Field presence support for proto3

[wfansh]

Google will update the proto3 syntax to support "optional" for field presence:

• Summary
  Add field presence to proto3 in a backward-compatible way by introducing the optional keyword with the same semantics as proto2.

• .proto syntax changes
  Users can put optional on any proto3 field, with syntax and semantics identical to proto2:

syntax = "proto3";
message Foo {
  // Optional fields track presence.
  optional int32 a = 1;
  optional int64 b = 2;
  optional string c = 3;
  optional double d = 4;

Public announcement will come soon. Can we support this for proto3?

Removing line 299 - 301 would be OK.
https://github.com/square/wire/blob/master/wire-library/wire-schema/src/commonMain/kotlin/com/squareup/wire/schema/internal/parser/ProtoParser.kt#L299

[oldergod]

Does this affect the encoding on the wire? what happens on a optional int32 of value 0? Same with decoding, is an optional int32 set to 0 or to null if there's no bytes for it?

[jwendel]

Looks like the doc has been posted describing this: https://github.com/protocolbuffers/protobuf/blob/master/docs/field_presence.md

[wfansh]

Yes. That's the doc.

[wfansh]

Here is another update.
protocolbuffers/protobuf#1606 (comment)

https://github.com/protocolbuffers/protobuf/blob/master/docs/implementing_proto3_presence.md

[wfansh]

Thanks guys!