library: Track const instability by maybe_uninit_slice

Co-authored-by: Ralf Jung <[email protected]>

Author: workingjubilee

library/core/src/mem/maybe_uninit.rs

@@ -1004,9 +1004,9 @@ impl<T> MaybeUninit<T> {
     /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
     #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
-    #[rustc_const_stable(
-        feature = "const_maybe_uninit_assume_init",
-        since = "CURRENT_RUSTC_VERSION"
+    #[rustc_const_unstable(
+        feature = "maybe_uninit_slice",
+        issue = "63569"
     )]
     #[inline(always)]
     pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {