Sync from Piper @306496510

PROTOBUF_SYNC_PIPER

google/protobuf/descriptor.proto

@@ -217,10 +217,21 @@ message FieldDescriptorProto {
   // If true, this is a proto3 "optional". When a proto3 field is optional, it
   // tracks presence regardless of field type.
   //
-  // For message fields this doesn't create any semantic change, since
-  // non-repeated message fields always track presence. However it still
+  // When proto3_optional is true, this field must be belong to a oneof to
+  // signal to old proto3 clients that presence is tracked for this field. This
+  // oneof is known as a "synthetic" oneof, and this field must be its sole
+  // member (each proto3 optional field gets its own synthetic oneof). Synthetic
+  // oneofs exist in the descriptor only, and do not generate any API. Synthetic
+  // oneofs must be ordered after all "real" oneofs.
+  //
+  // For message fields, proto3_optional doesn't create any semantic change,
+  // since non-repeated message fields always track presence. However it still
   // indicates the semantic detail of whether the user wrote "optional" or not.
-  // This can be useful for round-tripping the .proto file.
+  // This can be useful for round-tripping the .proto file. For consistency we
+  // give message fields a synthetic oneof also, even though it is not required
+  // to track presence. This is especially important because the parser can't
+  // tell if a field is a message or an enum, so it must always create a
+  // synthetic oneof.
   //
   // Proto2 optional fields do not set this flag, because they already indicate
   // optional with `LABEL_OPTIONAL`.