Rollup merge of rust-lang#97366 - WaffleLapkin:stabilize_array_slice_from_ref, r=dtolnay

Stabilize `{slice,array}::from_ref`

This PR stabilizes the following APIs as `const` functions in Rust `1.63`:
```rust
// core::array
pub const fn from_ref<T>(s: &T) -> &[T; 1];

// core::slice
pub const fn from_ref<T>(s: &T) -> &[T];
```

Note that the `mut` versions are not stabilized as unique references (`&mut _`) are [unstable in const context].

FCP: rust-lang#90206 (comment)

r? rust-lang/libs-api `@rustbot` label +T-libs-api -T-libs

[unstable in const context]: rust-lang#57349

Author: Dylan-DPC

core/src/array/mod.rs

@@ -92,7 +92,7 @@ where
 
 /// Converts a reference to `T` into a reference to an array of length 1 (without copying).
 #[stable(feature = "array_from_ref", since = "1.53.0")]
-#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
+#[rustc_const_stable(feature = "const_array_from_ref_shared", since = "1.63.0")]
 pub const fn from_ref<T>(s: &T) -> &[T; 1] {
     // SAFETY: Converting `&T` to `&[T; 1]` is sound.
     unsafe { &*(s as *const T).cast::<[T; 1]>() }

core/src/slice/raw.rs

@@ -144,7 +144,7 @@ pub const unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a m
 
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
-#[rustc_const_unstable(feature = "const_slice_from_ref", issue = "90206")]
+#[rustc_const_stable(feature = "const_slice_from_ref_shared", since = "1.63.0")]
 #[must_use]
 pub const fn from_ref<T>(s: &T) -> &[T] {
     array::from_ref(s)