutable. To fix this error, add the keyword -`mut` after the keyword `let` when declaring the variable. For example: - -``` -fn main() { - let mut x = 3; - x = 5; -} -``` -"##, - -E0386: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error occurs when an attempt is made to mutate the target of a mutable -reference stored inside an immutable container. - -For example, this can happen when storing a `&mut` inside an immutable `Box`: - -``` -let mut x: i64 = 1; -let y: Box<_> = Box::new(&mut x); -**y = 2; // error, cannot assign to data in an immutable container -``` - -This error can be fixed by making the container mutable: - -``` -let mut x: i64 = 1; -let mut y: Box<_> = Box::new(&mut x); -**y = 2; -``` - -It can also be fixed by using a type with interior mutability, such as `Cell` -or `RefCell`: - -``` -use std::cell::Cell; - -let x: i64 = 1; -let y: Box> = Box::new(Cell::new(x)); -y.set(2); -``` -"##, - -E0387: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error occurs when an attempt is made to mutate or mutably reference data -that a closure has captured immutably. - -Erroneous code example: - -```compile_fail -// Accepts a function or a closure that captures its environment immutably. -// Closures passed to foo will not be able to mutate their closed-over state. -fn foo(f: F) { } - -// Attempts to mutate closed-over data. Error message reads: -// `cannot assign to data in a captured outer variable...` -fn mutable() { - let mut x = 0u32; - foo(|| x = 2); -} - -// Attempts to take a mutable reference to closed-over data. Error message -// reads: `cannot borrow data mutably in a captured outer variable...` -fn mut_addr() { - let mut x = 0u32; - foo(|| { let y = &mut x; }); -} -``` - -The problem here is that foo is defined as accepting a parameter of type `Fn`. -Closures passed into foo will thus be inferred to be of type `Fn`, meaning that -they capture their context immutably. - -If the definition of `foo` is under your control, the simplest solution is to -capture the data mutably. This can be done by defining `foo` to take FnMut -rather than Fn: - -``` -fn foo(f: F) { } -``` - -Alternatively, we can consider using the `Cell` and `RefCell` types to achieve -interior mutability through a shared reference. Our example's `mutable` -function could be redefined as below: - -``` -use std::cell::Cell; - -fn foo(f: F) { } - -fn mutable() { - let x = Cell::new(0u32); - foo(|| x.set(2)); -} -``` - -You can read more about cell types in the API documentation: - -https://doc.rust-lang.org/std/cell/ -"##, - -E0388: r##" -#### Note: this error code is no longer emitted by the compiler. -"##, - -E0389: r##" -#### Note: this error code is no longer emitted by the compiler. - -An attempt was made to mutate data using a non-mutable reference. This -commonly occurs when attempting to assign to a non-mutable reference of a -mutable reference (`&(&mut T)`). - -Erroneous code example: - -```compile_fail -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy = FancyNum{ num: 5 }; - let fancy_ref = &(&mut fancy); - fancy_ref.num = 6; // error: cannot assign to data in a `&` reference - println!("{}", fancy_ref.num); -} -``` - -Here, `&mut fancy` is mutable, but `&(&mut fancy)` is not. Creating an -immutable reference to a value borrows it immutably. There can be multiple -references of type `&(&mut T)` that point to the same value, so they must be -immutable to prevent multiple mutable references to the same value. - -To fix this, either remove the outer reference: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy = FancyNum{ num: 5 }; - - let fancy_ref = &mut fancy; - // `fancy_ref` is now &mut FancyNum, rather than &(&mut FancyNum) - - fancy_ref.num = 6; // No error! - - println!("{}", fancy_ref.num); -} -``` - -Or make the outer reference mutable: - -``` -struct FancyNum { - num: u8 -} - -fn main() { - let mut fancy = FancyNum{ num: 5 }; - - let fancy_ref = &mut (&mut fancy); - // `fancy_ref` is now &mut(&mut FancyNum), rather than &(&mut FancyNum) - - fancy_ref.num = 6; // No error! - - println!("{}", fancy_ref.num); -} -``` -"##, - -E0492: r##" -A borrow of a constant containing interior mutability was attempted. - -Erroneous code example: - -```compile_fail,E0492 -use std::sync::atomic::AtomicUsize; - -const A: AtomicUsize = AtomicUsize::new(0); -static B: &'static AtomicUsize = &A; -// error: cannot borrow a constant which may contain interior mutability, -// create a static instead -``` - -A `const` represents a constant value that should never change. If one takes -a `&` reference to the constant, then one is taking a pointer to some memory -location containing the value. Normally this is perfectly fine: most values -can't be changed via a shared `&` pointer, but interior mutability would allow -it. That is, a constant value could be