Add support for making functions `const`
PR https://github.com/rust-lang/rust/pull/64906 adds the ability to write `const extern fn` and `const unsafe extern fn`, which will allow manys functions in `libc` to become `const`.
This is particuarly useful for functions which correspond to C macros (e.g. `CMSG_SPACE`). In C, these macros are constant expressions, allowing them to be used when declaring arrays. However, since the corresponding `libc` functions are not `const`, writing equivalent Rust code is impossible. Users must either perform an unecessary heap allocation, or pull in `bindgen` to evaluate the macro for specific values (e.g. `CMSG_SPACE(1)`).
However, the syntax `const extern fn` is not currently parsed by rust. To allow libc to use this without breaking backwards compatibility (i.e. bumping the minimum Rust version), I've taken the following approach:
1. A new off-by-default feature `extern-const-fn` is added to `libc`.
2. The internal `f!` macro has two versions, selected at compile-time by a `cfg_if`. When `extern-const-fn` is enabled, the declared `f!` macro passes through the `const` keyword from the macro user to the final definition (`pub const unsafe extern fn foo`. When `extern-const-fn` is disabled, the `const` keyword passed by the macro user is discarded, resulting in a plain `pub extern const fn` being declared.
Unfortunately, I couldn't manage to get `macro_rules` to accept a normal `const` token in the proper place (after `pub`). I had to resort to placing it in curly brackets:
```rust
pub {const} fn foo(val: u8) -> i8 {
}
```
The `f!` macro then translates this to a function definition with `const` in the proper position.
I'd appreciate it if someone who's more familiar with `macro_rules!` could see if I missed a way to get the desired syntax.
This adds more WASI support, and in particular adds support for WASI
being a target_os rather than a target_env, which relates to this PR:
https://github.com/rust-lang/rust/pull/60117
Lu, Wangshan
Guillaume Gomez
Yuki Okushi
Amanieu d'Antras
Josh Triplett
Thiébaud Weksteen
Sébastien Marie
Sam Balana
Daniil Bondarev
Glenn Hope
Jake Goulding
Andrew Walbran
Jeremy Soller
Tom Eccles
Alexander Batischev
Andre Nathan
Noah Gold
gnzlbg
bors
Aaron Hill
Wang Xuerui
Dan Gohman
Jeremy Soller
Alex Crichton
leo60228