Googler-on-googler trolling. The internal version of the build rule they're discussing has some busted dependencies. Because of historical cruft, it brings in a lot of unrelated crap. For example, building a C++ binary might build bring in Haskell dependencies because of a protocol buffer. This is being fixed, but it's been a thorn in our sides for years.
To expand a bit, the proto_library rules were "famously" broken in that it never knew which outputs were actually used (Or so i believe that was the cause, it's been a while)
Thus, it assumed the output was needed for any language specified in the proto library rule. So if your proto library rule offered a haskell version, anything using that proto library rule would do what was necessary (usually, building haskell stuff) to make that output.
The more people offered more languages in their rules, the worse it got.
I believe this was eventually fixed (i stopped coding in the main repo years ago, these days, i only hack on open source projects).
I expect one reason the rule was not open sourced was to not expose this cruft to the rest of the world :)
