Pre-RFC improved ergonomics for `!`

ais523:

The best way to avoid this error when using Infallible is match _b {}

Yes, there are a few ways in stable to note that the final line is unreachable. In any case, changing to ! goes from "error if you don't" to "lint if you do", and therefore drastically changes expectations on how TryTypeFoo<!>? is handled.

(I wouldn't describe anything discussed here as a bug or error, just an ergonomics concern)