Is it possible to create a generic service that is capable of processing messages?

[zakhenry]

I attempting to create a service that can process messages without being aware of the protobuf types. I want the raw message bytes and will be sending them on to a separate service for processing, and then returning the response bytes to be handled as grpc response.

Starting with unary for now, I've been trying to construct a generic service to pass to the tonic::server::grpc::unary method as follows:

struct GenericService();
impl tonic::server::UnaryService<Box<dyn Message>> for GenericService {
    type Response = Box<dyn Message>;
    type Future = tonic::codegen::BoxFuture<
        tonic::Response<Self::Response>,
        tonic::Status,
    >;
    fn call(
        &mut self,
        request: tonic::Request<Box<dyn Message>>,
    ) -> Self::Future {
        let fut = async move {

            // process generic request and send generic response
        };
        Box::pin(fut)
    }
}

unfortunately this does not compile

error[E0277]: the size for values of type `(dyn prost::Message + 'static)` cannot be known at compilation time
   --> src/lib.rs:342:28
    |
342 |             let res = grpc.unary(method, req_reconstructed).await;
    |                            ^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `(dyn prost::Message + 'static)`, which is required by `ProstCodec<_, _>: Codec`
    = help: the trait `Codec` is implemented for `ProstCodec<T, U>`
    = note: required for `Box<(dyn prost::Message + 'static)>` to implement `prost::Message`
    = note: required for `ProstCodec<Box<(dyn prost::Message + 'static)>, ()>` to implement `Codec`
note: required by a bound in `tonic::server::Grpc::<T>::unary`
   --> /Users/zak/.cargo/registry/src/index.crates.io-6f17d22bba15001f/tonic-0.12.1/src/server/grpc.rs:47:8
    |
47  |     T: Codec,
    |        ^^^^^ required by this bound in `Grpc::<T>::unary`
...
222 |     pub async fn unary<S, B>(
    |                  ----- required by a bound in this associated function

which does surprise me as I though wrapping dyn Message in a Box would work? Any pointers greatly appreciated!

[zakhenry]

The alternative that I pursued was to bypass the unary function entirely, which would actually simplify things, however as tonic::server::grpc::map_response is not public that is a non-starter (I think?)