fn do_work(mut _n: u32) {
    // u32, makes a copy of the value of _n
    _n += 5;
}

fn main() {
    let mut x = 5; // read the warning, x doesn't need to be mut, this is because u32 will make a copy once inside do_work()

    do_work(x);

    println!("{}", x);
}