fix bootstrap not knowing const_mut_refs is stable

Author: ultrabear

library/core/src/array/mod.rs

@@ -147,6 +147,7 @@ pub const fn from_ref<T>(s: &T) -> &[T; 1] {
 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
 #[stable(feature = "array_from_ref", since = "1.53.0")]
 #[rustc_const_stable(feature = "const_array_from_ref", since = "CURRENT_RUSTC_VERSION")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
 pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }

library/core/src/slice/raw.rs

@@ -204,6 +204,7 @@ pub const fn from_ref<T>(s: &T) -> &[T] {
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
 #[rustc_const_stable(feature = "const_slice_from_ref", since = "CURRENT_RUSTC_VERSION")]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
 #[must_use]
 pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
     array::from_mut(s)