What's the easiest way to do partial message decoding?

[radix]

Let's say I want to decide whether to process a protobuf message based on one or two fields, and I would like to make that decision before going to the effort of parsing the whole message.

Is there anything Prost provides that could make this easier? Do I just need to define an additional message type that includes only the subset of fields (with identical indices) and parse with that type instead? Is there any way I can say "parse the X field out of this message" or something?