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Split implied and super predicate queries, then allow elaborator to filter only supertraits #107614
Split implied and super predicate queries, then allow elaborator to filter only supertraits #107614
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Some changes occurred to the core trait solver cc @rust-lang/initiative-trait-system-refactor |
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I like this. I'll try to review it this weekend if @oli-obk doesn't beat me to it. Thanks for putting up with my (likely not that helpful) comment of "seems like a there's a better solution". (I still think there's probably some way to not have to filter |
I'd be happy to hear what you have in mind, but I'm not sure if I fully understand -- the filtering has to happen somewhere, it's just that I'm able to move this filtering "into" these two queries, instead of having to do it after elaboration. Anyways, this also probably needs a perf run. @bors try @rust-timer queue |
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⌛ Trying commit af7d9b076bbbd92e19184af024a637826afe9201 with merge 90322d178945ecd686f6c925d195723e108d67dd... |
idk, maybe two different Elaborator traits? Or two different interfaces to that? |
☀️ Try build successful - checks-actions |
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Finished benchmarking commit (90322d178945ecd686f6c925d195723e108d67dd): comparison URL. Overall result: no relevant changes - no action neededBenchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf. @bors rollup=never Instruction countThis benchmark run did not return any relevant results for this metric. Max RSS (memory usage)This benchmark run did not return any relevant results for this metric. CyclesResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
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another alternative would be to rip out trait aliases entirely? 😁 Making a PR and making an lang FCP to unaccept the RFC |
@rustbot label: -T-compiler +T-types |
@rustbot author Waiting to answer the question about "do we want trait aliases at all" |
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💔 Test failed - checks-actions |
@bors retry |
☀️ Test successful - checks-actions |
Finished benchmarking commit (9be9b5e): comparison URL. Overall result: no relevant changes - no action needed@rustbot label: -perf-regression Instruction countThis benchmark run did not return any relevant results for this metric. Max RSS (memory usage)ResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
CyclesResultsThis is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.
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Ignore own item bounds in parent alias types in `for_each_item_bound` Fixes rust-lang#120912 I want to get a vibe check on this approach, which is very obviously a hack, but I believe something that is forwards-compatible with a more thorough solution and "good enough for now". The problem here is that for a really deep rigid associated type, we are now repeatedly considering unrelated item bounds from the parent alias types, meaning we're doing a *lot* of extra work in the MIR inliner for deeply substituted rigid projections. This feels intimately related to rust-lang#107614. In that PR, we split *supertrait* bounds (bound which share the same `Self` type as the predicate which is being elaborated) and *implied* bounds (anything that is implied by elaborating the predicate). The problem here is related to the fact that we don't maintain the split between these two for `item_bounds`. If we did, then when recursing into a parent alias type, we'd want to consider only the bounds that are given by [`PredicateFilter::All`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/astconv/enum.PredicateFilter.html#variant.SelfOnly) **except** those given by [`PredicateFilter::SelfOnly`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/astconv/enum.PredicateFilter.html#variant.SelfOnly).
Ignore own item bounds in parent alias types in `for_each_item_bound` Fixes rust-lang#120912 I want to get a vibe check on this approach, which is very obviously a hack, but I believe something that is forwards-compatible with a more thorough solution and "good enough for now". The problem here is that for a really deep rigid associated type, we are now repeatedly considering unrelated item bounds from the parent alias types, meaning we're doing a *lot* of extra work in the MIR inliner for deeply substituted rigid projections. This feels intimately related to rust-lang#107614. In that PR, we split *supertrait* bounds (bound which share the same `Self` type as the predicate which is being elaborated) and *implied* bounds (anything that is implied by elaborating the predicate). The problem here is related to the fact that we don't maintain the split between these two for `item_bounds`. If we did, then when recursing into a parent alias type, we'd want to consider only the bounds that are given by [`PredicateFilter::All`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/astconv/enum.PredicateFilter.html#variant.SelfOnly) **except** those given by [`PredicateFilter::SelfOnly`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_hir_analysis/astconv/enum.PredicateFilter.html#variant.SelfOnly).
