Delete a node in singly linked list in Rust

Question

I am new to Rust and want to write linked list in Rust to have fun. I am confused about how to delete a node in the linked list. Here is my simple code.

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

struct LinkedList {
    head: Option<Box<Node>>,
}

impl LinkedList {
    fn remove(&mut self, v: usize) -> Option<usize> {
        let mut current_node: &mut Option<Box<Node>> = &mut self.head;
        loop {
           match current_node {
                None => break,
                Some(node) => {
                    if node.v == v {
                        // current_node = what? 
                        // ???????????????
                        break;
                    } else {
                        current_node = &mut node.next;
                    }
                },
            };
        }

        match current_node.take().map(|x| *x) {
            Some(node) => {
                *current_node = node.next;
                return Some(node.v)
            },
            None => None,
        }
    }
}

And here is the rust playground. I am using the nightly version and edition = 2018. In the loop, I try to find the node whose next node contains the value that I search for. However, I am confused about what to write in the ?? position.

Answer 1

There isn't really code that can go in that space to fix it; you'll need to make some bigger changes.

One of the problems is that you have mutably borrowed the current node in current_node, but then need to mutate it while that reference still exists.

Making use of non-lexical lifetimes in Edition 2018, you can do:

impl LinkedList {
    fn remove(&mut self, v: usize) -> Option<usize> {
        let mut current = &mut self.head;
        loop {
            match current {
                None => return None,
                Some(node) if node.v == v => {
                    *current = node.next.take();
                    return Some(v);
                },
                Some(node) => {
                    current = &mut node.next;
                }
            }
        }
    }
}

Somehow, using the match guard if node.v == v to make two match arms, instead of using an if condition inside one match arm, lets the borrower checker deduce that this is safe. I'm not sure why the if statement inside the match arm isn't allowed - there are some of the opinion that this could be a bug.