edition = "2023";

message OptionalFields {
  message SubMessage {
    string a = 1 [features.field_presence = IMPLICIT];
  }

  SubMessage a = 1;
  string e = 2;
  uint32 r = 3 [features.field_presence = LEGACY_REQUIRED];
  uint32 i = 4  [features.field_presence = IMPLICIT];
}