mutated. On the other hand, a `static` is -explicitly a single memory location, which can be mutated at will. - -So, in order to solve this error, either use statics which are `Sync`: - -``` -use std::sync::atomic::AtomicUsize; - -static A: AtomicUsize = AtomicUsize::new(0); -static B: &'static AtomicUsize = &A; // ok! -``` - -You can also have this error while using a cell type: - -```compile_fail,E0492 -use std::cell::Cell; - -const A: Cell = Cell::new(1); -const B: &Cell = &A; -// error: cannot borrow a constant which may contain interior mutability, -// create a static instead - -// or: -struct C { a: Cell } - -const D: C = C { a: Cell::new(1) }; -const E: &Cell = &D.a; // error - -// or: -const F: &C = &D; // error -``` - -This is because cell types do operations that are not thread-safe. Due to this, -they don't implement Sync and thus can't be placed in statics. - -However, if you still wish to use these types, you can achieve this by an unsafe -wrapper: - -``` -use std::cell::Cell; -use std::marker::Sync; - -struct NotThreadSafe { - value: Cell, -} - -unsafe impl Sync for NotThreadSafe {} - -static A: NotThreadSafe = NotThreadSafe { value : Cell::new(1) }; -static B: &'static NotThreadSafe = &A; // ok! -``` - -Remember this solution is unsafe! You will have to ensure that accesses to the -cell are synchronized. -"##, - -E0493: r##" -A type with a `Drop` implementation was destructured when trying to initialize -a static item. - -Erroneous code example: - -```compile_fail,E0493 -enum DropType { - A, -} - -impl Drop for DropType { - fn drop(&mut self) {} -} - -struct Foo { - field1: DropType, -} - -static FOO: Foo = Foo { ..Foo { field1: DropType::A } }; // error! -``` - -The problem here is that if the given type or one of its fields implements the -`Drop` trait, this `Drop` implementation cannot be called during the static -type initialization which might cause a memory leak. To prevent this issue, -you need to instantiate all the static type's fields by hand. - -``` -enum DropType { - A, -} - -impl Drop for DropType { - fn drop(&mut self) {} -} - -struct Foo { - field1: DropType, -} - -static FOO: Foo = Foo { field1: DropType::A }; // We initialize all fields - // by hand. -``` -"##, - -E0499: r##" -A variable was borrowed as mutable more than once. - -Erroneous code example: - -```compile_fail,E0499 -let mut i = 0; -let mut x = &mut i; -let mut a = &mut i; -x; -// error: cannot borrow `i` as mutable more than once at a time -``` - -Please note that in rust, you can either have many immutable references, or one -mutable reference. Take a look at -https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html for more -information. Example: - - -``` -let mut i = 0; -let mut x = &mut i; // ok! - -// or: -let mut i = 0; -let a = &i; // ok! -let b = &i; // still ok! -let c = &i; // still ok! -b; -a; -``` -"##, - -E0500: r##" -A borrowed variable was used by a closure. - -Erroneous code example: - -```compile_fail,E0500 -fn you_know_nothing(jon_snow: &mut i32) { - let nights_watch = &jon_snow; - let starks = || { - *jon_snow = 3; // error: closure requires unique access to `jon_snow` - // but it is already borrowed - }; - println!("{}", nights_watch); -} -``` - -In here, `jon_snow` is already borrowed by the `nights_watch` reference, so it -cannot be borrowed by the `starks` closure at the same time. To fix this issue, -you can create the closure after the borrow has ended: - -``` -fn you_know_nothing(jon_snow: &mut i32) { - let nights_watch = &jon_snow; - println!("{}", nights_watch); - let starks = || { - *jon_snow = 3; - }; -} -``` - -Or, if the type implements the `Clone` trait, you can clone it between -closures: - -``` -fn you_know_nothing(jon_snow: &mut i32) { - let mut jon_copy = jon_snow.clone(); - let starks = || { - *jon_snow = 3; - }; - println!("{}", jon_copy); -} -``` -"##, - -E0501: r##" -This error indicates that a mutable variable is being used while it is still -captured by a closure. Because the closure has borrowed the variable, it is not -available for use until the closure goes out of scope. - -Note that a capture will either move or borrow a variable, but in this -situation, the closure is borrowing the variable. Take a look at -http://rustbyexample.com/fn/closures/capture.html for more information about -capturing. - -Erroneous code example: - -```compile_fail,E0501 -fn inside_closure(x: &mut i32) { - // Actions which require unique access -} - -fn outside_closure(x: &mut i32) { - // Actions which require unique access -} - -fn foo(a: &mut i32) { - let mut bar = || { - inside_closure(a) - }; - outside_closure(a); // error: cannot borrow `*a` as mutable because previous - // closure requires unique access. - bar(); -} -``` - -To fix this error, you can finish using the closure before using the captured -variable: - -``` -fn