Stabilize const_mem_zeroed

Make `core::mem::zeroed` const stable. Newly stable API:

    // core::mem
    pub const unsafe fn zeroed<T>() -> T;

This is stabilized with `const_maybe_uninit_zeroed` since it is a simple
wrapper.

In order to make this possible, intrinsics `assert_zero_valid` was made
const stable under `const_assert_type2`.
`assert_mem_uninitialized_valid` was also made const stable since it is
under the same gate.

library/core/src/intrinsics.rs

@@ -1072,15 +1072,15 @@ extern "rust-intrinsic" {
     /// zero-initialization: This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
-    #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")]
+    #[rustc_const_stable(feature = "const_assert_type2", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
     pub fn assert_zero_valid<T>();
 
     /// A guard for `std::mem::uninitialized`. This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
-    #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")]
+    #[rustc_const_stable(feature = "const_assert_type2", since = "CURRENT_RUSTC_VERSION")]
     #[rustc_safe_intrinsic]
     #[rustc_nounwind]
     pub fn assert_mem_uninitialized_valid<T>();

library/core/src/mem/mod.rs

@@ -647,7 +647,8 @@ pub const fn needs_drop<T: ?Sized>() -> bool {
 #[allow(deprecated)]
 #[rustc_diagnostic_item = "mem_zeroed"]
 #[track_caller]
-pub unsafe fn zeroed<T>() -> T {
+#[rustc_const_stable(feature = "const_mem_zeroed", since = "CURRENT_RUSTC_VERSION")]
+pub const unsafe fn zeroed<T>() -> T {
     // SAFETY: the caller must guarantee that an all-zero value is valid for `T`.
     unsafe {
         intrinsics::assert_zero_valid::<T>();

tests/ui/consts/assert-type-intrinsics.rs

@@ -1,5 +1,4 @@
 #![feature(never_type)]
-#![feature(const_assert_type2)]
 #![feature(core_intrinsics)]
 
 use std::intrinsics;