E0524 - Error codes index
Description: Book listing all Rust error codes

# Error code E0524

A variable which requires unique access is being used in more than one closure at the same time.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn set(x: &mut isize) {
        *x += 4;
    }

    fn dragoooon(x: &mut isize) {
        let mut c1 = || set(x);
        let mut c2 = || set(x); // error!

        c2();
        c1();
    }
    }

To solve this issue, multiple solutions are available. First, is it required for this variable to be used in more than one closure at a time? If it is the case, use reference counted types such as `Rc` (or `Arc` if it runs concurrently):

    #![allow(unused)]
    fn main() {
    use std::rc::Rc;
    use std::cell::RefCell;

    fn set(x: &mut isize) {
        *x += 4;
    }

    fn dragoooon(x: &mut isize) {
        let x = Rc::new(RefCell::new(x));
        let y = Rc::clone(&x);
        let mut c1 = || { let mut x2 = x.borrow_mut(); set(&mut x2); };
        let mut c2 = || { let mut x2 = y.borrow_mut(); set(&mut x2); }; // ok!

        c2();
        c1();
    }
    }

If not, just run closures one at a time:

    #![allow(unused)]
    fn main() {
    fn set(x: &mut isize) {
        *x += 4;
    }

    fn dragoooon(x: &mut isize) {
        { // This block isn't necessary since non-lexical lifetimes, it's just to
          // make it more clear.
            let mut c1 = || set(&mut *x);
            c1();
        } // `c1` has been dropped here so we're free to use `x` again!
        let mut c2 = || set(&mut *x);
        c2();
    }
    }