Support Private Enum Variants

There should be a way to mark enum variants as private.

Possible syntax:

enum MyThing {
  priv OneState(Foo),
  priv OtherState(Bar),
}

Motivation

Type authors should be able to use an enum to represent their type without exposing that to users. Today if a type is an enum that is visible to users so almost any change is a breaking change (I think only adding an additional private field to a struct-style variant is not breaking).

For example if the leading example was written today without priv a user would be able to do:

match v {
    MyThing::OneState(..) => true,
    MyThing::OtherState(..) => false,
};

Possible Alternatives

#[doc(hidden)]

enum MyThing {
  #[doc(hidden)] OneState(Foo),
  #[doc(hidden)] OtherState(Bar),
}

This hides it from documentation but doesn't actually protect against using it. It is possible that the user copies an old code sample, guesses a name without knowing it is private or an IDE autocompletes it and the user will unknowingly be relying on a (logically) private API.

Internal Enum

enum MyThingImpl {
  OneState(Foo),
  OtherState(Bar),
}

pub struct MyThing(MyThingImpl);

This works but results in tedious code. Internals must be accessed as self.0 and if you want to derive traits this needs to be done on both the wrapper and the impl.

Related: https://github.com/rust-lang/rfcs/pull/2028 https://github.com/rust-lang/rust/issues/32770

The issue names no files, tests, or implementation entry points. Start by reviewing the proposed syntax, motivation, alternatives, and related RFC links; done would require an agreed design for private enum variants rather than only documentation hiding.