Add protoc plugin handling to the prost-build Config and basic plugin

[dfreese]

This refactors some of the private logic within prost-build's lib.rs so
that it is compatible with generating the logic necessary to run a
plugin for protoc or a build.rs file. In both cases, the output can be
changed based on how the Config is built. This adds the capability for
paramters passed in via protoc to a plugin to allow adjust this
configuration, but the mechanics of that are left as a TODO until the
design is solidified.

This based on #312 to take care of rustfmt issues and #313 for some CI issues.

[hrvolapeter]

Hi @dfreese what's the current status of this PR, is there maybe something I could help with?

[dfreese]

@hrvolapeter I ended up developing a different approach that used prost-build directly, rather than needing this. It wasn't ideal, as it prost-build needs to write to a file.

From that experience, I don't think this is the best approach. Ideally, more information about the code generation from prost-build would be exposed, and this could be built on top of it. I have something locally that I can look at trying to wrap up and put up as a PR.

I'll start taking a look if we can open source what we ended up doing for a Bazel/Prost/Tonic integration.

[dfreese]

@hrvolapeter the approach I was hoping to take is my generate branch here:
dfreese@ab69bf0

The TODO there is the main thing I thing I was hoping add before submitting it as a PR, otherwise I end up needing to re-implement a lot of the internal logic of prost-build. Let me know if you'd want to help with that approach, or want further clarification on why I was going that route.

[samschlegel]

I'll start taking a look if we can open source what we ended up doing for a Bazel/Prost/Tonic integration.

Definitely interested in this as I'm about to go down this path myself 😄

[epigramengineer]

@dfreese @samschlegel did either of you make (publicly available) progress on this? Attempting to do the same thing myself and hate reinventing the wheel.

[dfreese]

@dfreese @samschlegel did either of you make (publicly available) progress on this? Attempting to do the same thing myself and hate reinventing the wheel.

Forgot to come back here, but I put up an initial attempt to export what we use here:
bazelbuild/rules_rust#479

[dfreese]

Closing this for now. I don't think the plugin approach is worthwhile given the functionality to create more useful binaries directly in rust.

That being said, the extern_fields logic is difficult to get right unless you know exactly how prost goes about it's naming of things. If those could be exposed as public, that would be a huge help. I can take a look at if there's a clear way to do that.