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

impl E {
    fn is_unit_type(&self) -> bool {
        match self {
            E::A | E::B | E::C => true,
            _ => false
        }
    }
}

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