inside_closure(x: &mut i32) {} -fn outside_closure(x: &mut i32) {} - -fn foo(a: &mut i32) { - let mut bar = || { - inside_closure(a) - }; - bar(); - // borrow on `a` ends. - outside_closure(a); // ok! -} -``` - -Or you can pass the variable as a parameter to the closure: - -``` -fn inside_closure(x: &mut i32) {} -fn outside_closure(x: &mut i32) {} - -fn foo(a: &mut i32) { - let mut bar = |s: &mut i32| { - inside_closure(s) - }; - outside_closure(a); - bar(a); -} -``` - -It may be possible to define the closure later: - -``` -fn inside_closure(x: &mut i32) {} -fn outside_closure(x: &mut i32) {} - -fn foo(a: &mut i32) { - outside_closure(a); - let mut bar = || { - inside_closure(a) - }; - bar(); -} -``` -"##, - -E0502: r##" -This error indicates that you are trying to borrow a variable as mutable when it -has already been borrowed as immutable. - -Erroneous code example: - -```compile_fail,E0502 -fn bar(x: &mut i32) {} -fn foo(a: &mut i32) { - let ref y = a; // a is borrowed as immutable. - bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed - // as immutable - println!("{}", y); -} -``` - -To fix this error, ensure that you don't have any other references to the -variable before trying to access it mutably: - -``` -fn bar(x: &mut i32) {} -fn foo(a: &mut i32) { - bar(a); - let ref y = a; // ok! - println!("{}", y); -} -``` - -For more information on the rust ownership system, take a look at -https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html. -"##, - -E0503: r##" -A value was used after it was mutably borrowed. - -Erroneous code example: - -```compile_fail,E0503 -fn main() { - let mut value = 3; - // Create a mutable borrow of `value`. - let borrow = &mut value; - let _sum = value + 1; // error: cannot use `value` because - // it was mutably borrowed - println!("{}", borrow); -} -``` - -In this example, `value` is mutably borrowed by `borrow` and cannot be -used to calculate `sum`. This is not possible because this would violate -Rust's mutability rules. - -You can fix this error by finishing using the borrow before the next use of -the value: - -``` -fn main() { - let mut value = 3; - let borrow = &mut value; - println!("{}", borrow); - // The block has ended and with it the borrow. - // You can now use `value` again. - let _sum = value + 1; -} -``` - -Or by cloning `value` before borrowing it: - -``` -fn main() { - let mut value = 3; - // We clone `value`, creating a copy. - let value_cloned = value.clone(); - // The mutable borrow is a reference to `value` and - // not to `value_cloned`... - let borrow = &mut value; - // ... which means we can still use `value_cloned`, - let _sum = value_cloned + 1; - // even though the borrow only ends here. - println!("{}", borrow); -} -``` - -You can find more information about borrowing in the rust-book: -http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html -"##, - -E0504: r##" -#### Note: this error code is no longer emitted by the compiler. - -This error occurs when an attempt is made to move a borrowed variable into a -closure. - -Erroneous code example: - -```compile_fail -struct FancyNum { - num: u8, -} - -fn main() { - let fancy_num = FancyNum { num: 5 }; - let fancy_ref = &fancy_num; - - let x = move || { - println!("child function: {}", fancy_num.num); - // error: cannot move `fancy_num` into closure because it is borrowed - }; - - x(); - println!("main function: {}", fancy_ref.num); -} -``` - -Here, `fancy_num` is borrowed by `fancy_ref` and so cannot be moved into -the closure `x`. There is no way to move a value into a closure while it is -borrowed, as that would invalidate the borrow. - -If the closure can't outlive the value being moved, try using a reference -rather than moving: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let fancy_num = FancyNum { num: 5 }; - let fancy_ref = &fancy_num; - - let x = move || { - // fancy_ref is usable here because it doesn't move `fancy_num` - println!("child function: {}", fancy_ref.num); - }; - - x(); - - println!("main function: {}", fancy_num.num); -} -``` - -If the value has to be borrowed and then moved, try limiting the lifetime of -the borrow using a scoped block: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let fancy_num = FancyNum { num: 5 }; - - { - let fancy_ref = &fancy_num; - println!("main function: {}", fancy_ref.num); - // `fancy_ref` goes out of scope here - } - - let x = move || { - // `fancy_num` can be moved now (no more references exist) - println!("child function: {}", fancy_num.num); - }; - - x(); -} -``` - -If the lifetime of a reference isn't enough, such as in the case of threading, -consider using an `Arc` to create a reference-counted value: - -``` -use std::sync::Arc; -use std::thread; - -struct FancyNum { - num: u8, -} - -fn main() { - let fancy_ref1 = Arc::new(FancyNum { num: 5 }); - let fancy_ref2 = fancy_ref1.clone(); - - let x = thread::spawn(move || { - // `fancy_ref1` can be moved and has a `'static` lifetime - println!