[Pitch] `@OptionSet` macro

[tera]

@John_McCall

I'm not trying to argue that this is an inappropriate use of macros. I'm saying that I don't like OptionSet and don't want to encourage people to use that as a design pattern. The right language model for these types is a struct with a property for each option, not a collection of separate option values.

How does that reconcile with the fact that modern swift-only framework from Apple (SwiftUI) uses OptionSets massively? Being swift-only there's no Obj-C compatibility concerns in this case, just these:

• use bits for flags instead of full bytes to save space (compiler packing bools to bits is a pipe dream yet).

• keep the structs that hold option sets POD (if they were POD already)

• use compact syntax to represent set values, like so:

    SubView()
        .padding([.top, .trailing], 10)
        .padding(.all.subtracting(.top), 20)
  
    // vs somewhat mildly heavier:
  
    SubView()
        .padding(.init(top: true, trailing: true), 10)
        .padding(.init(top: true, trailing: true, bottom: true), 20)