Rust LinkedList 中的borrow 判断器错误的原因是什么

我正在编写一个fn来从LinkedList中的最后一个位置弹出元素，但是在这个场景中发生了一个双mut引用错误，我试图获取倒数第二个 node ，这样我就可以砍掉 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)
            }
        }
    }

但代码会在没有判断的情况下编译

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)
            }
        }
    }

误差率

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

因为我对Rust 还不熟悉，所以我好像搞不清楚这个问题.任何帮助都将不胜感激.

完整代码

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 }
    }
}