Stabilize const_maybe_uninit_zeroed

Make `MaybeUninit::zeroed` const stable. Newly stable API:

    // core::mem
    impl<T> MaybeUninit<T> {
        pub const fn zeroed() -> MaybeUninit<T>;
    }

Use of `const_mut_refs` should be acceptable since we do not leak the
mutability.

Tracking issue: rust-lang#91850

library/alloc/src/lib.rs

@@ -113,7 +113,6 @@
 #![feature(const_eval_select)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_write)]
-#![feature(const_maybe_uninit_zeroed)]
 #![feature(const_pin)]
 #![feature(const_refs_to_cell)]
 #![feature(const_size_of_val)]

library/core/src/mem/maybe_uninit.rs

@@ -374,6 +374,20 @@ impl<T> MaybeUninit<T> {
     /// assert_eq!(x, (0, false));
     /// ```
     ///
+    /// This can be used in const contexts, such as to indicate the end of plugin registration
+    /// arrays.
+    ///
+    /// ```
+    /// use std::mem::MaybeUninit;
+    ///
+    /// struct PluginInfo { id: u32, action: Option<fn(u32) -> u32> }
+    ///
+    /// static PLUGIN_LIST: [PluginInfo; 2] = [
+    ///     PluginInfo { id: 1, action: Some(|x| x + 5) },
+    ///     unsafe { MaybeUninit::zeroed().assume_init() }
+    /// ];
+    /// ```
+    ///
     /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()`
     /// when `0` is not a valid bit-pattern for the type:
     ///
@@ -387,17 +401,19 @@ impl<T> MaybeUninit<T> {
     /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant.
     /// // This is undefined behavior. ⚠️
     /// ```
-    #[stable(feature = "maybe_uninit", since = "1.36.0")]
-    #[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")]
-    #[must_use]
     #[inline]
+    #[must_use]
     #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
+    #[stable(feature = "maybe_uninit", since = "1.36.0")]
+    // 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 = "CURRENT_RUSTC_VERSION")]
     pub const fn zeroed() -> MaybeUninit<T> {
         let mut u = MaybeUninit::<T>::uninit();
         // SAFETY: `u.as_mut_ptr()` points to allocated memory.
-        unsafe {
-            u.as_mut_ptr().write_bytes(0u8, 1);
-        }
+        unsafe { u.as_mut_ptr().write_bytes(0u8, 1) };
         u
     }