1. impl Drop for List {
    2.     fn drop(&mut self) {
    3.         let mut cur_link = mem::replace(&mut self.head, Link::Empty);
    4.         // `while let` == "do this thing until this pattern doesn't match"
    5.         while let Link::More(mut boxed_node) = cur_link {
    6.             cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
    7.             // boxed_node goes out of scope and gets dropped here;
    8.             // but its Node's `next` field has been set to Link::Empty
    9.             // so no unbounded recursion occurs.
    10.         }
    11.     }
    12. }