Rollup merge of #69581 - RalfJung:align_to_mut, r=Centril

fix aliasing violation in align_to_mut

Fixes #68549

I decided to add the testcase here to make it all one PR, but if you prefer I can also add that test case in the Miri repo instead.

src/libcore/slice/mod.rs

@@ -2571,11 +2571,13 @@ impl<T> [T] {
  let (left, rest) = self.split_at_mut(offset);
  // now `rest` is definitely aligned, so `from_raw_parts_mut` below is okay
  let (us_len, ts_len) = rest.align_to_offsets::<U>();
+ let rest_len = rest.len();
  let mut_ptr = rest.as_mut_ptr();
+ // We can't use `rest` again after this, that would invalidate its alias `mut_ptr`!
  (
  left,
  from_raw_parts_mut(mut_ptr as *mut U, us_len),
- from_raw_parts_mut(mut_ptr.add(rest.len() - ts_len), ts_len),
+ from_raw_parts_mut(mut_ptr.add(rest_len - ts_len), ts_len),
  )
  }
  }

src/libcore/tests/slice.rs

@@ -1570,6 +1570,18 @@ fn test_align_to_empty_mid() {
  }
 }
 
+#[test]
+fn test_align_to_mut_aliasing() {
+ let mut val = [1u8, 2, 3, 4, 5];
+ // `align_to_mut` used to create `mid` in a way that there was some intermediate
+ // incorrect aliasing, invalidating the resulting `mid` slice.
+ let (begin, mid, end) = unsafe { val.align_to_mut::<[u8; 2]>() };
+ assert!(begin.len() == 0);
+ assert!(end.len() == 1);
+ mid[0] = mid[1];
+ assert_eq!(val, [3, 4, 3, 4, 5])
+}
+
 #[test]
 fn test_slice_partition_dedup_by() {
  let mut slice: [i32; 9] = [1, -1, 2, 3, 1, -5, 5, -2, 2];