Rollup merge of #90162 - WaffleLapkin:const_array_slice_from_ref_mut, r=oli-obk

Mark `{array, slice}::{from_ref, from_mut}` as const fn

This PR marks the following APIs as `const`:
```rust
// core::array
pub const fn from_ref<T>(s: &T) -> &[T; 1];
pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1];

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

Note that `from_ref` methods require `const_raw_ptr_deref` feature (which seems totally fine, since it's being stabilized, see #89551), `from_mut` methods require `const_mut_refs` (which seems fine too since this PR marks `from_mut` functions as const unstable).

r? ````@oli-obk````

library/core/src/array/mod.rs

@@ -85,14 +85,16 @@ 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")]
-pub fn from_ref<T>(s: &T) -> &[T; 1] {
+#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
+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]>() }
 }
 
 /// Converts a mutable reference to `T` into a mutable reference to an array of length 1 (without copying).
 #[stable(feature = "array_from_ref", since = "1.53.0")]
-pub fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
+#[rustc_const_unstable(feature = "const_array_from_ref", issue = "90206")]
+pub const fn from_mut<T>(s: &mut T) -> &mut [T; 1] {
     // SAFETY: Converting `&mut T` to `&mut [T; 1]` is sound.
     unsafe { &mut *(s as *mut T).cast::<[T; 1]>() }
 }

library/core/src/lib.rs

@@ -136,6 +136,8 @@
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]
 #![feature(variant_count)]
+#![feature(const_array_from_ref)]
+#![feature(const_slice_from_ref)]
 //
 // Language features:
 #![feature(abi_unadjusted)]

library/core/src/slice/raw.rs

@@ -138,12 +138,14 @@ pub unsafe fn from_raw_parts_mut<'a, T>(data: *mut T, len: usize) -> &'a mut [T]
 
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
-pub fn from_ref<T>(s: &T) -> &[T] {
+#[rustc_const_unstable(feature = "const_slice_from_ref", issue = "90206")]
+pub const fn from_ref<T>(s: &T) -> &[T] {
     array::from_ref(s)
 }
 
 /// Converts a reference to T into a slice of length 1 (without copying).
 #[stable(feature = "from_ref", since = "1.28.0")]
-pub fn from_mut<T>(s: &mut T) -> &mut [T] {
+#[rustc_const_unstable(feature = "const_slice_from_ref", issue = "90206")]
+pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
     array::from_mut(s)
 }

library/core/tests/array.rs

@@ -7,6 +7,11 @@ fn array_from_ref() {
     let value: String = "Hello World!".into();
     let arr: &[String; 1] = array::from_ref(&value);
     assert_eq!(&[value.clone()], arr);
+
+    const VALUE: &&str = &"Hello World!";
+    const ARR: &[&str; 1] = array::from_ref(VALUE);
+    assert_eq!(&[*VALUE], ARR);
+    assert!(core::ptr::eq(VALUE, &ARR[0]));
 }
 
 #[test]

library/core/tests/lib.rs

@@ -74,6 +74,8 @@
 #![feature(trusted_random_access)]
 #![feature(unsize)]
 #![feature(unzip_option)]
+#![feature(const_array_from_ref)]
+#![feature(const_slice_from_ref)]
 #![deny(unsafe_op_in_unsafe_fn)]
 
 extern crate test;

library/core/tests/slice.rs

@@ -2146,6 +2146,14 @@ fn test_slice_run_destructors() {
     assert_eq!(x.get(), 1);
 }
 
+#[test]
+fn test_const_from_ref() {
+    const VALUE: &i32 = &1;
+    const SLICE: &[i32] = core::slice::from_ref(VALUE);
+
+    assert!(core::ptr::eq(VALUE, &SLICE[0]))
+}
+
 #[test]
 fn test_slice_fill_with_uninit() {
     // This should not UB. See #87891