Macro on_receive_request 

macro_rules! on_receive_request {
    () => { ... };
}

This macro is used for the value of the to_future_hack parameter of Builder::on_receive_request and Builder::on_receive_request_from.

It expands to |f, req, responder, cx| Box::pin(f(req, responder, cx)).

This is needed until return-type notation is stabilized.