bors d742eedf90 Auto merge of #1536 - Aaron1011:feature/const-fn, r=gnzlbg

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.

2019-11-18 19:37:32 +00:00

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libc - Raw FFI bindings to platforms' system libraries

libc provides all of the definitions necessary to easily interoperate with C code (or "C-like" code) on each of the platforms that Rust supports. This includes type definitions (e.g. c_int), constants (e.g. EINVAL) as well as function headers (e.g. malloc).

This crate exports all underlying platform types, functions, and constants under the crate root, so all items are accessible as libc::foo. The types and values of all the exported APIs match the platform that libc is compiled for.

More detailed information about the design of this library can be found in its associated RFC.

Usage

Add the following to your Cargo.toml:

[dependencies]
libc = "0.2"

Features

std: by default libc links to the standard library. Disable this feature remove this dependency and be able to use libc in #![no_std] crates.
extra_traits: all structs implemented in libc are Copy and Clone. This feature derives Debug, Eq, Hash, and PartialEq.
const-extern-fn: Changes some extern fns into const extern fns. This features requires a nightly rustc
deprecated: use_std is deprecated, and is equivalent to std.

Rust version support

The minimum supported Rust toolchain version is Rust 1.13.0 . APIs requiring newer Rust features are only available on newer Rust toolchains:

Feature	Version
`union`	1.19.0
`const mem::size_of`	1.24.0
`repr(align)`	1.25.0
`extra_traits`	1.25.0
`core::ffi::c_void`	1.30.0
`repr(packed(N))`	1.33.0

Platform support

Platform-specific documentation (master branch).

See ci/build.sh for the platforms on which libc is guaranteed to build for each Rust toolchain. The test-matrix at Azure and Cirrus CI show the platforms in which libc tests are run.

License

This project is licensed under either of

at your option.

Contributing

We welcome all people who want to contribute. Please see the contributing instructions for more information.

Contributions in any form (issues, pull requests, etc.) to this project must adhere to Rust's Code of Conduct.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in libc by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

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