Rollup merge of #91851 - woppopo:const_maybeuninit_zeroed, r=yaahc

Make `MaybeUninit::zeroed` `const`

Tracking issue: #91850

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

library/core/src/mem/maybe_uninit.rs

@@ -394,10 +394,11 @@ impl<T> MaybeUninit<T> {
     /// // 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]
     #[rustc_diagnostic_item = "maybe_uninit_zeroed"]
-    pub fn zeroed() -> MaybeUninit<T> {
+    pub const fn zeroed() -> MaybeUninit<T> {
         let mut u = MaybeUninit::<T>::uninit();
         // SAFETY: `u.as_mut_ptr()` points to allocated memory.
         unsafe {