What would be a better way to implement .pop() in my single linked list in Rust?

Question

I've implemented my own version of a singly linked list in Rust as one of the challenges for me to learn it, and I'm satisfied with everything I have there except for the .pop() method. Using 2 while loops is very ugly and inefficient, but I found no other way to overcome the problem of setting the node at the index len() - 2 to None (popping the list), and using the data from the node at the index len() - 1 for the Some(data) return value (returns the element that was popped).

GitHub Link

pub struct SimpleLinkedList<T> {
    head: Option<Box<Node<T>>>,
}

struct Node<T> {
    data: T,
    next: Option<Box<Node<T>>>,
}

impl<T> Default for SimpleLinkedList<T> {
    fn default() -> Self {
        SimpleLinkedList { head: None }
    }
}

impl<T: Copy> Clone for SimpleLinkedList<T> {
    fn clone(&self) -> SimpleLinkedList<T> {
        let mut out: SimpleLinkedList<T> = SimpleLinkedList::new();
        let mut cur = &self.head;
        while let Some(node) = cur {
            cur = &node.next;
            out.push(node.data)
        }
        out
    }
}

impl<T> SimpleLinkedList<T> {
    pub fn new() -> Self {
        Default::default()
    }

    pub fn len(&self) -> usize {
        let mut c = 0;
        let mut cur = &self.head;
        while let Some(node) = cur {
            cur = &node.next;
            c += 1;
        }
        c
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    pub fn push(&mut self, _element: T) {
        let mut cur = &mut self.head;
        match cur {
            Some(_) => {
                while let Some(node) = cur {
                    cur = &mut node.next;
                }
            }
            None => (),
        }
        *cur = Some(Box::from(Node {
            data: _element,
            next: None,
        }));
    }

    pub fn pop(&mut self) -> Option<T>
    where
        T: Copy,
    {
        let length = &self.len();
        let mut cur = &mut self.head;
        let mut out = None;
        match cur {
            Some(_) if *length > 1usize => {
                let mut c = 0usize;
                while let Some(node) = cur {
                    cur = &mut node.next;
                    if c >= length - 1 {
                        out = Some(node.data);
                        break;
                    }
                    c += 1;
                }

                c = 0usize;
                cur = &mut self.head;
                while let Some(node) = cur {
                    cur = &mut node.next;
                    if c == length - 2 {
                        break;
                    }
                    c += 1;
                }
            }
            Some(node) => out = Some(node.data),
            None => (),
        }
        *cur = None;
        out
    }

    pub fn peek(&self) -> Option<&T> {
        let cur = &self.head;
        match cur {
            Some(node) => Some(&node.data),
            None => None,
        }
    }
}

impl<T: Copy> SimpleLinkedList<T> {
    pub fn rev(&self) -> SimpleLinkedList<T> {
        let mut clone = self.clone();
        let mut out: SimpleLinkedList<T> = SimpleLinkedList::new();
        while let Some(val) = clone.pop() {
            out.push(val)
        }
        out
    }
}

impl<'a, T: Copy> From<&'a [T]> for SimpleLinkedList<T> {
    fn from(_item: &[T]) -> Self {
        let mut out: SimpleLinkedList<T> = SimpleLinkedList::new();
        for &e in _item.iter() {
            out.push(e);
        }
        out
    }
}

impl<T> Into<Vec<T>> for SimpleLinkedList<T> {
    fn into(self) -> Vec<T> {
        let mut out: Vec<T> = Vec::new();
        let mut cur = self.head;
        while let Some(node) = cur {
            cur = node.next;
            out.push(node.data)
        }
        out
    }
}

Answer 1

inspired from here

fn pop(&mut self) -> Option<T> {
        let mut current: &mut Option<Box<Node<T>>> = &mut self.head;
        loop {
            // println!("curr: {:?}", current);
            match current {
                None => {
                    return None;
                }
                Some(node) if node.next.is_none() => {
                    let val = node.data;
                    *current = node.next.take();
                    return Some(val);
                }
                Some(ref mut node) => {
                    current = &mut node.next;
                }
            }
        }
    }

Answer 2

You can avoid re-traversing the list by keeping track of the last element you saw as you go (and then updating that at the end).

If you are naive about how you do that, you will run into trouble; your "previous" pointer retains ownership of the rest of the list and the borrow checker won't allow that. The trick is to break that link as you go - and to do that you can use the mem::replace function. Once you've done that, you have to put it back before you lose track of your previous node again.

Here's what that could look like in full (you'll have to forgive my liberal use of unwrap - I do think it makes things clearer):

pub fn pop(&mut self) -> Option<T>
    where T : Copy,
{
    use std::mem::replace;

    let curr = replace(&mut self.head, None);

    if curr.is_none() { // list started off empty; nothing to pop
        return None;
    }

    let mut curr = curr.unwrap(); // safe because of the check above

    if let None = curr.next { // popped the last element
        return Some(curr.data);
    }

    let mut prev_next = &mut self.head;

    while curr.next.is_some() {
        // Take ownership of the next element
        let nnext = replace(&mut curr.next, None).unwrap();

        // Update the previous element's "next" field
        *prev_next = Some(curr);

        // Progress to the next element
        curr = nnext;

        // Progress our pointer to the previous element's "next" field
        prev_next = &mut prev_next.as_mut().unwrap().next;

    }

    return Some(curr.data);
}

As an aside, all this pointer shuffling simplifies a lot if you're willing to change the interface a little so that we return a "new" list each time (taking ownership in the pop function), or use a persistent datastructure, as they do in Learning Rust with entirely too many linked lists (already mentioned in a comment):

pub fn pop_replace(self) -> (Option<T>, Self) {
    // freely mutate self and all the nodes
}

Which you would use like:

let elem, list = list.pop();