Emit a warning if a match is too complex

[GuillaumeGomez]

Based on the results of crater run from #121979, we can safely set this limit without introducing regressions in the current rust ecosystem.

Follow-up of #121917.

r? @Nadrieril

[rustbot]

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

[Nadrieril]

Suggested change

	// Default value to emit the warning for "too complex" match. We picked it based on
	// this crater run: <https://github.com/rust-lang/rust/pull/121979>.
	.or(Some(10_000_000));
	// Default value to emit the warning for "too complex" match. We picked it to warn after a second or two.
	.or(Some(10_000_000));

I don't think the crater matters much to future readers of this code, but the timing intent does.

[Nadrieril]

Actually you said that limit took more like 20s on your machine? I'd be curious to crater a lower limit then, because I'm afraid people won't wait 20s

[GuillaumeGomez]

Fine by me!

[Nadrieril]

Beautiful :D One last remark

[Nadrieril]

Dear T-lang, this PR adds a warning that cannot be silenced, triggered when a match takes a really long time to analyze (in the order of seconds). This is to help users figure out what's taking so long and fix it.

We could make the limit configurable or the warning allowable. I argue that's not necessary because crater showed zero regressions with the current limit, and it's be pretty easy in general to split up a match into smaller matches to avoid blowup.

We're still figuring out the exact limit, but does the team approve in principle?

[GuillaumeGomez]

Updated!

[GuillaumeGomez]

Fixed typo.

[traviscross]

Those interested in pushing the limits of exhaustiveness checking may be interested in:

• Compiling Rust is NP-hard
• rustc-sat