Pre-RFC improved ergonomics for `!`

ais523:

I'm not sure what happens if you write the equivalent using !. In any case, it would make sense to not lint on a 0-variant match in unreachable code

programmerjake:

directly matching ! doesn't trigger any lints on stable

But doing it in unreachable code does. (I just fixed the link to the second playground in my post above and tried it.)