What's the cause of this borrow checker error in Rust LinkedList?

I’m writing a fn to pop the element from last pos in a LinkedList, but a double mut ref error occurs in this scenario, where I’m trying to get the second last node so that I can chop off the tail

fn pop(&mut self) -> Option<T> {
        match self.head {
            None => None,
            Some(ref mut head) => {
                let mut curr = head;

                while let Some(ref mut next) = curr.next {
                    if next.next.is_none() {
                        break;
                    }

                    curr = next;
                }

                let taken = curr.next.take();
                taken.map(|x| x.data)
            }
        }
    }

but the code compiles when there is no check

fn pop(&mut self) -> Option<T> {
        match self.head {
            None => None,
            Some(ref mut head) => {
                let mut curr = head;

                while let Some(ref mut next) = curr.next {
                    // if next.next.is_none() {
                    //     break;
                    // }

                    curr = next;
                }

                let taken = curr.next.take();
                taken.map(|x| x.data)
            }
        }
    }

Error

error[E0499]: cannot borrow `curr.next` as mutable more than once at a time
  --> src\main.rs:70:29
   |
62 |                 while let Some(ref mut next) = curr.next {
   |                                ------------ first mutable borrow occurs here
...
70 |                 let taken = curr.next.take();
   |                             ^^^^^^^^^^^^^^^^
   |                             |
   |                             second mutable borrow occurs here
   |                             first borrow later used here

Can’t seem to figure out the issue as I’m new to rust. any help would be appreciated.

Full Code

use std::fmt::Debug;

#[derive(Debug)]
struct Node<T> {
    data: T,
    next: Option<Box<Node<T>>>,
}

#[derive(Debug)]
struct LinkedList<T> {
    head: Option<Box<Node<T>>>,
    len: usize,
}


impl<T: Debug> LinkedList<T> {
    fn new() -> Self {
        Self { head: None, len: 0 }
    }

    fn push(&mut self, data: T) {
        match self.head {
            None => self.head = Some(Box::new(Node::new(data))),
            Some(ref mut head) => {
                let mut curr = head;

                while let Some(ref mut next) = curr.next {
                    curr = next;
                }

                curr.next = Some(Box::new(Node::new(data)));
            }
        }

        self.len += 1;
    }

    // head -> 1 -> 2

    fn pop(&mut self) -> Option<T> {
        match self.head {
            None => None,
            Some(ref mut head) => {
                let mut curr = head;

                while let Some(ref mut next) = curr.next {
                    // if next.next.is_none() {
                    //     break;
                    // }

                    curr = next;
                }

                let taken = curr.next.take();
                taken.map(|x| x.data)
            }
        }
    }
}

impl<T> Node<T> {
    fn new(data: T) -> Self {
        Node { data, next: None }
    }
}

>Solution :

This is a well-known flaw of the borrow checker. Your code compiles with the next-gen Polonius borrow checker.

The fix for this is to not keep a mutable reference in the loop:

while curr.next.is_some() {
    curr = curr.next.as_mut().unwrap();
}

This is awkward and ugly, but this is what we can do now.