Breaking Down

pop_front should be the same basic logic as push_front, but backwards. Let's try:

pub fn pop_front(&mut self) -> Option<T> {
    // need to take the old head, ensuring it's -2
    self.head.take().map(|old_head| {                         // -1 old
        match old_head.borrow_mut().next.take() {
            Some(new_head) => {                               // -1 new
                // not emptying list
                new_head.borrow_mut().prev.take();            // -1 old
                self.head = Some(new_head);                   // +1 new
                // total: -2 old, +0 new
            None => {
                // emptying list
                self.tail.take();                             // -1 old
                // total: -2 old, (no new)
> cargo build

error[E0609]: no field `elem` on type `std::rc::Rc<std::cell::RefCell<fourth::Node<T>>>`
  --> src/
64 |             old_head.elem
   |                      ^^^^ unknown field

ACK. RefCells. Gotta borrow_mut again I guess...

pub fn pop_front(&mut self) -> Option<T> {
    self.head.take().map(|old_head| {
        match old_head.borrow_mut().next.take() {
            Some(new_head) => {
                self.head = Some(new_head);
            None => {
cargo build

error[E0507]: cannot move out of borrowed content
  --> src/
64 |             old_head.borrow_mut().elem
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot move out of borrowed content


cannot move out of borrowed content

Hrm... It seems that Box was really spoiling us. borrow_mut only gets us an &mut Node<T>, but we can't move out of that!

We need something that takes a RefCell<T> and gives us a T. Let's check the docs for something like that:

fn into_inner(self) -> T

Consumes the RefCell, returning the wrapped value.

That looks promising!

> cargo build

error[E0507]: cannot move out of an `Rc`
  --> src/
64 |             old_head.into_inner().elem
   |             ^^^^^^^^ cannot move out of an `Rc`

Ah dang. into_inner wants to move out the RefCell, but we can't, because it's in an Rc. As we saw in the previous chapter, Rc<T> only lets us get shared references into its internals. That makes sense, because that's the whole point of reference counted pointers: they're shared!

This was a problem for us when we wanted to implement Drop for our reference counted list, and the solution is the same: Rc::try_unwrap, which moves out the contents of an Rc if its refcount is 1.


Rc::try_unwrap returns a Result<T, Rc<T>>. Results are basically a generalized Option, where the None case has data associated with it. In this case, the Rc you tried to unwrap. Since we don't care about the case where it fails (if we wrote our program correctly, it has to succeed), we just call unwrap on it.

Anyway, let's see what compiler error we get next (let's face it, there's going to be one).

> cargo build

error[E0599]: no method named `unwrap` found for type `std::result::Result<std::cell::RefCell<fourth::Node<T>>, std::rc::Rc<std::cell::RefCell<fourth::Node<T>>>>` in the current scope
  --> src/
64 |             Rc::try_unwrap(old_head).unwrap().into_inner().elem
   |                                      ^^^^^^
   = note: the method `unwrap` exists but the following trait bounds were not satisfied:
           `std::rc::Rc<std::cell::RefCell<fourth::Node<T>>> : std::fmt::Debug`

UGH. unwrap on Result requires that you can debug-print the error case. RefCell<T> only implements Debug if T does. Node doesn't implement Debug.

Rather than doing that, let's just work around it by converting the Result to an Option with ok:



cargo build



We did it.

We implemented push and pop.

Let's test by stealing the old stack basic test (because that's all that we've implemented so far):

mod test {
    use super::List;

    fn basics() {
        let mut list = List::new();

        // Check empty list behaves right
        assert_eq!(list.pop_front(), None);

        // Populate list

        // Check normal removal
        assert_eq!(list.pop_front(), Some(3));
        assert_eq!(list.pop_front(), Some(2));

        // Push some more just to make sure nothing's corrupted

        // Check normal removal
        assert_eq!(list.pop_front(), Some(5));
        assert_eq!(list.pop_front(), Some(4));

        // Check exhaustion
        assert_eq!(list.pop_front(), Some(1));
        assert_eq!(list.pop_front(), None);
cargo test

     Running target/debug/lists-5c71138492ad4b4a

running 9 tests
test first::test::basics ... ok
test fourth::test::basics ... ok
test second::test::iter_mut ... ok
test second::test::basics ... ok
test fifth::test::iter_mut ... ok
test third::test::basics ... ok
test second::test::iter ... ok
test third::test::iter ... ok
test second::test::into_iter ... ok

test result: ok. 9 passed; 0 failed; 0 ignored; 0 measured

Nailed it.

Now that we can properly remove things from the list, we can implement Drop. Drop is a little more conceptually interesting this time around. Where previously we bothered to implement Drop for our stacks just to avoid unbounded recursion, now we need to implement Drop to get anything to happen at all.

Rc can't deal with cycles. If there's a cycle, everything will keep everything else alive. A doubly-linked list, as it turns out, is just a big chain of tiny cycles! So when we drop our list, the two end nodes will have their refcounts decremented down to 1... and then nothing else will happen. Well, if our list contains exactly one node we're good to go. But ideally a list should work right if it contains multiple elements. Maybe that's just me.

As we saw, removing elements was a bit painful. So the easiest thing for us to do is just pop until we get None:

impl<T> Drop for List<T> {
    fn drop(&mut self) {
        while self.pop_front().is_some() {}
cargo build

(We actually could have done this with our mutable stacks, but shortcuts are for people who understand things!)

We could look at implementing the _back versions of push and pop, but they're just copy-paste jobs which we'll defer to later in the chapter. For now let's look at more interesting things!