Matching on exhaustive enum errors with `const_if_match`

[jhpratt]

Example: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=13a9fbc4251d7db80f5d63b1dc35a98b

#![feature(const_if_match)]

enum E {
    A,
    B,
    C
}

const fn f(e: E) {
    match e {
        E::A => {}
        E::B => {}
        E::C => {}
    }
}

error[E0723]: const fn with unreachable code is not stable
  --> src/lib.rs:10:11
   |
10 |     match e {
   |           ^
   |
   = note: for more information, see issue https://github.com/rust-lang/rust/issues/57563
   = help: add `#![feature(const_fn)]` to the crate attributes to enable

CC: #49146, @ecstatic-morse

[ecstatic-morse]

The error is emitted from this match arm. Adding #![feature(const_fn)] does enable the example above. Interestingly, no Unreachable terminator appears in the MIR for that function. This suggests that we are running const checks before a cleanup pass that removes Unreachable terminators for exhaustive match expressions.

@oli-obk We have a few choices here. I am leaning towards just removing Unreachable from the min_const_fn checks. What do you think?

cc @rust-lang/wg-const-eval

[Centril]

I am leaning towards just removing Unreachable from the min_const_fn checks. What do you think?

Let's gate the removal on const_if_match for now to make sure we don't accidentally allow anything on stable.

[oli-obk]

I am leaning towards just removing Unreachable from the min_const_fn checks. What do you think?

Oh yea, I just left it in there so we don't accidentally allow something. I couldn't figure out how to trigger it, so I preferred to deny it.