Error code E0502

A variable already borrowed as immutable was borrowed as mutable.

Erroneous code example:

#![allow(unused)] fn main() { fn bar(x: &mut i32) {} fn foo(a: &mut i32) { let 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:

#![allow(unused)] fn main() { fn bar(x: &mut i32) {} fn foo(a: &mut i32) { bar(a); let y = &a; // ok! println!("{}", y); } }

For more information on Rust's ownership system, take a look at the References & Borrowing section of the Book.