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]
-pubfnreverse(&mutself){
+pubconstfnreverse(&mutself){
 let half_len = self.len() / 2;
 letRange{ start, end } = self.as_mut_ptr_range();
 
@@ -1011,15 +1012,16 @@ impl<T> [T] {
 revswap(front_half, back_half, half_len);
 
 #[inline]
-fnrevswap<T>(a:&mut[T],b:&mut[T],n:usize){
+constfnrevswap<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);
 
 letmut i = 0;
 while i < n {