("child thread: {}", fancy_ref1.num); - }); - - x.join().expect("child thread should finish"); - println!("main thread: {}", fancy_ref2.num); -} -``` -"##, - -E0505: r##" -A value was moved out while it was still borrowed. - -Erroneous code example: - -```compile_fail,E0505 -struct Value {} - -fn borrow(val: &Value) {} - -fn eat(val: Value) {} - -fn main() { - let x = Value{}; - let _ref_to_val: &Value = &x; - eat(x); - borrow(_ref_to_val); -} -``` - -Here, the function `eat` takes ownership of `x`. However, -`x` cannot be moved because the borrow to `_ref_to_val` -needs to last till the function `borrow`. -To fix that you can do a few different things: - -* Try to avoid moving the variable. -* Release borrow before move. -* Implement the `Copy` trait on the type. - -Examples: - -``` -struct Value {} - -fn borrow(val: &Value) {} - -fn eat(val: &Value) {} - -fn main() { - let x = Value{}; - - let ref_to_val: &Value = &x; - eat(&x); // pass by reference, if it's possible - borrow(ref_to_val); -} -``` - -Or: - -``` -struct Value {} - -fn borrow(val: &Value) {} - -fn eat(val: Value) {} - -fn main() { - let x = Value{}; - - let ref_to_val: &Value = &x; - borrow(ref_to_val); - // ref_to_val is no longer used. - eat(x); -} -``` - -Or: - -``` -#[derive(Clone, Copy)] // implement Copy trait -struct Value {} - -fn borrow(val: &Value) {} - -fn eat(val: Value) {} - -fn main() { - let x = Value{}; - let ref_to_val: &Value = &x; - eat(x); // it will be copied here. - borrow(ref_to_val); -} -``` - -You can find more information about borrowing in the rust-book: -http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html -"##, - -E0506: r##" -This error occurs when an attempt is made to assign to a borrowed value. - -Erroneous code example: - -```compile_fail,E0506 -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy_num = FancyNum { num: 5 }; - let fancy_ref = &fancy_num; - fancy_num = FancyNum { num: 6 }; - // error: cannot assign to `fancy_num` because it is borrowed - - println!("Num: {}, Ref: {}", fancy_num.num, fancy_ref.num); -} -``` - -Because `fancy_ref` still holds a reference to `fancy_num`, `fancy_num` can't -be assigned to a new value as it would invalidate the reference. - -Alternatively, we can move out of `fancy_num` into a second `fancy_num`: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy_num = FancyNum { num: 5 }; - let moved_num = fancy_num; - fancy_num = FancyNum { num: 6 }; - - println!("Num: {}, Moved num: {}", fancy_num.num, moved_num.num); -} -``` - -If the value has to be borrowed, try limiting the lifetime of the borrow using -a scoped block: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy_num = FancyNum { num: 5 }; - - { - let fancy_ref = &fancy_num; - println!("Ref: {}", fancy_ref.num); - } - - // Works because `fancy_ref` is no longer in scope - fancy_num = FancyNum { num: 6 }; - println!("Num: {}", fancy_num.num); -} -``` - -Or by moving the reference into a function: - -``` -struct FancyNum { - num: u8, -} - -fn main() { - let mut fancy_num = FancyNum { num: 5 }; - - print_fancy_ref(&fancy_num); - - // Works because function borrow has ended - fancy_num = FancyNum { num: 6 }; - println!("Num: {}", fancy_num.num); -} - -fn print_fancy_ref(fancy_ref: &FancyNum){ - println!("Ref: {}", fancy_ref.num); -} -``` -"##, - -E0507: r##" -You tried to move out of a value which was borrowed. - -This can also happen when using a type implementing `Fn` or `FnMut`, as neither -allows moving out of them (they usually represent closures which can be called -more than once). Much of the text following applies equally well to non-`FnOnce` -closure bodies. - -Erroneous code example: - -```compile_fail,E0507 -use std::cell::RefCell; - -struct TheDarkKnight; - -impl TheDarkKnight { - fn nothing_is_true(self) {} -} - -fn main() { - let x = RefCell::new(TheDarkKnight); - - x.borrow().nothing_is_true(); // error: cannot move out of borrowed content -} -``` - -Here, the `nothing_is_true` method takes the ownership of `self`. However, -`self` cannot be moved because `.borrow()` only provides an `&TheDarkKnight`, -which is a borrow of the content owned by the `RefCell`. To fix this error, -you have three choices: - -* Try to avoid moving the variable. -* Somehow reclaim the ownership. -* Implement the `Copy` trait on the type. - -Examples: - -``` -use std::cell::RefCell; - -struct TheDarkKnight; - -impl TheDarkKnight { - fn nothing_is_true(&self) {} // First case, we don't take ownership -} - -fn main() { - let x = RefCell::new(TheDarkKnight); - - x.borrow().nothing_is_true(); // ok! -} -``` - -Or: - -``` -use std::cell::RefCell; - -struct TheDarkKnight; - -impl TheDarkKnight { - fn nothing_is_true(self) {} -} - -fn