The intermediate implementation in 6.1 is totally off

[wf227]

This code is unfortunately completely misleading:

pub fn push(&mut self, elem: T) <here is no return code mentioned> {
        let new_tail = Box::new(Node {                   new_tail is holding the Box
            elem: elem,
            // When you push onto the tail, your next is always None
            next: None,
        });

        // Put the box in the right place, and then grab a reference to its Node
        let new_tail = match self.tail.take() {         new_tail is now something complete different and the Box is dropped
            Some(old_tail) => {
                // If the old tail existed, update it to point to the new tail
                old_tail.next = Some(new_tail);
                old_tail.next.as_deref_mut() <- but this is returning a mut reference
            }
            None => {
                // Otherwise, update the head to point to it
                self.head = Some(new_tail);
                self.head.as_deref_mut()     < - and this returns also a mut reference
            }

Is push() in this intermediate version meant to return something and is new_tail holding the box until it is linked and has a new owner?

[mwchase]

I'd recommend taking another look at the implementation (of the entire function), and try interpreting it from scratch.