Rust 迭代递归struct时无法获得可变引用：一次不能多次borrow可变引用

我试图迭代地浏览递归数据 struct ，以便在特定位置插入元素.就我有限的理解而言，这意味着对 struct 的根进行可变引用，然后依次替换为对其跟随者的引用:

type Link = Option<Box<Node>>;

struct Node {
    next: Link
}

struct Recursive {
    root: Link
}

impl Recursive {
    fn back(&mut self) -> &mut Link {
        let mut anchor = &mut self.root;
        while let Some(ref mut node) = *anchor {
            anchor = &mut node.next;
        }
        anchor
    }
}

(Rust playground link)

然而，这是失败的:

error[E0499]: cannot borrow `anchor.0` as mutable more than once at a time
  --> src/main.rs:14:24
   |
14 |         while let Some(ref mut node) = *anchor {
   |                        ^^^^^^^^^^^^
   |                        |
   |                        second mutable borrow occurs here
   |                        first mutable borrow occurs here
...
18 |     }
   |     - first borrow ends here

error[E0506]: cannot assign to `anchor` because it is borrowed
  --> src/main.rs:15:13
   |
14 |         while let Some(ref mut node) = *anchor {
   |                        ------------ borrow of `anchor` occurs here
15 |             anchor = &mut node.next;
   |             ^^^^^^^^^^^^^^^^^^^^^^^ assignment to borrowed `anchor` occurs here

error[E0499]: cannot borrow `*anchor` as mutable more than once at a time
  --> src/main.rs:17:9
   |
14 |         while let Some(ref mut node) = *anchor {
   |                        ------------ first mutable borrow occurs here
...
17 |         anchor
   |         ^^^^^^ second mutable borrow occurs here
18 |     }
   |     - first borrow ends here

这是有道理的，因为anchor和node都指的是同一个 struct ，但实际上，在分解它之后，我不再关心anchor.

如何使用安全防 rust 剂正确实施back()？