Add proto_toolchain rule

[Yannic]

No description provided.

[Yannic]

@comius This is a Starlark implementation of proto_toolchain that you asked for, PTAL (you only need to look at the last commit, the other commits come from #77 because there are merge conflicts with that PR. Will rebase once the other PR lands)

[comius]

Looks good. If you know any good alternatives for the transition, feel free to suggest them. Otherwise LGTM.

[Yannic]

(I'll fix the nits later today)

I agree that we should migrate away from the command-line flags, however, I'd like to start with using proto_toolchain to expose the command-line flags, then migrate the native rule to depend on proto_toolchain and, once that's complete, deprecate and remove the command-line flags. That avoids inconsistencies in the migration period.

[comius]

(I'll fix the nits later today)

I agree that we should migrate away from the command-line flags, however, I'd like to start with using proto_toolchain to expose the command-line flags, then migrate the native rule to depend on proto_toolchain and, once that's complete, deprecate and remove the command-line flags. That avoids inconsistencies in the migration period.

I would rather see that proto_toolchain rule looks like the final version, that is with a mandatory attribute for protoc, with a default value set. Then add a flag --incompatible_use_toolchain_resolution_for_proto_rules, which is used in the actual rule implementation and decides either to use the toolchain's protoc or the one from flag.

This direction won't block you, when you'll need to add multiple proto_toolchain definitions with protoc for different OS and CPUs.

You will still need an implicit dependency in the proto rules. But even before proto rules are in Starlark, we can flip the toolchain resolution flag and remove protoc flag.

[Yannic]

Sure, I can create something similar to what C++/Java do

[Yannic]

PTAL

Also, could you try to find a reviewer for #77? It seems like we only need a readability review, so I guess anyone with C++ readability could review if @lberki can't do it right now.

[comius]

Sure, I can create something similar to what C++/Java do

May I suggest that you write a proposal for proto rules toolchainization? See https://github.com/bazelbuild/proposals
With a clear plan, PRs might go through faster, going less back and forth.

[Yannic]

Sure, I can create something similar to what C++/Java do

May I suggest that you write a proposal for proto rules toolchainization? See https://github.com/bazelbuild/proposals
With a clear plan, PRs might go through faster, going less back and forth.

Done (https://docs.google.com/document/d/1go3UMwm2nM8JHhI3MZrtyoFEy3BYg5omGqQmOwcjmNE/edit) PTAL

[comius]

I'm closing this because it's outdated, and we probably don't need proto_toolchain rule.