Map transports not services

[rklaehn]

TLDR: instead of mapping the service, map the transport. This gets rid of the mapper in the service and makes the RpcClient/RpcChannel and friends again just pure newtype like things.

To make this work I had to make a bigger change. The various channel traits now have associated types In and Out, instead of type parameters.

• The three service types (RpcClient, RpcServer and RpcChannel) are now again pure newtypes over the underlying transports that just add the service type.

• map is available on RpcClient and RpcChannel, but unlike before it maps the underlying transport, so there is no third type parameter SC needed anymore

• the boxed transport is available by default, .boxed() is available on all transports including those that are already boxed. For RpcClient, RpcChannel and RpcServer, the transport type C is set by default to the boxed transport. That way in many cases you can avoid having to provide the transport type parameter at all, at the expense of a tiny bit of performance.

• also made the combined transport available by default - it does not have any dependencies, so it seems like having a feature flag for it is overkill.

[rklaehn]

Weird that the phantomdata is pub -> fixed

[dignifiedquire]

there is a lot of changes, is there a specific file showing the benefits of this change, to understand how it helps the user?

[rklaehn]

there is a lot of changes, is there a specific file showing the benefits of this change, to understand how it helps the user?

The closest thing to that is the smoke "test" in mapped.rs

As for a real use case: when extracting the rpc for blobs, I had the following problem. Either you have to drag around the service type parameter matching the transport channel SC everywhere, or you have to define a type alias. With this PR you simply do not have the SC type parameter anymore, so no need to drag it around.

E.g. you got blobs::Client::new. You can either make it take a RpcClient, having no type parameters at all. Then you create one by saying blobs::Client::new(iroh_rpc_client.map().boxed()).

...or take a RpcClient<BlobsService, C> where C is a channel that is compatible with BlobsService and defaults to BoxedServiceConnection. Then you can call it with blobs::Client::new(iroh_rpc_client.map()) and don't have the (small) boxing overhead.

In any case, you avoid having to have an open S: Service parameter at all in the iroh blobs client and service implementation, which at least while writing it I found horribly confusing...