Add _Enum! macros to replace _ arms when matching shared enums

[nicolas-guichard]

As the docs currently state about shared enums:

Pattern matching with match still works but will require you to write wildcard arms to handle the situation of an enum value that is not one of the listed variants.

This is unfortunate as there is no way to check that all listed enum variants are actually handled. IOW this code does not even warn:

mod ffi {
    enum Enum {
        A,
        B,
        C = 5,
    }
}

match t {
    Enum::A => {},
    Enum::B => {},
    _ => {},
}

This generates an extra _Enum! macro that matches all the ranges that are not listed by the enum. When using that macro instead of the _ catch-all, we get compile-time errors if we forget to match variants:

match t {
    Enum::A => {},
    Enum::B => {},
    _Enum!() => {},
}
// Error Enum{ repr: 5 } case not covered

The generated macro looks like:

macro_rules! _Enum {
    () => { Enum{ repr: 4..5 | 6.. } };
    ($i:ident) => { Enum { repr: $i @ 4..5 | 6.. } };
}

It can also be used to ergonomically retrieve the inner value in the unlisted case:

match t {
    Enum::A => {},
    Enum::B => {},
    Enum::C => {},
    _Enum!(value) => panic!("Unexpected Enum value {value}"),
}

I would have preferred something like Enum::unlisted!, but macro definitions are not supported in impls. I think _Enum carries the meaning of _ specific to Enum, but I'm open to other naming/namespacing ideas.

[nicolas-guichard]

NB: this TODO should obviously be fixed before this can be merged.