rustc: Add `const` globals to the language #17853

Remove a bunch of two-space tabs

This change is an implementation of [RFC 69][rfc] which adds a third kind of global to the language, `const`. This global is most similar to what the old `static` was, and if you're unsure about what to use then you should use a `const`. The semantics of these three kinds of globals are: * A `const` does not represent a memory location, but only a value. Constants are translated as rvalues, which means that their values are directly inlined at usage location (similar to a #define in C/C++). Constant values are, well, constant, and can not be modified. Any "modification" is actually a modification to a local value on the stack rather than the actual constant itself. Almost all values are allowed inside constants, whether they have interior mutability or not. There are a few minor restrictions listed in the RFC, but they should in general not come up too often. * A `static` now always represents a memory location (unconditionally). Any references to the same `static` are actually a reference to the same memory location. Only values whose types ascribe to `Sync` are allowed in a `static`. This restriction is in place because many threads may access a `static` concurrently. Lifting this restriction (and allowing unsafe access) is a future extension not implemented at this time. * A `static mut` continues to always represent a memory location. All references to a `static mut` continue to be `unsafe`. This is a large breaking change, and many programs will need to be updated accordingly. A summary of the breaking changes is: * Statics may no longer be used in patterns. Statics now always represent a memory location, which can sometimes be modified. To fix code, repurpose the matched-on-`static` to a `const`. static FOO: uint = 4; match n { FOO => { /* ... */ } _ => { /* ... */ } } change this code to: const FOO: uint = 4; match n { FOO => { /* ... */ } _ => { /* ... */ } } * Statics may no longer refer to other statics by value. Due to statics being able to change at runtime, allowing them to reference one another could possibly lead to confusing semantics. If you are in this situation, use a constant initializer instead. Note, however, that statics may reference other statics by address, however. * Statics may no longer be used in constant expressions, such as array lengths. This is due to the same restrictions as listed above. Use a `const` instead. [breaking-change] [rfc]: rust-lang/rfcs#246

The tables in libunicode are far too large to want to be inlined into any other program, so these tables are all going to remain `static`. For them to be legal, they cannot reference one another by value, but instead use references now. This commit also modifies the src/etc/unicode.py script to generate the right tables.

This crate is largely just one giant header file, so there's no need for any of these values to actually have a memory location, they're all just basically a regular #define.

This leaves the ziggurat tables as `pub static` as they're likely too large to want to go into the metadata anyway.

This commit repurposes most statics as constants in the standard library itself, with the exception of TLS keys which precisely have their own memory location as an implementation detail. This commit also rewrites the bitflags syntax to use `const` instead of `static`. All invocations will need to replace the word `static` with `const` when declaring flags. Due to the modification of the `bitflags!` syntax, this is a: [breaking-change]

This require a bit of finesse to work around the changes with libunicode, but nothing too major!

At the same time, migrate statics to constants.

Instead of returning &'static [u8], an invocation of `bytes!()` now returns `&'static [u8, ..N]` where `N` is the length of the byte vector. This should functionally be the same, but there are some cases where an explicit cast may be needed, so this is a: [breaking-change]

Additionally, add lots of tests for new functionality around statics and `static mut`.

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

rustc: Add `const` globals to the language #17853

rustc: Add `const` globals to the language #17853

Commits on Oct 9, 2014

rustc: Add const globals to the language #17853

rustc: Add const globals to the language #17853

Commits on Oct 9, 2014

rustc: Add `const` globals to the language #17853

rustc: Add `const` globals to the language #17853