Make from{,_mut}_ptr_range const

- `from_ptr_range` uses `#![feature(slice_from_ptr_range_const)]`
- `from_mut_ptr_range` uses `#![feature(slice_from_mut_ptr_range_const)]`

Author: WaffleLapkin

library/core/src/slice/raw.rs

@@ -213,7 +213,8 @@ pub const fn from_mut<T>(s: &mut T) -> &mut [T] {
 ///
 /// [valid]: ptr#safety
 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
-pub unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
+#[rustc_const_unstable(feature = "slice_from_ptr_range_const", issue = "89792")]
+pub const unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
     // SAFETY: the caller must uphold the safety contract for `from_ptr_range`.
     unsafe { from_raw_parts(range.start, range.end.sub_ptr(range.start)) }
 }
@@ -263,7 +264,8 @@ pub unsafe fn from_ptr_range<'a, T>(range: Range<*const T>) -> &'a [T] {
 ///
 /// [valid]: ptr#safety
 #[unstable(feature = "slice_from_ptr_range", issue = "89792")]
-pub unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
+#[rustc_const_unstable(feature = "slice_from_mut_ptr_range_const", issue = "89792")]
+pub const unsafe fn from_mut_ptr_range<'a, T>(range: Range<*mut T>) -> &'a mut [T] {
     // SAFETY: the caller must uphold the safety contract for `from_mut_ptr_range`.
     unsafe { from_raw_parts_mut(range.start, range.end.sub_ptr(range.start)) }
 }