Avoid fetching `@protobuf` unconditionally

[malkia]

It was removed from MODULE.bazel here - 4b1c73e -

But then added (I guess by accident) in the next change - 80b4e1a

cc: @comius @hvadehra

[pzembrod]

Thanks for the notice!

[hvadehra]

But then added (I guess by accident) in the next change

Why do you think this was an accident? If you look at the diff in 80b4e1a , cc_shared_library.bzl makes use of @com_google_protobuf

[malkia]

I thought the goal was to remove the protobuf dependency from rules_cc - If cc_shared_library needs it - are there plans to remove the need for this? At the end of the day is not the biggest deal ever (I do use protobuf / grpc) but sometimes you have a project where neither is used, and it's bit pointless that your CI has to pull something that does not get used.

Is it possible that cc_shared_library only needs this if certain aspect and/or only during testing (devonly of sorts)?

[hvadehra]

Ah, I see. While Bazel lazily fetches repos, (the transitive closure of) bzl files are loaded eagerly. Since the cc rules are going away in Bazel 9, but we'd like rules_cc to remain backwards-compatible with older Bazel versions, we're using a repo rule (the cc_compatibility_proxy) to switch between native and Starlark rules based on the Bazel version.

As currently implemented, there is a single proxy.bzl file that loads all the rules, so we have the following load graph

(e.g.) cc/cc_library.bzl -> proxy.bzl -> <all cc/private/rule_impl/*.bzl files, including cc_shared_library>

1. I imagine this could be fixed by having separate proxy files for each rule which would mean @protobuf is only fetched if one were to load cc/cc_shared_library.bzl. A small caveat is, cc/private/rules_impl/{cc_binary,cc_test}.bzl also load some symbols from cc_shared_library.bzl but that can be fixed by moving those to a separate bzl file.

2. Longer term, this problem will organically go away when Bazel 9 is the minimum supported version, and we can drop the compatibility proxy, in Dec 2027.

3. (This is just for completeness, we definitely don't want to do this) Have two separate releases for rules_cc for now on, the regular rules_cc only supports Bazel >= 9, and a legacy_rules_cc for Bazel <= 8.

I'm re-titling and re-opening this issue to track the fix.

Priority is now P3 to indicate that if someone wants to implement (1), I'm happy to review. Else we can wait for (2).

[malkia]

Thanks for the detailed explanation @hvadehra!