Put loop_modifies in cfg_attr(kani)

Author: tautschnig

library/core/src/slice/mod.rs

@@ -1012,7 +1012,12 @@ impl<T> [T] {
 
             let mut i = 0;
             #[safety::loop_invariant(i <= n)]
-            #[kani::loop_modifies(unsafe {slice::from_raw_parts_mut(a.as_mut_ptr(), n)}, unsafe {slice::from_raw_parts_mut(b.as_mut_ptr(), n)}, &i)]
+            #[cfg_attr(kani,
+                       kani::loop_modifies(
+                           unsafe {slice::from_raw_parts_mut(a.as_mut_ptr(), n)},
+                           unsafe {slice::from_raw_parts_mut(b.as_mut_ptr(), n)},
+                           &i)
+            )]
             while i < n {
                 mem::swap(&mut a[i], &mut b[n - 1 - i]);
                 i += 1;