Inner match doesn't narrow exhaustiveness scope based on outer binding

[HackAttack]

It seems like the following should compile

enum Enum {
    A,
    B,
    C,
}

fn main() {
    match Enum::A {
        x @ Enum::A | x @ Enum::B => match x {
            Enum::A => println!("a"),
            Enum::B => println!("b"),
            // _ => unreachable!(),  // Won't compile without this line
        }
        Enum::C => println!("c"),
    }
}

because the inner match only executes on A and B. But exhaustiveness is checked over the whole enum even though only a subset of variants are possible in this context.

Maybe related: #8391

[jonas-schievink]

Hi! This is currently working as intended, and changing this would be a language change. Additions or changes to the language need to go through our RFC process. Before an RFC is written, the feature can also be discussed in our internals forum to find other people interested in it.

Closing in favor of an RFC.