[Pitch] Generalize `conformance` macros as `extension` macros

[Douglas_Gregor]

@gwendal.roue

ow, maybe you know it: the @Model macro adds protocol conformances:

// Expansion of `@Model` adds at least PersistentModel
// and Observable conformances:
enum SchemaV1: VersionedSchema {
  final class Language {
    var name: String
  }
  extension Language: PersistentModel { }
  extension Language: Observable { }
}

Hey, but this code is not supposed to compile: extensions are only valid at file scope, right? So it's not possible to nest models in enums? What about this SwiftData slide, then?

The correct formulation here is

// Expansion of `@Model` adds at least PersistentModel
// and Observable conformances:
enum SchemaV1: VersionedSchema {
  final class Language {
    var name: String
  }
}

extension SchemaV1.Language: PersistentModel { }
extension SchemaV1.Language: Observable { }

i.e., this is expressible in the language, but the way it's presented on a slide and in "Expand Macro" is misleading. Cool, we can fix the presentation. It doesn't change the feature.

@gwendal.roue

May I try to suggest that the steering group tries harder, though. We all deserve the best. Macros do not operate at the level of expressible Swift code. Pretending the opposite is flatly wrong. Pick up the opportunities, and don't let half-baked and leaky abstractions rule your designs. Thanks.

I honestly have no notion of how to interpret this statement. I suspect you're extrapolating from the presentation error above regarding extensions on nested types, and also trying to refute my argument that macros shouldn't allow extensions on local types because you want that feature. But "Macros do not operate at the level of expressible Swift code" is a fairly sweeping statement that is at odds with the macro system vision and design, wholly unsubstantiated by the above. If you want to stick by that argument, please find some more substantive evidence and bring it to another thread: it's either a much bigger topic than extensions, or it's just those two things we talked about.

@gwendal.roue

May I try to suggest that the steering group tries harder, though. We all deserve the best. [...] Pretending the opposite is flatly wrong. Pick up the opportunities, and don't let half-baked and leaky abstractions rule your designs. Thanks.

The above is patronizing and rude. Please tone it down so we can have a productive discussion.

Doug