macro_rules! defn {
    ($x:ident ($($a:ident: $t:ty),*) -> $ot:ty $body:block) => {
        const FUNC_NAME: &'static str = stringify!($x);
        fn $x ($($a: $t),*) -> $ot $body
    }
}

defn! { fun () -> &'static str {
    FUNC_NAME
} }

pub fn main() {
    println!("fun: {}", fun());
}