Option

Look at Link from last chapter. Empty or Box<Node>. That's Option<Box<Node>> with the names changed. The standard library's Option already has every helper we hand-rolled with mem::replace and match.

1. 01
   Drop Link entirely. Option<Box<Node>> is the same shape — one tag bit, one pointer payload — and Rust knows the null-pointer optimization, so it's still one machine word.

   pub struct List {
       head: Link,
   }
   
   type Link = Option<Box<Node>>;
   
   struct Node {
       elem: i32,
       next: Link,
   }

2. 02
   Option::take is mem::replace(&mut x, None) with a shorter name. It pulls the value out and leaves None in place. Every mem::replace call in push, pop, and drop becomes a .take().

   pub fn push(&mut self, elem: i32) {
       let new_node = Box::new(Node {
           elem,
           next: self.head.take(),
       });
       self.head = Some(new_node);
   }

3. 03
   Option::map runs a closure on the inner value if there is one and gives you back a new Option. Our two-arm match in pop collapses to a single line.

   pub fn pop(&mut self) -> Option<i32> {
       self.head.take().map(|node| {
           self.head = node.next;
           node.elem
       })
   }