Remove internal and unstable MaybeUninit::UNINIT.

Looks like it is no longer necessary, as uninit_array() can be used
instead in the few cases where it was needed.

library/alloc/src/collections/btree/node.rs

@@ -78,8 +78,8 @@ unsafe fn new() -> Self {
         LeafNode {
             // As a general policy, we leave fields uninitialized if they can be, as this should
             // be both slightly faster and easier to track in Valgrind.
-            keys: [MaybeUninit::UNINIT; CAPACITY],
-            vals: [MaybeUninit::UNINIT; CAPACITY],
+            keys: MaybeUninit::uninit_array(),
+            vals: MaybeUninit::uninit_array(),
             parent: ptr::null(),
             parent_idx: MaybeUninit::uninit(),
             len: 0,
@@ -111,7 +111,7 @@ impl<K, V> InternalNode<K, V> {
     /// `len` of 0), there must be one initialized and valid edge. This function does not set up
     /// such an edge.
     unsafe fn new() -> Self {
-        InternalNode { data: unsafe { LeafNode::new() }, edges: [MaybeUninit::UNINIT; 2 * B] }
+        InternalNode { data: unsafe { LeafNode::new() }, edges: MaybeUninit::uninit_array() }
     }
 }
 

library/alloc/src/lib.rs

@@ -100,7 +100,6 @@
 #![feature(fn_traits)]
 #![feature(fundamental)]
 #![feature(inplace_iteration)]
-#![feature(internal_uninit_const)]
 #![feature(lang_items)]
 #![feature(layout_for_ptr)]
 #![feature(libc)]
@@ -134,7 +133,7 @@
 #![feature(unsized_locals)]
 #![feature(allocator_internals)]
 #![feature(slice_partition_dedup)]
-#![feature(maybe_uninit_extra, maybe_uninit_slice)]
+#![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_uninit_array)]
 #![feature(alloc_layout_extra)]
 #![feature(trusted_random_access)]
 #![feature(try_trait)]

library/core/src/mem/maybe_uninit.rs

@@ -305,14 +305,6 @@ pub const fn uninit() -> MaybeUninit<T> {
         unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
     }
 
-    /// A promotable constant, equivalent to `uninit()`.
-    #[unstable(
-        feature = "internal_uninit_const",
-        issue = "none",
-        reason = "hack to work around promotability"
-    )]
-    pub const UNINIT: Self = Self::uninit();
-
     /// Creates a new `MaybeUninit<T>` in an uninitialized state, with the memory being
     /// filled with `0` bytes. It depends on `T` whether that already makes for
     /// proper initialization. For example, `MaybeUninit<usize>::zeroed()` is initialized,