Mark slice::reverse unstably const

Author: okaneco

library/core/src/slice/mod.rs

@@ -987,8 +987,9 @@ impl<T> [T] {
     /// assert!(v == [3, 2, 1]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_reverse", issue = "135120")]
     #[inline]
-    pub fn reverse(&mut self) {
+    pub const fn reverse(&mut self) {
         let half_len = self.len() / 2;
         let Range { start, end } = self.as_mut_ptr_range();
 
@@ -1011,15 +1012,16 @@ impl<T> [T] {
         revswap(front_half, back_half, half_len);
 
         #[inline]
-        fn revswap<T>(a: &mut [T], b: &mut [T], n: usize) {
+        const fn revswap<T>(a: &mut [T], b: &mut [T], n: usize) {
             debug_assert!(a.len() == n);
             debug_assert!(b.len() == n);
 
             // Because this function is first compiled in isolation,
             // this check tells LLVM that the indexing below is
             // in-bounds. Then after inlining -- once the actual
             // lengths of the slices are known -- it's removed.
-            let (a, b) = (&mut a[..n], &mut b[..n]);
+            let (a, _) = a.split_at_mut(n);
+            let (b, _) = b.split_at_mut(n);
 
             let mut i = 0;
             while i < n {