…oli-obk Split an item bounds and an item's super predicates This is the moral equivalent of rust-lang#107614, but instead for predicates this applies to **item bounds**. This PR splits out the item bounds (i.e. *all* predicates that are assumed to hold for the alias) from the item *super predicates*, which are the subset of item bounds which share the same self type as the alias. ## Why? Much like rust-lang#107614, there are places in the compiler where we *only* care about super-predicates, and considering predicates that possibly don't have anything to do with the alias is problematic. This includes things like closure signature inference (which is at its core searching for `Self: Fn(..)` style bounds), but also lints like `#[must_use]`, error reporting for aliases, computing type outlives predicates. Even in cases where considering all of the `item_bounds` doesn't lead to bugs, unnecessarily considering irrelevant bounds does lead to a regression (rust-lang#121121) due to doing extra work in the solver. ## Example 1 - Trait Aliases This is best explored via an example: ``` type TAIT<T> = impl TraitAlias<T>; trait TraitAlias<T> = A + B where T: C; ``` The item bounds list for `Tait<T>` will include: * `Tait<T>: A` * `Tait<T>: B` * `T: C` While `item_super_predicates` query will include just the first two predicates. Side-note: You may wonder why `T: C` is included in the item bounds for `TAIT`? This is because when we elaborate `TraitAlias<T>`, we will also elaborate all the predicates on the trait. ## Example 2 - Associated Type Bounds ``` type TAIT<T> = impl Iterator<Item: A>; ``` The `item_bounds` list for `TAIT<T>` will include: * `Tait<T>: Iterator` * `<Tait<T> as Iterator>::Item: A` But the `item_super_predicates` will just include the first bound, since that's the only bound that is relevant to the *alias* itself. ## So what This leads to some diagnostics duplication just like rust-lang#107614, but none of it will be user-facing. We only see it in the UI test suite because we explicitly disable diagnostic deduplication. Regarding naming, I went with `super_predicates` kind of arbitrarily; this can easily be changed, but I'd consider better names as long as we don't block this PR in perpetuity.
…cates-always, r=<try> Encode implied predicates for traits In rust-lang#112629, we decided to make associated type bounds in the "supertrait" AST position *implied* even though they're not supertraits themselves. This means that the `super_predicates` and `implied_predicates` queries now differ for regular traits. The assumption that they didn't differ was hard-coded in rust-lang#107614, so in cross-crate positions this means that we forget the implied predicates from associated type bounds. This isn't unsound, just kind of annoying. This should be backported since associated type bounds are slated to stabilize for 1.78 -- either that, or associated type bounds can be reverted on beta and re-shipped in 1.79 with this patch. Fixes rust-lang#122859
…cates-always, r=oli-obk Encode implied predicates for traits In rust-lang#112629, we decided to make associated type bounds in the "supertrait" AST position *implied* even though they're not supertraits themselves. This means that the `super_predicates` and `implied_predicates` queries now differ for regular traits. The assumption that they didn't differ was hard-coded in rust-lang#107614, so in cross-crate positions this means that we forget the implied predicates from associated type bounds. This isn't unsound, just kind of annoying. This should be backported since associated type bounds are slated to stabilize for 1.78 -- either that, or associated type bounds can be reverted on beta and re-shipped in 1.79 with this patch. Fixes rust-lang#122859
…cates-always, r=oli-obk Encode implied predicates for traits In rust-lang#112629, we decided to make associated type bounds in the "supertrait" AST position *implied* even though they're not supertraits themselves. This means that the `super_predicates` and `implied_predicates` queries now differ for regular traits. The assumption that they didn't differ was hard-coded in rust-lang#107614, so in cross-crate positions this means that we forget the implied predicates from associated type bounds. This isn't unsound, just kind of annoying. This should be backported since associated type bounds are slated to stabilize for 1.78 -- either that, or associated type bounds can be reverted on beta and re-shipped in 1.79 with this patch. Fixes rust-lang#122859
…oli-obk Split an item bounds and an item's super predicates This is the moral equivalent of rust-lang#107614, but instead for predicates this applies to **item bounds**. This PR splits out the item bounds (i.e. *all* predicates that are assumed to hold for the alias) from the item *super predicates*, which are the subset of item bounds which share the same self type as the alias. ## Why? Much like rust-lang#107614, there are places in the compiler where we *only* care about super-predicates, and considering predicates that possibly don't have anything to do with the alias is problematic. This includes things like closure signature inference (which is at its core searching for `Self: Fn(..)` style bounds), but also lints like `#[must_use]`, error reporting for aliases, computing type outlives predicates. Even in cases where considering all of the `item_bounds` doesn't lead to bugs, unnecessarily considering irrelevant bounds does lead to a regression (rust-lang#121121) due to doing extra work in the solver. ## Example 1 - Trait Aliases This is best explored via an example: ``` type TAIT<T> = impl TraitAlias<T>; trait TraitAlias<T> = A + B where T: C; ``` The item bounds list for `Tait<T>` will include: * `Tait<T>: A` * `Tait<T>: B` * `T: C` While `item_super_predicates` query will include just the first two predicates. Side-note: You may wonder why `T: C` is included in the item bounds for `TAIT`? This is because when we elaborate `TraitAlias<T>`, we will also elaborate all the predicates on the trait. ## Example 2 - Associated Type Bounds ``` type TAIT<T> = impl Iterator<Item: A>; ``` The `item_bounds` list for `TAIT<T>` will include: * `Tait<T>: Iterator` * `<Tait<T> as Iterator>::Item: A` But the `item_super_predicates` will just include the first bound, since that's the only bound that is relevant to the *alias* itself. ## So what This leads to some diagnostics duplication just like rust-lang#107614, but none of it will be user-facing. We only see it in the UI test suite because we explicitly disable diagnostic deduplication. Regarding naming, I went with `super_predicates` kind of arbitrarily; this can easily be changed, but I'd consider better names as long as we don't block this PR in perpetuity.
