Final Code

Eight short sections, one working stack. The chapter is "bad" because we used i32 instead of a generic, reinvented Option, and skipped peek and iter. We'll fix all of that next chapter.

1. 01
   The whole module — keep it open in another tab while reading the next chapter; we'll refactor against this baseline.

   use std::mem;
   
   pub struct List {
       head: Link,
   }
   
   enum Link {
       Empty,
       More(Box<Node>),
   }
   
   struct Node {
       elem: i32,
       next: Link,
   }
   
   impl List {
       pub fn new() -> Self {
           List { head: Link::Empty }
       }
   
       pub fn push(&mut self, elem: i32) {
           let new_node = Box::new(Node {
               elem,
               next: mem::replace(&mut self.head, Link::Empty),
           });
           self.head = Link::More(new_node);
       }
   
       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)
               }
           }
       }
   }
   
   impl Drop for List {
       fn drop(&mut self) {
           let mut cur = mem::replace(&mut self.head, Link::Empty);
           while let Link::More(mut node) = cur {
               cur = mem::replace(&mut node.next, Link::Empty);
           }
       }
   }