fn drop(&mut self) {
                self.0.for_each(drop);

                let source_deque = unsafe { self.0.deque.as_mut() };

                // T = source_deque_tail; H = source_deque_head; t = drain_tail; h = drain_head
                //
                //        T   t   h   H
                // [. . . o o x x o o . . .]
                //
                let orig_tail = source_deque.tail;
                let drain_tail = source_deque.head;
                let drain_head = self.0.after_tail;
                let orig_head = self.0.after_head;

                let tail_len = count(orig_tail, drain_tail, source_deque.cap());
                let head_len = count(drain_head, orig_head, source_deque.cap());

                // Restore the original head value
                source_deque.head = orig_head;

                match (tail_len, head_len) {
                    (0, 0) => {
                        source_deque.head = 0;
                        source_deque.tail = 0;
                    }
                    (0, _) => {
                        source_deque.tail = drain_head;
                    }
                    (_, 0) => {
                        source_deque.head = drain_tail;
                    }
                    _ => unsafe {
                        if tail_len <= head_len {
                            source_deque.tail = source_deque.wrap_sub(drain_head, tail_len);
                            source_deque.wrap_copy(source_deque.tail, orig_tail, tail_len);
                        } else {
                            source_deque.head = source_deque.wrap_add(drain_tail, head_len);
                            source_deque.wrap_copy(drain_tail, drain_head, head_len);
                        }
                    },
                }
            }
Returns the two slices that cover the `VecDeque`'s valid rang
benchmarks/haystacks/code/rust-library.rs