Sorry should have mentioned this in my initial comment, but we actually explicitly ignore the CanIgnoreReturnValueSuggester from error prone. (see here for the original commit)

If we want to add @CanIgnoreReturnValue, I think we'd want to do it holistically. Or at least commit to doing it holistically. WDYT @jkwatson? Is it worth it to go through all the builders and add an annotation?