rust/src/librustc_error_codes/error_codes/E0502.md

This error indicates that you are trying to borrow a variable as mutable when it has already been borrowed as immutable.

Erroneous code example:

fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    let ref y = a; // a is borrowed as immutable.
    bar(a); // error: cannot borrow `*a` as mutable because `a` is also borrowed
            //        as immutable
    println!("{}", y);
}

To fix this error, ensure that you don't have any other references to the variable before trying to access it mutably:

fn bar(x: &mut i32) {}
fn foo(a: &mut i32) {
    bar(a);
    let ref y = a; // ok!
    println!("{}", y);
}

For more information on the rust ownership system, take a look at https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html.