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Auto merge of #79670 - Nadrieril:uninhabited-query, r=estebank

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Turn type inhabitedness into a query to fix `exhaustive_patterns` perf

We measured in https://github.com/rust-lang/rust/pull/79394 that enabling the [`exhaustive_patterns` feature](https://github.com/rust-lang/rust/issues/51085) causes significant perf degradation. It was conjectured that the culprit is type inhabitedness checking, and [I hypothesized](https://github.com/rust-lang/rust/pull/79394#issuecomment-733861149) 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](https://github.com/rust-lang/rustc-perf/pull/801). 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
This commit is contained in:
bors 2021-01-12 22:58:42 +00:00
commit 058a710165
15 changed files with 563 additions and 522 deletions
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9
compiler/rustc_middle/src/query/mod.rs
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@ -1319,6 +1319,15 @@ rustc_queries! {
eval_always
desc { |tcx| "computing visibility of `{}`", tcx.def_path_str(def_id) }
}
/// Computes the set of modules from which this type is visibly uninhabited.
/// To check whether a type is uninhabited at all (not just from a given module), you could
/// check whether the forest is empty.
query type_uninhabited_from(
key: ty::ParamEnvAnd<'tcx, Ty<'tcx>>
) -> ty::inhabitedness::DefIdForest {
desc { "computing the inhabitedness of `{:?}`", key }
}
}
Other {