From 5dfc2c82854eede618f35c4511e941506f0e76d0 Mon Sep 17 00:00:00 2001 From: Aaron Hill Date: Mon, 30 Sep 2019 16:02:01 -0400 Subject: [PATCH] Add support for declaring 'const fn' Add a new feature to enable this, since `const extern fn` support is unstable --- Cargo.toml | 1 + README.md | 3 ++ src/lib.rs | 1 + src/macros.rs | 98 ++++++++++++++++++++++++++++++++++---- src/unix/linux_like/mod.rs | 8 ++-- 5 files changed, 99 insertions(+), 12 deletions(-) diff --git a/Cargo.toml b/Cargo.toml index e48a34cdfe9a2..a0658132492b2 100644 --- a/Cargo.toml +++ b/Cargo.toml @@ -28,6 +28,7 @@ std = [] align = [] rustc-dep-of-std = ['align', 'rustc-std-workspace-core'] extra_traits = [] +const-extern-fn = [] # use_std is deprecated, use `std` instead use_std = [ 'std' ] diff --git a/README.md b/README.md index 5371916acfec3..dc1d6d05de029 100644 --- a/README.md +++ b/README.md @@ -35,6 +35,9 @@ libc = "0.2" * `extra_traits`: all `struct`s implemented in `libc` are `Copy` and `Clone`. This feature derives `Debug`, `Eq`, `Hash`, and `PartialEq`. +* `const-extern-fn`: Changes some `extern fn`s into `const extern fn`s. + This features requires a nightly rustc + * **deprecated**: `use_std` is deprecated, and is equivalent to `std`. ## Rust version support diff --git a/src/lib.rs b/src/lib.rs index 3255303e5a9b0..40625ac98dc69 100644 --- a/src/lib.rs +++ b/src/lib.rs @@ -34,6 +34,7 @@ #![no_std] #![cfg_attr(feature = "rustc-dep-of-std", no_core)] #![cfg_attr(target_os = "redox", feature(static_nobundle))] +#![cfg_attr(feature = "const-extern-fn", feature(const_extern_fn))] #[macro_use] mod macros; diff --git a/src/macros.rs b/src/macros.rs index 14a28046640c6..da39270633274 100644 --- a/src/macros.rs +++ b/src/macros.rs @@ -121,16 +121,96 @@ macro_rules! s_no_extra_traits { ); } -#[allow(unused_macros)] -macro_rules! f { - ($(pub fn $i:ident($($arg:ident: $argty:ty),*) -> $ret:ty { - $($body:stmt);* - })*) => ($( - #[inline] - pub unsafe extern fn $i($($arg: $argty),*) -> $ret { - $($body);* +// This is a pretty horrible hack to allow us to conditionally mark +// some functions as 'const', without requiring users of this macro +// to care about the "const-extern-fn" feature. +// +// When 'const-extern-fn' is enabled, we emit the captured 'const' keyword +// in the expanded function. +// +// When 'const-extern-fn' is disabled, we always emit a plain 'pub unsafe extern fn'. +// Note that the expression matched by the macro is exactly the same - this allows +// users of this macro to work whether or not 'const-extern-fn' is enabled +// +// Unfortunately, we need to duplicate most of this macro between the 'cfg_if' blocks. +// This is because 'const unsafe extern fn' won't even parse on older compilers, +// so we need to avoid emitting it at all of 'const-extern-fn'. +// +// Specifically, moving the 'cfg_if' into the macro body will *not* work. +// Doing so would cause the '#[cfg(feature = "const-extern-fn")]' to be emiited +// into user code. The 'cfg' gate will not stop Rust from trying to parse the +// 'pub const unsafe extern fn', so users would get a compiler error even when +// the 'const-extern-fn' feature is disabled +// +// Note that users of this macro need to place 'const' in a weird position +// (after the closing ')' for the arguments, but before the return type). +// This was the only way I could satisfy the following two requirements: +// 1. Avoid ambuguity errors from 'macro_rules!' (which happen when writing '$foo:ident fn' +// 2. Allow users of this macro to mix 'pub fn foo' and 'pub const fn bar' within the same +// 'f!' block +cfg_if! { + if #[cfg(feature = "const-extern-fn")] { + #[allow(unused_macros)] + macro_rules! f { + ($(pub $({$constness:ident})* fn $i:ident( + $($arg:ident: $argty:ty),* + ) -> $ret:ty { + $($body:stmt);* + })*) => ($( + #[inline] + pub $($constness)* unsafe extern fn $i($($arg: $argty),* + ) -> $ret { + $($body);* + } + )*) } - )*) + + #[allow(unused_macros)] + macro_rules! const_fn { + ($($({$constness:ident})* fn $i:ident( + $($arg:ident: $argty:ty),* + ) -> $ret:ty { + $($body:stmt);* + })*) => ($( + #[inline] + $($constness)* fn $i($($arg: $argty),* + ) -> $ret { + $($body);* + } + )*) + } + + } else { + #[allow(unused_macros)] + macro_rules! f { + ($(pub $({$constness:ident})* fn $i:ident( + $($arg:ident: $argty:ty),* + ) -> $ret:ty { + $($body:stmt);* + })*) => ($( + #[inline] + pub unsafe extern fn $i($($arg: $argty),* + ) -> $ret { + $($body);* + } + )*) + } + + #[allow(unused_macros)] + macro_rules! const_fn { + ($($({$constness:ident})* fn $i:ident( + $($arg:ident: $argty:ty),* + ) -> $ret:ty { + $($body:stmt);* + })*) => ($( + #[inline] + fn $i($($arg: $argty),* + ) -> $ret { + $($body);* + } + )*) + } + } } #[allow(unused_macros)] diff --git a/src/unix/linux_like/mod.rs b/src/unix/linux_like/mod.rs index 1568e4f8369f1..a6ec553eeeb87 100644 --- a/src/unix/linux_like/mod.rs +++ b/src/unix/linux_like/mod.rs @@ -1165,8 +1165,10 @@ pub const ARPHRD_IEEE802154: u16 = 804; pub const ARPHRD_VOID: u16 = 0xFFFF; pub const ARPHRD_NONE: u16 = 0xFFFE; -fn CMSG_ALIGN(len: usize) -> usize { - len + ::mem::size_of::() - 1 & !(::mem::size_of::() - 1) +const_fn! { + {const} fn CMSG_ALIGN(len: usize) -> usize { + len + ::mem::size_of::() - 1 & !(::mem::size_of::() - 1) + } } f! { @@ -1182,7 +1184,7 @@ f! { cmsg.offset(1) as *mut ::c_uchar } - pub fn CMSG_SPACE(length: ::c_uint) -> ::c_uint { + pub {const} fn CMSG_SPACE(length: ::c_uint) -> ::c_uint { (CMSG_ALIGN(length as usize) + CMSG_ALIGN(::mem::size_of::())) as ::c_uint }