第4节 push

pub fn push(&mut self, elem: i32) {
    let new_node = Box::new(Node {
        elem: elem,
        next: self.head,// 错误：不能移动借用的内容
    });
    self.head = Link::More(new_node);
}

用**mem::replace()**函数可以窃取借用的值，将其替换成另一个值。

pub fn replace<T>(dest: &mut T, src: T) -> T

use std::mem;
pub fn push(&mut self, elem: i32) {
    let new_node = Box::new(Node {
        elem: elem,
        next: mem::replace(&mut self.head, Link::Empty),
    });
    self.head = Link::More(new_node);
}