Rollup merge of #130542 - ultrabear:stabilize_const_maybeuninit_as_mut_ptr, r=workingjubilee

Stabilize const `MaybeUninit::as_mut_ptr`

This PR stabilizes the following APIs as const stable as of rust `1.83`:
```rs
impl<T> MaybeUninit<T> {
    pub const fn as_mut_ptr(&mut self) -> *mut T;
}
```
This is made possible by const_mut_refs being stabilized (yay).

FCP: #75251 [(comment)](#75251 (comment))

Author: workingjubilee

Diff for: library/alloc/src/lib.rs

@@ -110,7 +110,6 @@
 #![feature(const_cow_is_borrowed)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
-#![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_write)]
 #![feature(const_option)]
 #![feature(const_pin)]

Diff for: library/core/src/lib.rs

@@ -131,7 +131,6 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]
-#![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_nonnull_new)]
 #![feature(const_num_midpoint)]

Diff for: library/core/src/mem/maybe_uninit.rs

@@ -393,7 +393,6 @@ impl<T> MaybeUninit<T> {
     // These are OK to allow since we do not leak &mut to user-visible API
     #[rustc_allow_const_fn_unstable(const_mut_refs)]
     #[rustc_allow_const_fn_unstable(const_ptr_write)]
-    #[rustc_allow_const_fn_unstable(const_maybe_uninit_as_mut_ptr)]
     #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "1.75.0")]
     pub const fn zeroed() -> MaybeUninit<T> {
         let mut u = MaybeUninit::<T>::uninit();
@@ -570,7 +569,11 @@ impl<T> MaybeUninit<T> {
     /// (Notice that the rules around references to uninitialized data are not finalized yet, but
     /// until they are, it is advisable to avoid them.)
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
-    #[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
+    #[rustc_const_stable(
+        feature = "const_maybe_uninit_as_mut_ptr",
+        since = "CURRENT_RUSTC_VERSION"
+    )]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_mut_refs))]
     #[inline(always)]
     pub const fn as_mut_ptr(&mut self) -> *mut T {
         // `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.

Diff for: library/core/tests/lib.rs

@@ -26,7 +26,6 @@
 #![feature(const_ipv4)]
 #![feature(const_ipv6)]
 #![feature(const_likely)]
-#![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_nonnull_new)]
 #![feature(const_option)]
 #![feature(const_option_ext)]