Delete proto rules together or not at all about bazel-gazelle HOT 1 OPEN

Here's the use case: suppose we have a .proto file that's generated in a way that Gazelle doesn't know about. We need to add a # keep comment in the proto_library rule.

proto_library(
    name = "foo_proto",
    srcs = [
        "foo.proto", # keep
    ],
)

go_proto_library(
    name = "foo_go_proto",
    proto = ":foo_proto",
)

Since the generator doesn't know about the # keep comment, it will generate empty rules for both foo_proto and foo_go_proto. The merger considers all rules to be independent, so it will delete foo_go_proto but keep foo_proto because of the keep comment.

This gets more complicated when deps are involved. We'll need to mark all dependencies in foo_proto with # keep. I don't think the merger can reasonably translate the dependencies from proto to Go, and other phases shouldn't be concerned with deletion.

The best thing to do here is to use # keep liberally. This is a corner case where Gazelle can't get all the information it needs, so I don't think we can do much better without making the implementation complicated and inefficient.

cc @ianthehat Did I miss anything about this case? If not, I'll close as WAI.

from bazel-gazelle.