unassigned values for enums should be smarter about finding new options

Someone at the rust meetup pointed out to me that code like this does not compile today:

#[repr(u16)]
enum Foo {
    A,
    B = 14,
    C,
    D = 15,
}

The issue is that we attempt to assign the value 15 to C based on it falling immediately after B.

The workaround I suggested was to move the C to the end of the series (or elsewhere), so that it would be assigned a value other than 15.

It seems we could backwards-compatibly support this, by being smarter in how we search for values to assign to otherwise unassigned variants.

(However, it is noteworthy that the semantics of the above enum that I am suggesting does not match that of C semantics, at least not according to gcc, which happily assigns 15 to both C and D, as the C language has no problem with that.)

The issue names no source file, test, or entry point. Start by locating Rust's enum discriminant-assignment rules and the relevant implementation; done means the backwards-compatible value-search semantics are agreed and the example behavior is covered by focused tests.