Partially stabilize maybe_uninit_extra

This covers:

    impl<T> MaybeUninit<T> {
        pub unsafe fn assume_init_read(&self) -> T { ... }
        pub unsafe fn assume_init_drop(&mut self) { ... }
    }

It does not cover the const-ness of `write` under
`const_maybe_uninit_write` nor the const-ness of
`assume_init_read` (this commit adds
`const_maybe_uninit_assume_init_read` for that).

FCP: https://github.com/rust-lang/rust/issues/63567#issuecomment-958590287.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

library/alloc/src/lib.rs

@@ -119,7 +119,6 @@
 #![feature(inplace_iteration)]
 #![feature(iter_advance_by)]
 #![feature(layout_for_ptr)]
-#![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_slice)]
 #![cfg_attr(test, feature(new_uninit))]
 #![feature(nonnull_slice_from_raw_parts)]

library/core/src/mem/maybe_uninit.rs

@@ -330,7 +330,7 @@ impl<T> MaybeUninit<T> {
     /// # Examples
     ///
     /// ```no_run
-    /// #![feature(maybe_uninit_uninit_array, maybe_uninit_extra, maybe_uninit_slice)]
+    /// #![feature(maybe_uninit_uninit_array, maybe_uninit_slice)]
     ///
     /// use std::mem::MaybeUninit;
     ///
@@ -662,7 +662,6 @@ impl<T> MaybeUninit<T> {
     /// Correct usage of this method:
     ///
     /// ```rust
-    /// #![feature(maybe_uninit_extra)]
     /// use std::mem::MaybeUninit;
     ///
     /// let mut x = MaybeUninit::<u32>::uninit();
@@ -683,7 +682,6 @@ impl<T> MaybeUninit<T> {
     /// *Incorrect* usage of this method:
     ///
     /// ```rust,no_run
-    /// #![feature(maybe_uninit_extra)]
     /// use std::mem::MaybeUninit;
     ///
     /// let mut x = MaybeUninit::<Option<Vec<u32>>>::uninit();
@@ -693,8 +691,8 @@ impl<T> MaybeUninit<T> {
     /// // We now created two copies of the same vector, leading to a double-free ⚠️ when
     /// // they both get dropped!
     /// ```
-    #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
-    #[rustc_const_unstable(feature = "maybe_uninit_extra", issue = "63567")]
+    #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init_read", issue = "63567")]
     #[inline(always)]
     #[track_caller]
     pub const unsafe fn assume_init_read(&self) -> T {
@@ -728,7 +726,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init`]: MaybeUninit::assume_init
     /// [`Vec<T>`]: ../../std/vec/struct.Vec.html
-    #[unstable(feature = "maybe_uninit_extra", issue = "63567")]
+    #[stable(feature = "maybe_uninit_extra", since = "1.60.0")]
     pub unsafe fn assume_init_drop(&mut self) {
         // SAFETY: the caller must guarantee that `self` is initialized and
         // satisfies all invariants of `T`.

library/core/tests/lib.rs

@@ -15,6 +15,7 @@
 #![feature(const_convert)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
+#![feature(const_maybe_uninit_assume_init_read)]
 #![feature(const_num_from_num)]
 #![feature(const_ptr_read)]
 #![feature(const_ptr_write)]
@@ -46,7 +47,6 @@
 #![feature(slice_take)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_array_assume_init)]
-#![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_write_slice)]
 #![feature(min_specialization)]
 #![feature(numfmt)]

library/std/src/lib.rs

@@ -297,7 +297,6 @@
 #![feature(llvm_asm)]
 #![feature(log_syntax)]
 #![feature(map_try_insert)]
-#![feature(maybe_uninit_extra)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_write_slice)]