Auto merge of rust-lang#60445 - RalfJung:maybe-uninit, r=Centril

stabilize core parts of MaybeUninit

and deprecate mem::uninitialized in the future (1.40.0). This is part of implementing rust-lang/rfcs#1892.

Also expand the documentation a bit.

This type is currently primarily useful when dealing with partially initialized arrays. In libstd, it is used e.g. in `BTreeMap` (with some unstable APIs that however can all be replaced, less ergonomically, by stable ones). What we stabilize should also be enough for `SmallVec` (Cc @bluss).

Making this useful for structs requires rust-lang/rfcs#2582 or a commitment that references to uninitialized data are not insta-UB.

src/liballoc/lib.rs

@@ -109,7 +109,7 @@
 #![feature(rustc_const_unstable)]
 #![feature(const_vec_new)]
 #![feature(slice_partition_dedup)]
-#![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
+#![feature(maybe_uninit_extra, maybe_uninit_slice, maybe_uninit_array)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
 #![feature(iter_nth_back)]

src/libcore/lib.rs

@@ -125,7 +125,7 @@
 #![feature(structural_match)]
 #![feature(abi_unadjusted)]
 #![feature(adx_target_feature)]
-#![feature(maybe_uninit, maybe_uninit_slice, maybe_uninit_array)]
+#![feature(maybe_uninit_slice, maybe_uninit_array)]
 #![feature(external_doc)]
 
 #[prelude_import]