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?
Unfortunately there is no good way in stable Rust to do that without dynamic dispatch. You have to use
dyn Stream
, andfutures
providesBoxStream
for that:If you use nightly, you can use the unstable type_alias_impl_trait feature to avoid the overhead of dynamic dispatch: