don't duplicate slice panic_bounds_check

library/core/src/slice/mod.rs

@@ -560,8 +560,8 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn swap(&mut self, a: usize, b: usize) {
-        assert_in_bounds(self.len(), a);
-        assert_in_bounds(self.len(), b);
+        let _ = &self[a];
+        let _ = &self[b];
 
         // SAFETY: we just checked that both `a` and `b` are in bounds
         unsafe { self.swap_unchecked(a, b) }
@@ -598,8 +598,8 @@ impl<T> [T] {
     pub unsafe fn swap_unchecked(&mut self, a: usize, b: usize) {
         #[cfg(debug_assertions)]
         {
-            assert_in_bounds(self.len(), a);
-            assert_in_bounds(self.len(), b);
+            let _ = &self[a];
+            let _ = &self[b];
         }
 
         let ptr = self.as_mut_ptr();
@@ -3502,12 +3502,6 @@ impl<T> [T] {
     }
 }
 
-fn assert_in_bounds(len: usize, idx: usize) {
-    if idx >= len {
-        panic!("index out of bounds: the len is {} but the index is {}", len, idx);
-    }
-}
-
 trait CloneFromSpec<T> {
     fn spec_clone_from(&mut self, src: &[T]);
 }