1 改类型声明

pub struct List<T> {
    head: Link<T>,
}
type Link<T> = Option<Box<Node<T>>>;
struct Node<T> {
    elem: T,
    next: Link<T>,
}

2 修改类型实现

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None }
    }
    pub fn push(&mut self, val: T) {
        let node = Node {
            elem: val,
            next: self.head.take(),
        };
        self.head = Some(Box::new(node));
    }
    pub fn pop(&mut self) -> Option<T> {
        // 使用 Option::map()
        // 注意: map() 要求所有权;node 的类型是 Box<Node>
        self.head.take().map(|node| {
            self.head = node.next;
            node.elem
        })
    }
}
impl<T> Drop for List<T> {
    fn drop(&mut self) {
        let mut cur_link = self.head.take();
        while let Some(mut boxed_node) = cur_link {
            cur_link = boxed_node.next.take();
        }
    }
}