Explicit match on uninhabited type shouldn't warn

[RalfJung]

So say I have a function that takes an uninhabited type (common e.g. when implementing a trait for !). I would like to make it explicit that I am using "ex falso quod libet" reasoning, so I write

fn foo(x: !) -> bool {
  match x {}
}

Unfortunately, now Rust warns:

warning: unreachable expression
 --> src/lib.rs:4:3
  |
4 |   match x {}
  |   ^^^^^^^^^^
  |
  = note: #[warn(unreachable_code)] on by default

It seems it wants me to instead write

fn foo(x: !) -> bool {
    x
}

I think that's bad style, now I have to think much harder to figure out what is going on ("oh, there's a coercion from uninhabited types to bool"). The original code I wrote uses a construct which only works for uninhabited types, explicitly documenting intent.

I think the compiler should not print a warning for that code. Empty matches are a useful, explicit and constructive way of saying that we are dealing with uninhabited data, we shouldn't discourage them in favor of implicit coercions that require way more context to comprehend.

[nikomatsakis]

(I agree)

[estebank]

CC #55123