#[derive(Debug)]
struct A {
    x: usize,
    #[cfg(debug)]
    y: usize,
}

fn main() {
    #[cfg(debug)] {
        println!("DEBUG!");
        let a = A {x: 1, y: 2};
    }
    #[cfg(not(debug))]
    let a = A {x: 1};

    println!("{:?}", a);
}