proto3Optional property
If true, this is a proto3 "optional".
When a proto3 field is optional, it tracks presence regardless of field
type. When proto3_optional is true, this field must 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. 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.
Implementation
core.bool? proto3Optional;