Cannot obtain a mutable reference when iterating a recursive structure: cannot borrow as mutable more than once at a time

Question

I\'m trying to navigate a recursive data structure iteratively in order to insert elements at a certain position. To my limited understanding, this means taking a mutable re

Answer 1

It is possible... but I wish I had a more elegant solution.

The trick is NOT to borrow from anchor, and therefore to juggle between two accumulators:

• one holding the reference to the current node
• the other being assigned the reference to the next node

This leads me to:

impl Recursive {
    fn back(&mut self) -> &mut Link {
        let mut anchor = &mut self.root;

        loop {
            let tmp = anchor;
            if let Some(ref mut node) = *tmp {
                anchor = &mut node.next;
            } else {
                anchor = tmp;
                break;
            }
        }

        anchor
    }
}

Not exactly pretty, but this is something the borrow checker can get behind so ¯\_(ツ)_/¯.

@ker has improved on this by creating an unnamed temporary:

impl Recursive {
    fn back(&mut self) -> &mut Link {
        let mut anchor = &mut self.root;

        loop {
            match {anchor} {
                &mut Some(ref mut node) => anchor = &mut node.next,
                other => return other,
            }
        }
    }
}

The trick here is that using {anchor} moves the content of anchor into an unnamed temporary on which the match executes. Therefore, in the match block we are not borrowing from anchor but from the temporary, leaving us free to modify anchor. See the related blog post Stuff the Identity Function Does (in Rust).