pub fn pop(&mut self) -> Option<i32> {
    match mem::replace(&mut self.head, Link::Empty) {
        Link::Empty => None,
        Link::More(boxed_node) => {
            let node = *boxed_node;
            self.head = node.next;
            Some(node.elem)
        }
    }
}

pub fn pop(&mut self) -> Option<i32> {
    match mem::replace(&mut self.head, Link::Empty) {
        Link::Empty => None,
        Link::More(node) => {
            self.head = node.next;
            Some(node.elem)
        }
    }
}

struct Node {
    elem: i32,
    next: Link,
}

enum Link {
    Empty,
    More(Box<Node>),
}