Gate emitting 'const extern fn' behind a feature

Author: Aaron1011

Cargo.toml

@@ -28,6 +28,7 @@ std = []
 align = []
 rustc-dep-of-std = ['align', 'rustc-std-workspace-core']
 extra_traits = []
+const-fn = []
 # use_std is deprecated, use `std` instead
 use_std = [ 'std' ]
 

src/lib.rs

@@ -34,7 +34,7 @@
 #![no_std]
 #![cfg_attr(feature = "rustc-dep-of-std", no_core)]
 #![cfg_attr(target_os = "redox", feature(static_nobundle))]
-#![feature(const_extern_fn)]
+#![cfg_attr(feature = "const-fn", feature(const_extern_fn))]
 
 #[macro_use]
 mod macros;

src/macros.rs

@@ -121,16 +121,31 @@ macro_rules! s_no_extra_traits {
     );
 }
 
-#[allow(unused_macros)]
-macro_rules! f {
-    ($(pub fn $i:ident($($arg:ident: $argty:ty),*) $($constness:ident)* -> $ret:ty  {
-        $($body:stmt);*
-    })*) => ($(
-        #[inline]
-        pub $($constness)* unsafe extern fn $i($($arg: $argty),*) -> $ret {
-            $($body);*
+cfg_if! {
+    if #[cfg(feature = "const-fn")] {
+        #[allow(unused_macros)]
+        macro_rules! f {
+            ($(pub fn $i:ident($($arg:ident: $argty:ty),*) $($constness:ident)* -> $ret:ty  {
+                $($body:stmt);*
+            })*) => ($(
+                #[inline]
+                pub $($constness)* unsafe extern fn $i($($arg: $argty),*) -> $ret {
+                    $($body);*
+                }
+            )*)
         }
-    )*)
+    } else {
+        macro_rules! f {
+            ($(pub fn $i:ident($($arg:ident: $argty:ty),*) $($constness:ident)* -> $ret:ty  {
+                $($body:stmt);*
+            })*) => ($(
+                #[inline]
+                pub unsafe extern fn $i($($arg: $argty),*) -> $ret {
+                    $($body);*
+                }
+            )*)
+        }
+    }
 }
 
 #[allow(unused_macros)]