RFC: Open up your enum with an unnamed variant

[kupiakos]

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Enable ranges of enum discriminants to be claimed ahead of time with an unnamed enum variant. This requires all users of that enum to consider those values as valid - including within the declaring crate.

This is useful for any situation in which recompiling an enum to handle new discriminant values is infeasible, and using a newtype integer is unergonomic due to the type's intended nature as a set of enumerated values. This includes FFI, Protobuf, and embedded syscalls.

If an enum is valid for every discriminant (it has no niches), it is an open enum and may be as cast from its explicit backing integer. For example:

#[repr(u32)]     // Fruit is represented by any `u32` due to the unnamed *open variant* below.
enum Fruit {
    Apple,       // Apple is represented with 0u32.
    Orange,      // Orange is represented with 1u32.
    Banana = 4,  // Banana is represented with 4u32.
    _ = ..,      // Unnamed variants in `Fruit` are represented with the
                 // remaining discriminants in `u32`.
}
// Using an `as` cast from `u32`, since it is an open enum.
let fruit = 3 as Fruit;

// Does not match any of the known variants.
match fruit {
    Fruit::Apple | Fruit::Orange | Fruit::Banana => unreachable!(),
    // This wildcard branch is required for all exhaustive matches of `Fruit`.
    // `fruit` preserves its value casting back to `u32`.
    _ => assert_eq!(fruit as u32, 3),
}

While writing this RFC I was not aware of the recent work on a similar RFC by @madsmtm. I considered using an attribute to make an open enum before I had learned about this RFC - I explain why I chose unnamed variants instead in the Alternatives.

@rustbot label T-lang

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Rendered.