We can't use the same names for both with/without-parameter functions because there is no function overloading in C. In patch v3-0001 I've replaced the "dual set of macros", with a single inline function of a different name, and one set of space-saving macros.
I think it's a good idea to add worker type field. Trying to deduce it based on the contents of the structure isn't good. RelOptInfo, for example, has RelOptKind. But RelOptInfo has become really large with members required by all RelOptKinds crammed under the same structure. If we can avoid that here at the beginning itself, that will be great. May be we should create a union of type specific members while we are introducing the type. This will also provide some protection against unrelated members being (mis)used in the future.