How to restrict a protocol to value types only?

Question

Similar in spirit to this question, except, instead of limiting a protocol to only classes, I want to be able to define a protocol that can only be adopted by enums, structs

Answer 1

I couldn't find the answer, but through playing around, I've come to the conclusion that you can't. Being that to restrict a protocol to just classes, you precede it with class like

protocol SomeProto: class {
    func structYourStuff() -> Void
}

Making the assumption that this would be consistent among other types, I tried

protocol SomeProto: struct {
    func structYourStuff() -> Void
}

But Xcode gave me five different errors on one line, which brings me to the conclusion that you can't. I could be completely wrong though, I only started learning Swift about a week ago