Consider an `IntoRequest` trait about tonic

[carllerche]

For this to work out, there would need to be two traits. IntoRequest and IntoStreamingRequest. Otherwise, you get into coherence problems.

Sketch:

struct Request<T>(T);

trait Message: std::fmt::Display {}

impl Message for &'static str {}

trait IntoRequest {
    type Message: Message;
    
    fn into_request(self) -> Request<Self::Message>;
}

trait IntoStreamingRequest {
    type Stream: Iterator<Item = Self::Message>;
    type Message: Message;
    
    fn into_streaming_request(self) -> Request<Self::Stream>;
}

impl<T: Message> IntoRequest for Request<T> {
    type Message = T;
    
    fn into_request(self) -> Self {
        self
    }
}

impl<T: Iterator> IntoStreamingRequest for Request<T> where T::Item: Message {
    type Stream = T;
    type Message = T::Item;
    
    fn into_streaming_request(self) -> Self {
        self
    }
}

impl<T: Iterator> IntoStreamingRequest for T where T::Item: Message {
    type Stream = T;
    type Message = T::Item;
    
    fn into_streaming_request(self) -> Request<Self> {
        Request(self)
    }
}

fn say_hello<T: IntoStreamingRequest<Message = &'static str>>(t: T) {
    println!("== say hello");
    for msg in t.into_streaming_request().0 {
        println!(" + {}", msg);
    }
}

fn main() {
    say_hello(vec!["hello", "world"].into_iter());
    say_hello(Request(vec!["hello", "world"].into_iter()));
}

[ChetanBhasin]

Hey! I'm fairly new to this project, but the library seems fantastic so far!

This looks like a nice first issue. I was hoping to take a crack at it sometime this week or over the weekend if a PR is welcome. 😃

[hawkw]

@ChetanBhasin I'm not sure if this is actually going to be easy, or even possible — I took a crack at it earlier, and ran into some issues with type inference. If you want to take a look at it, you may have more success than I did, but I just wanted to warn you.

[hawkw]

Oh, the solution of adding a separate trait for streaming requests is obvious in hindsight, nice!