Drain only needs a shared reference

src/liballoc/collections/vec_deque.rs

@@ -1019,19 +1019,17 @@ impl<T> VecDeque<T> {
  // the drain is complete and the Drain destructor is run.
  self.head = drain_tail;
 
- // `deque` and `ring` overlap in what they point to, so we must make sure
- // that `ring` is "derived-from" `deque`, or else even just creating ring
- // from `self` already invalidates `deque`.
- let deque = NonNull::from(&mut *self);
-
  Drain {
- deque,
+ deque: NonNull::from(&mut *self),
  after_tail: drain_head,
  after_head: head,
  iter: Iter {
  tail: drain_tail,
  head: drain_head,
- ring: unsafe { (&mut *deque.as_ptr()).buffer_as_mut_slice() },
+ // Crucially, we only create shared references from `self` here and read from
+ // it. We do not write to `self` nor reborrow to a mutable reference.
+ // Hence the raw pointer we created above, for `deque`, remains valid.
+ ring: unsafe { self.buffer_as_slice() },
  },
  }
  }