How to refer to type of impl output in Rust?

I'm trying to implement a stream in Rust for use in a tonic GRPC handler and encountered this difficulty: most ways of creating streams don't have easily-expressible types, yet the GRPC trait I need to implement requires a specific Stream type. Something like this (simplified):

// trait to implement
trait GrpcHandler {
  type RespStream: futures::Stream<ResponseType> + Send + 'static
  fn get_resp_stream() -> Self::RespStream;
}

// a start at implementing it
impl GrpcHandler for MyHandler {
  type RespStream = ???; // what do I put here?
  fn get_resp_stream() -> Self::RespStream {
    futures::stream::unfold((), |_| async {
      tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
      Some((ResponseType {}, ()))
    })
  }
}

I know the type of my stream is technically something like Unfold<(), ComplicatedFnSignatureWithImpl, ComplicatedFutureSignatureWithImpl>, but even if I typed that whole thing in, the compiler wouldn't be happy about it being an opaque type. How can I refer to the type of this stream?

Solution

Unfortunately there is no good way in stable Rust to do that without dynamic dispatch. You have to use dyn Stream, and futures provides BoxStream for that:

impl GrpcHandler for MyHandler {
    type RespStream = futures::stream::BoxStream<'static, ResponseType>;
    fn get_resp_stream() -> Self::RespStream {
        futures::stream::unfold((), |_| async {
            tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
            Some((ResponseType {}, ()))
        })
        .boxed()
    }
}

If you use nightly, you can use the unstable type_alias_impl_trait feature to avoid the overhead of dynamic dispatch:

#![feature(type_alias_impl_trait)]

impl GrpcHandler for MyHandler {
    type RespStream = impl futures::Stream<Item = ResponseType> + Send + 'static;
    fn get_resp_stream() -> Self::RespStream {
        futures::stream::unfold((), |_| async {
            tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
            Some((ResponseType {}, ()))
        })
    }
}