wants_id.rs

trait Identity {
    fn call<T>(&self, T) -> T;
}

macro_rules! id {
    ($closure:expr) => {{
        struct ThisID;
        impl Identity for ThisID {
            fn call<T>(&self, t: T) -> T {
                $closure(t)
            }
        }
        ThisID
    }}
}

fn wants_id<F: Identity>(f: F) {
    f.call(5);
    f.call("Hello");
}

fn main() {
    wants_id(id!(|a| a));
}