trait SupportedFn<X> {
    fn arity(self) -> usize;
}

// Support functions with 1 argument
impl<T, R, F: Fn(T) -> R> SupportedFn<(T, R)> for F {
    fn arity(self) -> usize { 1 }
}

// Support functions with 2 arguments
impl<T, U, R, F: Fn(T, U) -> R> SupportedFn<(T, R, U)> for F {
    fn arity(self) -> usize { 2 }
}

// Unfortunately, this does not work for real functions. :(
fn f1(x: &u32) {}
fn main() {
    println!("f1 takes {} args", f1.arity());
}