Add feature gate for &mut in const fns

Author: pvdrz

src/librustc_feature/active.rs

@@ -527,6 +527,9 @@ declare_features! (
     /// Allows the use of `#[cfg(sanitize = "option")]`; set when -Zsanitizer is used.
     (active, cfg_sanitize, "1.41.0", Some(39699), None),
 
+    /// Allows using `&mut` in constant functions.
+    (active, const_fn_mut_refs, "1.41.0", None, None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/libsyntax_pos/symbol.rs

@@ -203,6 +203,7 @@ symbols! {
         const_constructor,
         const_extern_fn,
         const_fn,
+        const_fn_mut_refs,
         const_fn_union,
         const_generics,
         const_if_match,