macro_rules! once {
    ($exp:expr) => {
        once!((), $exp)
    };
    ($type:ty, $exp:expr) => {{
        static ONCE: once_cell::sync::OnceCell<$type> = once_cell::sync::OnceCell::new();
        ONCE.get_or_init(|| $exp)
    }};
}

fn main() {
    // If you run this, you can see that the expressions are executed only once.

    once();
    once();
    once();

    assert_eq!(42, calc());
    assert_eq!(42, calc());
    assert_eq!(42, calc());
}

fn once() {
    once!(println!("Doing it only once"));
}

fn calc() -> u8 {
    *once!(u8, {
        println!("Calculating the value");

        3 * 2 * 7 * 5 / 4 - 10
    })
}