I would like to support preserving unknown fields for ProtoBuf, here is an issue for discussion beforehand.

Motivation

Basically it is required in protobuf spec, so it needs to be implemented to make kotlinx.serialization a valid protobuf parser.

Basic design

A new helper class will be introduced

interface UnknownFieldsHolder: Map<ProtoId, ProtoField>

Users can add to class property according to their needs:

@Serializable
data class ForwardCompatibleData(
    @ProtoNumber(1) val name: String,
    // No @ProtoNumber needed
    // Maybe introducing a new annotation?
    val unknownFields: UnknownFieldsHolder,
)

Behaviours

• The ProtoBuf format should keeps all undeclare fields in the UnknownFieldsHolder when deserializing;
• All data in the UnknownFieldsHolder should be written back to wire as is during serializing;
• If new fields are added in oneof field, the new value will be stored in UnknownFieldsHolder, leaving the @ProtoOneOf property to be null;
• Other formats should not deseralize or serialize this property.

Further more

Enumeration

Enums in protobuf can also have unknown values in old version of code, but it is much more easier to store it in sealed interface / objects styles:

sealed interface IPhoneType {
  val value: Int
}
data object WorkPhone: IPhoneType {
  override val value: Int
    get() = 1
}
data object HomePhone: IPhoneType {
  override val value: Int
    get() = 2
}
data class UnknownPhoneType(override val value: Int): IPhoneType

Other formats

Do other formats need similar functionality?