Conditionaly adding services to router

[hlb8122]

Feature Request

Motivation

I have a server which will enjoy up to 5 different services, each of which should be able to be enable/disabled at boot time. Currently there seems to be no way to do this without 2^5 different branches.

Proposal

Diesel has a similar problem when conditionally constructing queries, their solution is to provide a into_boxed() method.

A similar method could be added to Router.

A subsequent problem is that there is no way to generate a router with no services from a Server - this would also be required to conditionally add services. This could be done via a into_router() method on the Server.

The potential drawbacks here are that doing this may have unintended overhead due to the required boxing?

[LucioFranco]

So I unfortunately think this is not possible right now due to our https://docs.rs/tonic/0.1.1/tonic/transport/trait.NamedService.html#associatedconstant.NAME trait since it contains a const type. This makes it hard to implement Either for services. We also can't do the same for a trait object since the type system relies upon knowing the type to generate the correct path check.

So to implement this we would need to make a breaking change. So I'm curious if maybe using a macro could help here? If you end up running into many branches you may wan to use another type of router than the one tonic provides.

[hlb8122]

@LucioFranco I think a macro would be great substitute as long as n isn't too large, yes. I appreciate the modularity of tonic allowing me to switch out the router if I wish.

Wondering about another possibility:
Instead of conditionally adding a service X you conditionally switch X out for an "empty service" with the same signature which always returns unavailable (or whatever is appropriate). Does that sound feasible?

[LucioFranco]

I am not sure that will work but I'd be open to a PR for that!