Option<T>. Protobuf "supports" this particular case via other means: messages are always implicitly nullable, and the WKTs cover primitives, which don't count

Option<T>. Protobuf "supports" this particular case via other means: messages are always implicitly nullable, and the WKTs cover primitives, which don't count as messages. Map<K, V>: Protobuf has dedicated syntax for this case.