Rollup merge of rust-lang#96099 - clarfonthey:maybeuninit_array_cleanup, r=dtolnay

MaybeUninit array cleanup

* Links `MaybeUninit::uninit_array` to meta-tracking issue
* Links `MaybeUninit::array_assume_init` to meta-tracking issue
* Unstably constifies `MaybeUninit::array_assume_init`

Another thing worth mentioning: this splits the const feature flag for `maybe_uninit_uninit_array` into `const_maybe_uninit_uninit_array` to avoid weird cases where only one gets stabilised.

Note that it may be desired to keep the `array_assume_init` method linked to its dedicated issue, but at least for now, I decided to link to the meta-tracking issue so that all of the methods lead users to the same place. But I can revert that bit if desired.

The meta-tracking issue that I filed is rust-lang#96097.

Author: Dylan-DPC

library/core/src/lib.rs

@@ -117,6 +117,7 @@
 #![feature(const_intrinsic_copy)]
 #![feature(const_intrinsic_forget)]
 #![feature(const_likely)]
+#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_num_from_num)]

library/core/src/mem/maybe_uninit.rs

@@ -350,13 +350,13 @@ impl<T> MaybeUninit<T> {
     /// let mut buf: [MaybeUninit<u8>; 32] = MaybeUninit::uninit_array();
     /// let data = read(&mut buf);
     /// ```
-    #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
-    #[rustc_const_unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
+    #[unstable(feature = "maybe_uninit_uninit_array", issue = "96097")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_uninit_array", issue = "96097")]
     #[must_use]
     #[inline(always)]
-    pub const fn uninit_array<const LEN: usize>() -> [Self; LEN] {
+    pub const fn uninit_array<const N: usize>() -> [Self; N] {
         // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
-        unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
+        unsafe { MaybeUninit::<[MaybeUninit<T>; N]>::uninit().assume_init() }
     }
 
     /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
@@ -942,19 +942,24 @@ impl<T> MaybeUninit<T> {
     ///
     /// assert_eq!(array, [0, 1, 2]);
     /// ```
-    #[unstable(feature = "maybe_uninit_array_assume_init", issue = "80908")]
+    #[unstable(feature = "maybe_uninit_array_assume_init", issue = "96097")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_array_assume_init", issue = "96097")]
     #[inline(always)]
     #[track_caller]
-    pub unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
+    pub const unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
         // SAFETY:
         // * The caller guarantees that all elements of the array are initialized
         // * `MaybeUninit<T>` and T are guaranteed to have the same layout
         // * `MaybeUninit` does not drop, so there are no double-frees
         // And thus the conversion is safe
-        unsafe {
+        let ret = unsafe {
             intrinsics::assert_inhabited::<[T; N]>();
             (&array as *const _ as *const [T; N]).read()
-        }
+        };
+
+        // FIXME: required to avoid `~const Destruct` bound
+        super::forget(array);
+        ret
     }
 
     /// Assuming all the elements are initialized, get a slice to them.