Implement the new proto3 field presence feature

[garyp]

See protocolbuffers/protobuf#1606 (comment) and https://github.com/protocolbuffers/protobuf/blob/master/docs/implementing_proto3_presence.md for details.

When this feature is in use, pbandk should generate code for primitive fields using a nullable type that defaults to null, rather than a non-null type that defaults to empty string, zero, false, etc.

E.g., current code for this protobuf definition:

syntax = "proto3";

message Foo {
  int32 foo = 1;
}

will look like:

data class Foo(val foo: Int = 0)

Using the new optional presence feature, this protobuf definition:

syntax = "proto3";

message Foo {
  optional int32 foo = 1;
}

should look like:

data class Foo(val foo: Int? = null)

[Maragues]

Hi, I'm experiencing a problem with nullability generation. My scenario is a bit different in that I use .ptbtxt as input, not .proto, and protobuf-gradle-plugin.

Given this input

file {
  name: "data_models.proto"
  package: "my.package"
  message_type {
    name: "User"
    field {
      name: "name"
      number: 2
      label: LABEL_OPTIONAL
      type: TYPE_STRING
    }

I get this kotlin class

@pbandk.Export
data class User(
    val name: String = "",

I need name to be of type String?, not String.

I'm using pbandk 0.14.3. This is the protoc command

/.../protoc-3.21.9-osx-x86_64.exe
--plugin=protoc-gen-pbandk=<some-module-path>>/scripts/protoc-gen-pbandk-jvm-0.14.3-jvm8-generateProto-trampoline.sh
--pbandk_out=log=debug:<some-module-path>>/generated/source/proto/main/pbandk
--descriptor_set_in=/tmp/data_models.pb
data_models.proto

Would you expect this to generate null types, or do .proto files produce something different than text format?

---

Technical details, in case it's relevant

I parse pbtxt to a FileDescriptorSet, and write it to a temporary file. I checked that the FieldDescriptorProto.label is still LABEL_OPTIONAL

file.inputStream()
  .use { inputStream: FileInputStream ->
    FileDescriptorSet.parseFrom(inputStream.readAllBytes()).also {
      it.getFile(0).messageTypeList.forEach { message ->
        println("Message ${message.name}")

        message.fieldList.forEach { field: FieldDescriptorProto ->
            println("\t${field.name}, label ${field.label}")
        }
      }
    }
  }

prints

Message User
        name, label LABEL_OPTIONAL