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

Author: WaffleLapkin

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 = "none")]
+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 = "none")]
+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

@@ -135,6 +135,8 @@
 #![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 = "none")]
+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 = "none")]
+pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
     array::from_mut(s)
 }