[Pitch] Reflection

The type system representation proposed here recently came up in the review of SE-0385:

I agree that we should settle on one family of types to represent the type system that can be used throughout various language features, including the Reflection APIs proposed here, custom reflection metadata as proposed in SE-0385, and possibly other areas of the language that need to express requirements on declarations in Swift source code.

For example, I'm working on the design and implementation for attached macros, which would also benefit from a way to specify requirements on the "attached to" declaration, very similar to what's possible in SE-0385. Consider the @Persisted property wrapper from the Realm package. It may be possible to express this API as a macro that generates accessors directly, rather than using a property wrapper:

@attached(accessor) macro Persisted = #externalMacro(...)

class Dog {
  @Persisted var name: String
  @Persisted var age: Int

The @Persisted property wrapper today requires the wrapped value to conform to _Persistable. There's currently no way to express this requirement in an attached macro declaration. We could invent an ad-hoc representation of the various kinds of declarations, mirroring what SE-0385 did with the attachedTo: initializer parameter, but I think it would be much better to have a consistent representation across these various features.