main() { - let x = RefCell::new(TheDarkKnight); - let x = x.into_inner(); // we get back ownership - - x.nothing_is_true(); // ok! -} -``` - -Or: - -``` -use std::cell::RefCell; - -#[derive(Clone, Copy)] // we implement the Copy trait -struct TheDarkKnight; - -impl TheDarkKnight { - fn nothing_is_true(self) {} -} - -fn main() { - let x = RefCell::new(TheDarkKnight); - - x.borrow().nothing_is_true(); // ok! -} -``` - -Moving a member out of a mutably borrowed struct will also cause E0507 error: - -```compile_fail,E0507 -struct TheDarkKnight; - -impl TheDarkKnight { - fn nothing_is_true(self) {} -} - -struct Batcave { - knight: TheDarkKnight -} - -fn main() { - let mut cave = Batcave { - knight: TheDarkKnight - }; - let borrowed = &mut cave; - - borrowed.knight.nothing_is_true(); // E0507 -} -``` - -It is fine only if you put something back. `mem::replace` can be used for that: - -``` -# struct TheDarkKnight; -# impl TheDarkKnight { fn nothing_is_true(self) {} } -# struct Batcave { knight: TheDarkKnight } -use std::mem; - -let mut cave = Batcave { - knight: TheDarkKnight -}; -let borrowed = &mut cave; - -mem::replace(&mut borrowed.knight, TheDarkKnight).nothing_is_true(); // ok! -``` - -You can find more information about borrowing in the rust-book: -http://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html -"##, - -E0508: r##" -A value was moved out of a non-copy fixed-size array. - -Erroneous code example: - -```compile_fail,E0508 -struct NonCopy; - -fn main() { - let array = [NonCopy; 1]; - let _value = array[0]; // error: cannot move out of type `[NonCopy; 1]`, - // a non-copy fixed-size array -} -``` - -The first element was moved out of the array, but this is not -possible because `NonCopy` does not implement the `Copy` trait. - -Consider borrowing the element instead of moving it: - -``` -struct NonCopy; - -fn main() { - let array = [NonCopy; 1]; - let _value = &array[0]; // Borrowing is allowed, unlike moving. -} -``` - -Alternatively, if your type implements `Clone` and you need to own the value, -consider borrowing and then cloning: - -``` -#[derive(Clone)] -struct NonCopy; - -fn main() { - let array = [NonCopy; 1]; - // Now you can clone the array element. - let _value = array[0].clone(); -} -``` -"##, - -E0509: r##" -This error occurs when an attempt is made to move out of a value whose type -implements the `Drop` trait. - -Erroneous code example: - -```compile_fail,E0509 -struct FancyNum { - num: usize -} - -struct DropStruct { - fancy: FancyNum -} - -impl Drop for DropStruct { - fn drop(&mut self) { - // Destruct DropStruct, possibly using FancyNum - } -} - -fn main() { - let drop_struct = DropStruct{fancy: FancyNum{num: 5}}; - let fancy_field = drop_struct.fancy; // Error E0509 - println!("Fancy: {}", fancy_field.num); - // implicit call to `drop_struct.drop()` as drop_struct goes out of scope -} -``` - -Here, we tried to move a field out of a struct of type `DropStruct` which -implements the `Drop` trait. However, a struct cannot be dropped if one or -more of its fields have been moved. - -Structs implementing the `Drop` trait have an implicit destructor that gets -called when they go out of scope. This destructor may use the fields of the -struct, so moving out of the struct could make it impossible to run the -destructor. Therefore, we must think of all values whose type implements the -`Drop` trait as single units whose fields cannot be moved. - -This error can be fixed by creating a reference to the fields of a struct, -enum, or tuple using the `ref` keyword: - -``` -struct FancyNum { - num: usize -} - -struct DropStruct { - fancy: FancyNum -} - -impl Drop for DropStruct { - fn drop(&mut self) { - // Destruct DropStruct, possibly using FancyNum - } -} - -fn main() { - let drop_struct = DropStruct{fancy: FancyNum{num: 5}}; - let ref fancy_field = drop_struct.fancy; // No more errors! - println!("Fancy: {}", fancy_field.num); - // implicit call to `drop_struct.drop()` as drop_struct goes out of scope -} -``` - -Note that this technique can also be used in the arms of a match expression: - -``` -struct FancyNum { - num: usize -} - -enum DropEnum { - Fancy(FancyNum) -} - -impl Drop for DropEnum { - fn drop(&mut self) { - // Destruct DropEnum, possibly using FancyNum - } -} - -fn main() { - // Creates and enum of type `DropEnum`, which implements `Drop` - let drop_enum = DropEnum::Fancy(FancyNum{num: 10}); - match drop_enum { - // Creates a reference to the inside of `DropEnum::Fancy` - DropEnum::Fancy(ref fancy_field) => // No error! - println!