trait MyTrait<I, O> {
    type MyType;

    fn my_method(&self);

}

impl<F, I, O> MyTrait<I, O> for F
where
    F: Fn(I) -> O,
{
    type MyType = F::Output;

    fn my_method(&self) {
        println!("hello my_method");
    }


}

fn main() {
    (|_: &str| {4}).my_method();
}