I reject the idea of having to use a compiler for ProtoBuf. I think you should be able to Marshal and Unmarshal just like JSON. And really, that should be possible. If they had only added a single extra wiretype for messages, ProtoBuf would be more or less self describing. At any rate, I want a package that can decode arbitrary ProtoBuf, and can also encode some Map or Struct into ProtoBuf as well.
If you use generic add with method interface, it would allow recurive slices. We could solve this by also adding type interface, but its probably simpler to just not use generics for this.
I tried, but Unmarshal is way faster:
Benchmark_Decode-12 367295 ns/op 1481091 B/op 2588 allocs/op
Benchmark_Unmarshal-12 86033 ns/op 80245 B/op 1104 allocs/op