Mutably borrowing in match statement and result

Question

I'm trying to determine if a container has an object and return the found object if it does, or add it if it doesn't.

I've found Rust borrow mutable self inside match expression which has an answer which says what I am trying to do can't (couldn't?) be done.

In my situation, I've got some objects that have vectors of children. I don't want to expose the internals of my object, because I may want to change the representation underneath.

How can you resolve the need to mutably borrow in different match arms in Rust? seems to suggest I may be able to do what I want if I get the lifetimes correct, but I haven't been able to figure out how.

Here's a representation of the issue I'm having:

fn find_val<'a>(container: &'a mut Vec<i32>, to_find: i32) -> Option<&'a mut i32> {
    for item in container.iter_mut() {
        if *item == to_find {
            return Some(item);
        }
    }

    None
}

fn main() {
    let mut container = Vec::<i32>::new();
    container.push(1);
    container.push(2);
    container.push(3);

    let to_find = 4;

    match find_val(&mut container, to_find) {
        Some(x) => {
            println!("Found {}", x);
        }
        _ => {
            container.push(to_find);
            println!("Added {}", to_find);
        }
    }
}

playground

The error I get is:

error[E0499]: cannot borrow `container` as mutable more than once at a time
  --> src/main.rs:24:13
   |
19 |     match find_val(&mut container, to_find) {
   |                         --------- first mutable borrow occurs here
...
24 |             container.push(to_find);
   |             ^^^^^^^^^ second mutable borrow occurs here
...
27 |     }
   |     - first borrow ends here

score 2 · Accepted Answer · edited Mar 13 '18 at 12:56

Put the change in a function, and use early return instead of an else branch:

fn find_val_or_insert(container: &mut Vec<i32>, to_find: i32) {
    if let Some(x) = find_val(&container, to_find) {
        println!("Found {}", x);
        return; // <- return here instead of an else branch
    }
    container.push(to_find);
    println!("Added {}", to_find);
}

See also Mutable borrow more than once and How to update-or-insert on a Vec?

Mutably borrowing in match statement and result

1 Answers1