E0501 - Error codes index
Description: Book listing all Rust error codes

# Error code E0501

A mutable variable is used but it is already captured by a closure.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn inside_closure(x: &mut i32) {
        // Actions which require unique access
    }

    fn outside_closure(x: &mut i32) {
        // Actions which require unique access
    }

    fn foo(a: &mut i32) {
        let mut bar = || {
            inside_closure(a)
        };
        outside_closure(a); // error: cannot borrow `*a` as mutable because previous
                            //        closure requires unique access.
        bar();
    }
    }

This error indicates that a mutable variable is used while it is still captured by a closure. Because the closure has borrowed the variable, it is not available until the closure goes out of scope.

Note that a capture will either move or borrow a variable, but in this situation, the closure is borrowing the variable. Take a look at the chapter on Capturing in Rust By Example for more information.

To fix this error, you can finish using the closure before using the captured variable:

    #![allow(unused)]
    fn main() {
    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        let mut bar = || {
            inside_closure(a)
        };
        bar();
        // borrow on `a` ends.
        outside_closure(a); // ok!
    }
    }

Or you can pass the variable as a parameter to the closure:

    #![allow(unused)]
    fn main() {
    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        let mut bar = |s: &mut i32| {
            inside_closure(s)
        };
        outside_closure(a);
        bar(a);
    }
    }

It may be possible to define the closure later:

    #![allow(unused)]
    fn main() {
    fn inside_closure(x: &mut i32) {}
    fn outside_closure(x: &mut i32) {}

    fn foo(a: &mut i32) {
        outside_closure(a);
        let mut bar = || {
            inside_closure(a)
        };
        bar();
    }
    }