SE-0402: Generalize `conformance` macros as `extension` macros

[Douglas_Gregor]

@s-k

First of all: I think the general idea of the proposal is great.

If I am not mistaken, the need to list all possible conformances in the macro declaration under conformances: is a restriction that was not present in the original conformance macros. I can imagine different scenarios where listing the conformances in advance is not possible. For example, the conformance may need to be different for every type the macro has been attached to.

I would suggest that the requirement is dropped or an arbitrary option is added such as for names:.

Having a macro introduce arbitrary conformances are something we were trying to get away from. "Does type X conform to protocol P?" is a question often asked in the compiler, and a macro that can possibly answer that question for every P will be expanded nearly all the time. That's both a compile-time performance issue, and also can introduce cyclic dependencies in type checking when that macro has interesting arguments.

We could allow the same prefixed(X) and suffixed(X) forms in the conformances list that we do for names, so one can generate a protocol and a conformance to it together, with a name derived from the entity to which the macro is attached.

Doug