fn next(&mut self) -> Option<&'a mut T> {
        if self.len == 0 {
            None
        } else {
            self.head.map(|node| unsafe {
                // Need an unbound lifetime to get 'a
                let node = &mut *node.as_ptr();
                self.len -= 1;
                self.head = node.next;
                &mut node.element
            })
        }
    }
A cursor over a `LinkedList` with editing operations.

A `Cursor` is like an iterator, except that it can freely seek back-and-forth, and can
safely mutate the list during iteration. This is because the lifetime of its yielded
references is tied to its own lifetime, instead of just the underlying list. This means
cursors cannot yield multiple elements at once.

Cursors always
benchmarks/haystacks/code/rust-library.rs