Set true to use the old proto1 MessageSet wire format for extensions. This is provided for backwards-compatibility with the MessageSet wire format. You should not use this for any other reason: It's less efficient, has fewer features, and is more complicated.

The message must be defined exactly as follows:

message Foo {
  option message_set_wire_format = true;
  extensions 4 to max;

Note that the message cannot have any defined fields; MessageSets only have extensions.

All extensions of your type must be singular messages; e.g. they cannot be int32s, enums, or repeated messages.

Because this is an option, the above two restrictions are not enforced by the protocol compiler.

MessageSetWireFormat is referenced in 1 repository