…e1-dead,fmease Assert that `explicit_super_predicates_of` and `explicit_item_super_predicates` truly only contains bounds for the type itself We distinguish _implied_ predicates (anything that is implied from elaborating a trait bound) from _super_ predicates, which are are the subset of implied predicates that share the same self type as the trait predicate we're elaborating. This was originally done in rust-lang#107614, which fixed a large class of ICEs and strange errors where the compiler expected the self type of a trait predicate not to change when elaborating super predicates. Specifically, super predicates are special for various reasons: they're the valid candidates for trait upcasting, are the only predicates we elaborate when doing closure signature inference, etc. So making sure that we get this list correct and don't accidentally "leak" any other predicates into this list is quite important. This PR adds some debug assertions that we're in fact not doing so, and it fixes an oversight in the effect desugaring rework.
…e1-dead,fmease Assert that `explicit_super_predicates_of` and `explicit_item_super_predicates` truly only contains bounds for the type itself We distinguish _implied_ predicates (anything that is implied from elaborating a trait bound) from _super_ predicates, which are are the subset of implied predicates that share the same self type as the trait predicate we're elaborating. This was originally done in rust-lang#107614, which fixed a large class of ICEs and strange errors where the compiler expected the self type of a trait predicate not to change when elaborating super predicates. Specifically, super predicates are special for various reasons: they're the valid candidates for trait upcasting, are the only predicates we elaborate when doing closure signature inference, etc. So making sure that we get this list correct and don't accidentally "leak" any other predicates into this list is quite important. This PR adds some debug assertions that we're in fact not doing so, and it fixes an oversight in the effect desugaring rework.
Rollup merge of rust-lang#130666 - compiler-errors:super-bounds, r=fee1-dead,fmease Assert that `explicit_super_predicates_of` and `explicit_item_super_predicates` truly only contains bounds for the type itself We distinguish _implied_ predicates (anything that is implied from elaborating a trait bound) from _super_ predicates, which are are the subset of implied predicates that share the same self type as the trait predicate we're elaborating. This was originally done in rust-lang#107614, which fixed a large class of ICEs and strange errors where the compiler expected the self type of a trait predicate not to change when elaborating super predicates. Specifically, super predicates are special for various reasons: they're the valid candidates for trait upcasting, are the only predicates we elaborate when doing closure signature inference, etc. So making sure that we get this list correct and don't accidentally "leak" any other predicates into this list is quite important. This PR adds some debug assertions that we're in fact not doing so, and it fixes an oversight in the effect desugaring rework.
Split the
super_predicates_of
query into a newimplied_predicates_of
query. The former now only returns the real supertraits of a trait alias, and the latter now returns the implied predicates (which include all of thewhere
clauses of the trait alias). The behavior of these queries is identical for regular traits.Now that the two queries are split, we can add a new filter method to the elaborator,
filter_only_self()
, which can be used in instances that we need only the supertrait predicates, such as during the elaboration used in closure signature deduction. This toggles the usage ofsuper_predicates_of
instead ofimplied_predicates_of
during elaboration of a trait predicate.This supersedes #104745, and fixes the four independent bugs identified in that PR.
Fixes #104719
Fixes #106238
Fixes #110023
Fixes #109514
r? types