enum E {
    A,
    B,
    C,
    D(i32),
}

macro_rules! unit_type {
    () => (E::A | E::B | E::C)
}

fn main() {
    match &E::A {
        E::D(i) => println!("{}", i),
        unit_type!() => println!("unit type"),
        // is there a way to let the compiler know there are no more possible matches?
    }
}