("It was fancy-- {}!", fancy_field.num), - } - // implicit call to `drop_enum.drop()` as drop_enum goes out of scope -} -``` -"##, - -E0510: r##" -Cannot mutate place in this match guard. - -When matching on a variable it cannot be mutated in the match guards, as this -could cause the match to be non-exhaustive: - -```compile_fail,E0510 -let mut x = Some(0); -match x { - None => (), - Some(_) if { x = None; false } => (), - Some(v) => (), // No longer matches -} -``` - -Here executing `x = None` would modify the value being matched and require us -to go "back in time" to the `None` arm. -"##, - -E0515: r##" -Cannot return value that references local variable - -Local variables, function parameters and temporaries are all dropped before the -end of the function body. So a reference to them cannot be returned. - -Erroneous code example: - -```compile_fail,E0515 -fn get_dangling_reference() -> &'static i32 { - let x = 0; - &x -} -``` - -```compile_fail,E0515 -use std::slice::Iter; -fn get_dangling_iterator<'a>() -> Iter<'a, i32> { - let v = vec![1, 2, 3]; - v.iter() -} -``` - -Consider returning an owned value instead: - -``` -use std::vec::IntoIter; - -fn get_integer() -> i32 { - let x = 0; - x -} - -fn get_owned_iterator() -> IntoIter { - let v = vec![1, 2, 3]; - v.into_iter() -} -``` -"##, - -E0524: r##" -A variable which requires unique access is being used in more than one closure -at the same time. - -Erroneous code example: - -```compile_fail,E0524 -fn set(x: &mut isize) { - *x += 4; -} - -fn dragoooon(x: &mut isize) { - let mut c1 = || set(x); - let mut c2 = || set(x); // error! - - c2(); - c1(); -} -``` - -To solve this issue, multiple solutions are available. First, is it required -for this variable to be used in more than one closure at a time? If it is the -case, use reference counted types such as `Rc` (or `Arc` if it runs -concurrently): - -``` -use std::rc::Rc; -use std::cell::RefCell; - -fn set(x: &mut isize) { - *x += 4; -} - -fn dragoooon(x: &mut isize) { - let x = Rc::new(RefCell::new(x)); - let y = Rc::clone(&x); - let mut c1 = || { let mut x2 = x.borrow_mut(); set(&mut x2); }; - let mut c2 = || { let mut x2 = y.borrow_mut(); set(&mut x2); }; // ok! - - c2(); - c1(); -} -``` - -If not, just run closures one at a time: - -``` -fn set(x: &mut isize) { - *x += 4; -} - -fn dragoooon(x: &mut isize) { - { // This block isn't necessary since non-lexical lifetimes, it's just to - // make it more clear. - let mut c1 = || set(&mut *x); - c1(); - } // `c1` has been dropped here so we're free to use `x` again! - let mut c2 = || set(&mut *x); - c2(); -} -``` -"##, - -E0579: r##" -When matching against an exclusive range, the compiler verifies that the range -is non-empty. Exclusive range patterns include the start point but not the end -point, so this is equivalent to requiring the start of the range to be less -than the end of the range. - -Erroneous code example: - -```compile_fail,E0579 -#![feature(exclusive_range_pattern)] - -fn main() { - match 5u32 { - // This range is ok, albeit pointless. - 1 .. 2 => {} - // This range is empty, and the compiler can tell. - 5 .. 5 => {} // error! - } -} -``` -"##, - -E0594: r##" -A non-mutable value was assigned a value. - -Erroneous code example: - -```compile_fail,E0594 -struct SolarSystem { - earth: i32, -} - -let ss = SolarSystem { earth: 3 }; -ss.earth = 2; // error! -``` - -To fix this error, just declare `ss` as mutable by using the `mut` keyword: - -``` -struct SolarSystem { - earth: i32, -} - -let mut ss = SolarSystem { earth: 3 }; // declaring `ss` as mutable -ss.earth = 2; // ok! -``` -"##, - -E0595: r##" -#### Note: this error code is no longer emitted by the compiler. - -Closures cannot mutate immutable captured variables. - -Erroneous code example: - -```compile_fail,E0594 -let x = 3; // error: closure cannot assign to immutable local variable `x` -let mut c = || { x += 1 }; -``` - -Make the variable binding mutable: - -``` -let mut x = 3; // ok! -let mut c = || { x += 1 }; -``` -"##, - -E0596: r##" -This error occurs because you tried to mutably borrow a non-mutable variable. - -Erroneous code example: - -```compile_fail,E0596 -let x = 1; -let y = &mut x; // error: cannot borrow mutably -``` - -In here, `x` isn't mutable, so when we try to mutably borrow it in `y`, it -fails. To fix this error, you need to make `x` mutable: - -``` -let mut x = 1; -let y = &mut x; // ok! -``` -"##, - -E0597: r##" -This error occurs because a value was dropped while it was still borrowed - -Erroneous code example: - -```compile_fail,E0597 -struct Foo<'a> { - x: Option<&'a u32>, -} - -let mut x = Foo { x: None }; -{ - let y = 0; - x.x = Some(&y); // error: `y` does not live long enough -} -println!("{:?}", x.x); -``` - -In here, `y` is dropped at the end of the inner scope, but it is borrowed by -`x` until the `println`. To fix the previous example, just remove the scope -so that `y` isn't dropped until after the println - -``` -struct Foo<'a> { - x: Option<&'a u32>, -} - -let mut x = Foo { x: None }; - -let y = 0; -x.x = Some(&y); - -println!("{:?