Resolved some of the comments

* Undo fn -> const fn for some fns.
* Split feature gate.
* Made all three intrinsics const

library/core/src/intrinsics.rs

@@ -815,19 +815,21 @@ extern "rust-intrinsic" {
     /// This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
+    #[rustc_const_unstable(feature = "const_assert_type", issue = "none")]
     pub fn assert_inhabited<T>();
 
     /// A guard for unsafe functions that cannot ever be executed if `T` does not permit
     /// zero-initialization: This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_assert_type", issue = "none")]
     pub fn assert_zero_valid<T>();
 
     /// A guard for unsafe functions that cannot ever be executed if `T` has invalid
     /// bit patterns: This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_assert_type", issue = "none")]
     pub fn assert_uninit_valid<T>();
 
     /// Gets a reference to a static `Location` indicating where it was called.

library/core/src/lib.rs

@@ -69,6 +69,7 @@
 #![feature(asm)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_alloc_layout)]
+#![feature(const_assert_type)]
 #![feature(const_discriminant)]
 #![feature(const_cell_into_inner)]
 #![feature(const_checked_int_methods)]
@@ -101,7 +102,7 @@
 #![feature(const_type_name)]
 #![feature(const_likely)]
 #![feature(const_unreachable_unchecked)]
-#![feature(const_maybe_assume_init)]
+#![feature(const_maybe_uninit_assume_init)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

library/core/src/mem/maybe_uninit.rs

@@ -314,8 +314,7 @@ impl<T> MaybeUninit<T> {
     /// let data = read(&mut buf);
     /// ```
     #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
-    #[rustc_allow_const_fn_unstable(const_maybe_assume_init)]
+    #[rustc_const_unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
     #[inline(always)]
     pub const fn uninit_array<const LEN: usize>() -> [Self; LEN] {
         // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
@@ -505,7 +504,7 @@ impl<T> MaybeUninit<T> {
     /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
     /// ```
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
     #[inline(always)]
     #[rustc_diagnostic_item = "assume_init"]
     pub const unsafe fn assume_init(self) -> T {
@@ -813,7 +812,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init_ref`]: MaybeUninit::assume_init_ref
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
     #[inline(always)]
     pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
         // SAFETY: casting slice to a `*const [T]` is safe since the caller guarantees that
@@ -835,7 +834,7 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
     #[inline(always)]
     pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
         // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
@@ -845,17 +844,15 @@ impl<T> MaybeUninit<T> {
 
     /// Gets a pointer to the first element of the array.
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
     #[inline(always)]
-    pub const fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
+    pub fn slice_as_ptr(this: &[MaybeUninit<T>]) -> *const T {
         this.as_ptr() as *const T
     }
 
     /// Gets a mutable pointer to the first element of the array.
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
-    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
     #[inline(always)]
-    pub const fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
+    pub fn slice_as_mut_ptr(this: &mut [MaybeUninit<T>]) -> *mut T {
         this.as_mut_ptr() as *mut T
     }
 }