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Remove the `anon` query modifier Prior experiments: - https://github.com/rust-lang/rust/pull/152268 - https://github.com/rust-lang/rust/pull/153996 [Zulip thread: *Removing the `anon` query modifier*](https://rust-lang.zulipchat.com/#narrow/channel/582699-t-compiler.2Fquery-system/topic/Removing.20the.20.60anon.60.20query.20modifier/with/580760962) --- There are currently three queries that use the `anon` modifier: - `check_representability` - `check_representability_adt_ty` - `erase_and_anonymize_regions_ty` It seems that none of them are using `anon` in an essential way. According to comments and tests, the *representability* queries mainly care about not being eligible for forcing (despite having a recoverable key type), so that if a cycle does occur then the entire cycle will be on the query stack. Replacing `anon` with a new `no_force` modifier gives a modest perf improvement. The `erase_and_anonymize_regions_ty` query appears to be using `anon` to reduce the dep-graph overhead of a query that is expected to not call any other queries (and thus have no dependencies). Replacing `anon` with either `no_hash` or nothing appears to give only a very small perf hit on `cargo` benchmarks, which is justified by the fact that it lets us remove a lot of machinery for anonymous queries. We still need to retain some of the machinery for anonymous *tasks*, because the non-query task `DepKind::TraitSelect` still uses it. --- I have some ideas for a follow-up that will reduce dep-graph overhead by replacing *all* zero-dependency nodes with a singleton node, but I want to keep that separate in case it causes unexpected issues and needs to be bisected or reverted.