}", x.x); -``` -"##, - -E0626: r##" -This error occurs because a borrow in a generator persists across a -yield point. - -Erroneous code example: - -```compile_fail,E0626 -# #![feature(generators, generator_trait, pin)] -# use std::ops::Generator; -# use std::pin::Pin; -let mut b = || { - let a = &String::new(); // <-- This borrow... - yield (); // ...is still in scope here, when the yield occurs. - println!("{}", a); -}; -Pin::new(&mut b).resume(); -``` - -At present, it is not permitted to have a yield that occurs while a -borrow is still in scope. To resolve this error, the borrow must -either be "contained" to a smaller scope that does not overlap the -yield or else eliminated in another way. So, for example, we might -resolve the previous example by removing the borrow and just storing -the integer by value: - -``` -# #![feature(generators, generator_trait, pin)] -# use std::ops::Generator; -# use std::pin::Pin; -let mut b = || { - let a = 3; - yield (); - println!("{}", a); -}; -Pin::new(&mut b).resume(); -``` - -This is a very simple case, of course. In more complex cases, we may -wish to have more than one reference to the value that was borrowed -- -in those cases, something like the `Rc` or `Arc` types may be useful. - -This error also frequently arises with iteration: - -```compile_fail,E0626 -# #![feature(generators, generator_trait, pin)] -# use std::ops::Generator; -# use std::pin::Pin; -let mut b = || { - let v = vec![1,2,3]; - for &x in &v { // <-- borrow of `v` is still in scope... - yield x; // ...when this yield occurs. - } -}; -Pin::new(&mut b).resume(); -``` - -Such cases can sometimes be resolved by iterating "by value" (or using -`into_iter()`) to avoid borrowing: - -``` -# #![feature(generators, generator_trait, pin)] -# use std::ops::Generator; -# use std::pin::Pin; -let mut b = || { - let v = vec![1,2,3]; - for x in v { // <-- Take ownership of the values instead! - yield x; // <-- Now yield is OK. - } -}; -Pin::new(&mut b).resume(); -``` - -If taking ownership is not an option, using indices can work too: - -``` -# #![feature(generators, generator_trait, pin)] -# use std::ops::Generator; -# use std::pin::Pin; -let mut b = || { - let v = vec![1,2,3]; - let len = v.len(); // (*) - for i in 0..len { - let x = v[i]; // (*) - yield x; // <-- Now yield is OK. - } -}; -Pin::new(&mut b).resume(); - -// (*) -- Unfortunately, these temporaries are currently required. -// See . -``` -"##, - -E0712: r##" -This error occurs because a borrow of a thread-local variable was made inside a -function which outlived the lifetime of the function. - -Erroneous code example: - -```compile_fail,E0712 -#![feature(thread_local)] - -#[thread_local] -static FOO: u8 = 3; - -fn main() { - let a = &FOO; // error: thread-local variable borrowed past end of function - - std::thread::spawn(move || { - println!("{}", a); - }); -} -``` -"##, - -E0713: r##" -This error occurs when an attempt is made to borrow state past the end of the -lifetime of a type that implements the `Drop` trait. - -Erroneous code example: - -```compile_fail,E0713 -#![feature(nll)] - -pub struct S<'a> { data: &'a mut String } - -impl<'a> Drop for S<'a> { - fn drop(&mut self) { self.data.push_str("being dropped"); } -} - -fn demo<'a>(s: S<'a>) -> &'a mut String { let p = &mut *s.data; p } -``` - -Here, `demo` tries to borrow the string data held within its -argument `s` and then return that borrow. However, `S` is -declared as implementing `Drop`. - -Structs implementing the `Drop` trait have an implicit destructor that -gets called when they go out of scope. This destructor gets exclusive -access to the fields of the struct when it runs. - -This means that when `s` reaches the end of `demo`, its destructor -gets exclusive access to its `&mut`-borrowed string data. allowing -another borrow of that string data (`p`), to exist across the drop of -`s` would be a violation of the principle that `&mut`-borrows have -exclusive, unaliased access to their referenced data. - -This error can be fixed by changing `demo` so that the destructor does -not run while the string-data is borrowed; for example by taking `S` -by reference: - -``` -pub struct S<'a> { data: &'a mut String } - -impl<'a> Drop for S<'a> { - fn drop(&mut self) { self.data.push_str("being dropped"); } -} - -fn demo<'a>(s: &'a mut S<'a>) -> &'a mut String { let p = &mut *(*s).data; p } -``` - -Note that this approach needs a reference to S with lifetime `'a`. -Nothing shorter than `'a` will suffice: a shorter lifetime would imply -that after `demo` finishes executing, something else (such as the -destructor!) could access `s.data` after the end of that shorter -lifetime, which would again violate the `&mut`-borrow's exclusive -access. -"##, - -E0716: r##" -This error indicates that a temporary value is being dropped -while a borrow is still in