Add support for declaring 'const fn'

Add a new feature to enable this, since `const extern fn` support is unstable
rust-lang · bors · Nov 18, 2019 · Sep 30, 2019 · Oct 29, 2019 · Oct 29, 2019
commit 5dfc2c82854eede618f35c4511e941506f0e76d0
diff --git 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
@@ -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
@@ -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
@@ -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
@@ -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::<usize>() - 1 & !(::mem::size_of::<usize>() - 1)
+const_fn! {
+    {const} fn CMSG_ALIGN(len: usize) -> usize {
+        len + ::mem::size_of::<usize>() - 1 & !(::mem::size_of::<usize>() - 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::<cmsghdr>()))
             as ::c_uint
     }