Allow explicit matches on ! without warning

[varkor]

It's now possible to explicitly match on ! without an unreachable code warning. This seems desirable as promoting explicitness.

Fixes #55116.

[rust-highfive]

r? @nikomatsakis

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[RalfJung]

What about this? I think it should get the warning.

fn foo(x: !) {
    return;
    match x {}
}

Somehow it "manages" to not warn about fn foo(x: !) { x } even though the x here is just as unreachable, can't match x get a similar exception?

[varkor]

I've spotted that this the root cause here is actually different than I originally thought; the fix should be simpler and address those other cases.

[varkor]

Turns out match was being special-cased before...

[RalfJung]

LGTM :) Except of course that the test is failing...?

[nikomatsakis]

r=me with new test

[nikomatsakis]

It would be nice to test

fn main() {
  match (return) {
    () => ()
  }
}

too (this should warn, I think, on the arm?)

[varkor]

At the moment, this won't warn at all. I'll try to get the arms warning.

[varkor]

The arm warning message could be improved in the future, but this is definitely an uncommon case, so the new warning should suffice for now.

@bors r=nikomatsakis

[bors]

📌 Commit 2634646 has been approved by nikomatsakis

[bors]

⌛ Testing commit 2634646 with merge 22cc2ae...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing 22cc2ae to master...