Replace `MaybeUninit::uninit_array()` with array repeat expression.

This is possible now that inline const blocks are stable; the idea was
even mentioned as an alternative when `uninit_array()` was added:
<https://github.com/rust-lang/rust/pull/65580#issuecomment-544200681>

> if it’s stabilized soon enough maybe it’s not worth having a
> standard library method that will be replaceable with
> `let buffer = [MaybeUninit::<T>::uninit(); $N];`

Const array repetition and inline const blocks are now stable (in the
next release), so that circumstance has come to pass, and we no longer
have reason to want `uninit_array()` other than convenience. Therefore,
let’s evaluate the inconvenience by not using `uninit_array()` in
the standard library, before potentially deleting it entirely.

3 files changed, 2 insertions, 3 deletions

diff --git a/library/alloc/src/collections/vec_deque/into_iter.rs b/library/alloc/src/collections/vec_deque/into_iter.rs
index 692af7c197a..4747517393c 100644
--- a/library/alloc/src/collections/vec_deque/into_iter.rs
+++ b/library/alloc/src/collections/vec_deque/into_iter.rs
@@ -132,7 +132,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     fn next_chunk<const N: usize>(
         &mut self,
     ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>> {
-        let mut raw_arr = MaybeUninit::uninit_array();
+        let mut raw_arr = [const { MaybeUninit::uninit() }; N];
         let raw_arr_ptr = raw_arr.as_mut_ptr().cast();
         let (head, tail) = self.inner.as_slices();
 
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index ecb019b49c6..a1a2404a2ee 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -135,7 +135,6 @@
 #![feature(layout_for_ptr)]
 #![feature(local_waker)]
 #![feature(maybe_uninit_slice)]
-#![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(panic_internals)]
 #![feature(pattern)]
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index c4798933770..3bd89eaa6cb 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -254,7 +254,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn next_chunk<const N: usize>(&mut self) -> Result<[T; N], core::array::IntoIter<T, N>> {
-        let mut raw_ary = MaybeUninit::uninit_array();
+        let mut raw_ary = [const { MaybeUninit::uninit() }; N];
 
         let len = self.len();