Rollup merge of rust-lang#100663 - clarfonthey:const-reverse, r=scottmcm

Make slice::reverse const

I remember this not being doable for some reason before, but decided to try it again and everything worked out in the tests.

Author: matthiaskrgr

Diff for: library/core/src/slice/mod.rs

@@ -674,8 +674,9 @@ impl<T> [T] {
     /// assert!(v == [3, 2, 1]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_reverse", issue = "100784")]
     #[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();
 
@@ -698,18 +699,20 @@ impl<T> [T] {
         revswap(front_half, back_half, half_len);
 
         #[inline]
-        fn revswap<T>(a: &mut [T], b: &mut [T], n: usize) {
-            debug_assert_eq!(a.len(), n);
-            debug_assert_eq!(b.len(), n);
+        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]);
 
-            for i in 0..n {
+            let mut i = 0;
+            while i < n {
                 mem::swap(&mut a[i], &mut b[n - 1 - i]);
+                i += 1;
             }
         }
     }