Github Turn type inhabitedness into a query to fix `exhaustive_patterns` perf by...

Nadrieril commented on Dec 4, 2020 •

We measured in #79394 that enabling the exhaustive_patterns feature causes significant perf degradation. It was conjectured that the culprit is type inhabitedness checking, and I hypothesized that turning this computation into a query would solve most of the problem.

This PR turns tcx.is_ty_uninhabited_from into a query, and I measured a 25% perf gain on the benchmark that stress-tests exhaustiveness_patterns. This more than compensates for the 30% perf hit I measured when creating it. We'll have to measure enabling the feature again, but I suspect this fixes the perf regression entirely.
I'd like a perf run on this PR obviously.
I made small atomic commits to help reviewing. The first one is just me discovering the "revisions" feature of the testing framework.

I believe there's a push to move things out of rustc_middle because it's huge. I guess inhabitedness/mod.rs could be moved out, but it's quite small. DefIdForest might be movable somewhere too. I don't know what the policy is for that.

Ping @camelid since you were interested in following along
@rustbot modify labels: +A-exhaustiveness-checking