With RFC 4 (private fields), the unit tuple is a private field, so it can't be instantiated.

It's true that it can't be emulated exactly, it's mostly just a rough approximation. It's a good point though, and I hadn't realized it. 

Today, if you have a private variant (and therefore can't match against it), you can never exhaustively match on the enum outside the module. This means that enums with private variants can have variants added backwards-compatibly. With this RFC, no enum could have variants added backwards compatibility (guaranteed) because you could still list the public name.