add test for bytewise ptr::swap of a pointer

Author: RalfJung

Diff for: library/core/tests/lib.rs

@@ -18,6 +18,7 @@
 #![feature(const_eval_select)]
 #![feature(const_heap)]
 #![feature(const_nonnull_new)]
+#![feature(const_swap)]
 #![feature(const_trait_impl)]
 #![feature(core_intrinsics)]
 #![feature(core_io_borrowed_buf)]

Diff for: library/core/tests/ptr.rs

@@ -897,6 +897,25 @@ fn test_const_copy() {
     };
 }
 
+#[test]
+fn test_const_swap() {
+    const {
+        let mut ptr1 = &1;
+        let mut ptr2 = &666;
+
+        // Swap ptr1 and ptr2, bytewise. `swap` does not take a count
+        // so the best we can do is use an array.
+        type T = [u8; mem::size_of::<&i32>()];
+        unsafe {
+            ptr::swap(ptr::from_mut(&mut ptr1).cast::<T>(), ptr::from_mut(&mut ptr2).cast::<T>());
+        }
+
+        // Make sure they still work.
+        assert!(*ptr1 == 666);
+        assert!(*ptr2 == 1);
+    };
+}
+
 #[test]
 fn test_null_array_as_slice() {
     let arr: *mut [u8; 4] = null_mut();

Diff for: tests/ui/consts/missing_span_in_backtrace.rs

@@ -1,7 +1,7 @@
 //@ compile-flags: -Z ui-testing=no
 
 
-#![feature(const_swap)]
+#![feature(const_swap_nonoverlapping)]
 use std::{
     mem::{self, MaybeUninit},
     ptr,