# Error code E0596

This error occurs because you tried to mutably borrow a non-mutable variable.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    let x = 1;
    let y = &mut x; // error: cannot borrow mutably
    }

In here, `x` isn't mutable, so when we try to mutably borrow it in `y`, it fails. To fix this error, you need to make `x` mutable:

    #![allow(unused)]
    fn main() {
    let mut x = 1;
    let y = &mut x; // ok!
    }