Support for proto-loader "virtual" oneof field

[jtimmons]

The @grpc/proto-loader library (documentation) supports a oneofs config option which adds some additional information to a deserialized message to identify which field in a oneof (if any) the user specified. My team makes heavy use of this feature, so not having that information in our typedefs is a bit of a hinderance to us switching over to ts-proto.

Proposal

Introduce a new oneof=virtual_properties option which would behave the same as the existing properties option but add in the proto-loader "virtual" property as described below.

Example

Given the following proto message definition:

message ExampleMessage {
  oneof contact {
    string phoneNumber = 1;
    string email = 2;
  }
}

deserializing a message with the email field specified results in the following object. Note the contact field which proto-loader populates to let you know which field in the oneof the user actually provided:

{
  contact: "email",
  email: "[email protected]"
}

specifying this in the interface generated by ts-proto could look like the following:

export interface ExampleMessage {
  phoneNumber: string | undefined;
  email: string | undefined;
  contact: 'phoneNumber' | 'email'; // new
}

even better would be to use a discriminating union similar to the oneof=union option but that might be a good amount of work. If people agree with this as a proposal I'd be happy to take a stab at it