Add fuel to match checking

[Nadrieril]

Exhaustiveness checking is NP-hard hence can take extremely long to check some specific matches. This PR makes ehxaustiveness bail after a set number of steps. I chose a bound that takes ~100ms on my machine, which should be more than enough for normal matches.

I'd like someone with less recent hardware to run the test to see if that limit is low enough for them. Also curious if the r-a team thinks this is a good ballpark or if we should go lower/higher. I don't have much data on how complex real-life matches get, but we can definitely go lower than 500 000 steps.

The second commit is a drive-by soundness fix which doesn't matter much today but will matter once min_exhaustive_patterns is stabilized.

Fixes #9528 cc @matklad

[Veykril]

Running time cargo test -p ide-diagnostics exponential_match --release gives me ~344ms (debug is more like 2 seconds). I think that should be fine. Might be good to add a

    if skip_slow_tests() {
        return;
    }

in front given usually the test runs in debug. With that it gets skipped by default when people run the test suite (while CI will still always run it).

[Nadrieril]

I'll make the test even bigger actually, to be sure it hangs CI if we regress ^^

[Veykril]

Thanks 👍
@bors r+

[bors]

📌 Commit 08a5f1e has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 08a5f1e with merge a2f73d3...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing a2f73d3 to master...