ErrorProne flags Subscribe annotated methods as UnusedMethod

[bright-tools]

We've found that ErrorProne flags all @Subscribe annotated methods with an UnusedMethod warning

ErrorProne has a Keep annotation which could be applied to the Subscribe annotation in order to avoid this.

The alternatives seem to be suppressing the warning for each method or adding the subscribe annotation to a growing exemption list.

Would you be open to a PR to annotate Subscribe with @keep?

[cpovirk]

Thanks. It looks like UnusedMethod has a special case for @Subscribe already, but it's accidentally set up only for our internal builds.

That special case predates the creation of @Keep, so @Keep wasn't an option back then. We'll either add @Keep now or make the special case public.

[cpovirk]

(I should always take EventBus bugs as an opportunity to point people to our docs that recommend against using it if you can help it. But we should fix this regardless.)

[cpovirk]

This will be fixed in the next Error Prone release.

It's probably true that @Keep would be better. Hopefully that, like the special case in Error Prone, would work even if users omit error_prone_annotations from their own builds (though we recommend against that). I went with the fix on the Error Prone side just because it was slightly easier, especially to handle more annotations than just Guava's.