Stabilize const_fn_union

Author: jhpratt

core/src/lib.rs

@@ -86,7 +86,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(const_panic)]
 #![feature(const_pin)]
-#![feature(const_fn_union)]
+#![cfg_attr(bootstrap, feature(const_fn_union))]
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]

core/src/slice/mod.rs

@@ -100,7 +100,7 @@ impl<T> [T] {
     #[rustc_const_stable(feature = "const_slice_len", since = "1.39.0")]
     #[inline]
     // SAFETY: const sound because we transmute out the length field as a usize (which it must be)
-    #[rustc_allow_const_fn_unstable(const_fn_union)]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_union))]
     pub const fn len(&self) -> usize {
         // FIXME: Replace with `crate::ptr::metadata(self)` when that is const-stable.
         // As of this writing this causes a "Const-stable functions can only call other