active use. - -Erroneous code example: - -```compile_fail,E0716 -fn foo() -> i32 { 22 } -fn bar(x: &i32) -> &i32 { x } -let p = bar(&foo()); - // ------ creates a temporary -let q = *p; -``` - -Here, the expression `&foo()` is borrowing the expression -`foo()`. As `foo()` is a call to a function, and not the name of -a variable, this creates a **temporary** -- that temporary stores -the return value from `foo()` so that it can be borrowed. -You could imagine that `let p = bar(&foo());` is equivalent -to this: - -```compile_fail,E0597 -# fn foo() -> i32 { 22 } -# fn bar(x: &i32) -> &i32 { x } -let p = { - let tmp = foo(); // the temporary - bar(&tmp) -}; // <-- tmp is freed as we exit this block -let q = p; -``` - -Whenever a temporary is created, it is automatically dropped (freed) -according to fixed rules. Ordinarily, the temporary is dropped -at the end of the enclosing statement -- in this case, after the `let`. -This is illustrated in the example above by showing that `tmp` would -be freed as we exit the block. - -To fix this problem, you need to create a local variable -to store the value in rather than relying on a temporary. -For example, you might change the original program to -the following: - -``` -fn foo() -> i32 { 22 } -fn bar(x: &i32) -> &i32 { x } -let value = foo(); // dropped at the end of the enclosing block -let p = bar(&value); -let q = *p; -``` - -By introducing the explicit `let value`, we allocate storage -that will last until the end of the enclosing block (when `value` -goes out of scope). When we borrow `&value`, we are borrowing a -local variable that already exists, and hence no temporary is created. - -Temporaries are not always dropped at the end of the enclosing -statement. In simple cases where the `&` expression is immediately -stored into a variable, the compiler will automatically extend -the lifetime of the temporary until the end of the enclosing -block. Therefore, an alternative way to fix the original -program is to write `let tmp = &foo()` and not `let tmp = foo()`: - -``` -fn foo() -> i32 { 22 } -fn bar(x: &i32) -> &i32 { x } -let value = &foo(); -let p = bar(value); -let q = *p; -``` - -Here, we are still borrowing `foo()`, but as the borrow is assigned -directly into a variable, the temporary will not be dropped until -the end of the enclosing block. Similar rules apply when temporaries -are stored into aggregate structures like a tuple or struct: - -``` -// Here, two temporaries are created, but -// as they are stored directly into `value`, -// they are not dropped until the end of the -// enclosing block. -fn foo() -> i32 { 22 } -let value = (&foo(), &foo()); -``` -"##, - -E0723: r##" -An feature unstable in `const` contexts was used. - -Erroneous code example: - -```compile_fail,E0723 -trait T {} - -impl T for () {} - -const fn foo() -> impl T { // error: `impl Trait` in const fn is unstable - () -} -``` - -To enable this feature on a nightly version of rustc, add the `const_fn` -feature flag: - -``` -#![feature(const_fn)] - -trait T {} - -impl T for () {} - -const fn foo() -> impl T { - () -} -``` -"##, - -E0729: r##" -Support for Non-Lexical Lifetimes (NLL) has been included in the Rust compiler -since 1.31, and has been enabled on the 2015 edition since 1.36. The new borrow -checker for NLL uncovered some bugs in the old borrow checker, which in some -cases allowed unsound code to compile, resulting in memory safety issues. - -### What do I do? - -Change your code so the warning does no longer trigger. For backwards -compatibility, this unsound code may still compile (with a warning) right now. -However, at some point in the future, the compiler will no longer accept this -code and will throw a hard error. - -### Shouldn't you fix the old borrow checker? - -The old borrow checker has known soundness issues that are basically impossible -to fix. The new NLL-based borrow checker is the fix. - -### Can I turn these warnings into errors by denying a lint? - -No. - -### When are these warnings going to turn into errors? - -No formal timeline for turning the warnings into errors has been set. See -[GitHub issue 58781](https://github.com/rust-lang/rust/issues/58781) for more -information. - -### Why do I get this message with code that doesn't involve borrowing? - -There are some known bugs that trigger this message. -"##, - -; - -// E0008, // cannot bind by-move into a pattern guard -// E0298, // cannot compare constants -// E0299, // mismatched types between arms -// E0471, // constant evaluation error (in pattern) -// E0385, // {} in an aliasable location - E0521, // borrowed data escapes outside of closure -// E0526, // shuffle indices are not constant -// E0598, // lifetime of {} is too short to guarantee its contents can be... - E0625, // thread-local statics cannot be accessed at compile-time -}