rt_lists/
third.rs

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type Link<T> = Option<Box<Node<T>>>;

struct  Node<T> {
  elem: T,
  next: Link<T>
}

pub struct List<T> {
  head: Link<T>
}

impl<T> List<T> {
  pub fn new() -> Self {
    List { head: None }
  }

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

    self.head = Some(new_node);
  }

  pub fn pop(&mut self) -> Option<T> {
    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();
    }
  }
}