Add `AliasKind::Weak` for type aliases. #108860

oli-obk · 2023-03-07T12:08:52Z

type Foo<T: Debug> = Bar<T>; does not check T: Debug at use sites of Foo<NotDebug>, because in contrast to a

trait Identity {
    type Identity;
}
impl<T: Debug> Identity for T {
    type Identity = T;
}
<NotDebug as Identity>::Identity

type aliases do not exist in the type system, but are expanded to their aliased type immediately when going from HIR to the type layer.

Similarly:

a private type alias for a public type is a completely fine thing, even though it makes it a bit hard to write out complex times sometimes
rustdoc expands the type alias, even though often times users use them for documentation purposes
diagnostics show the expanded type, which is confusing if the user wrote a type alias and the diagnostic talks about another type that they don't know about.

For type alias impl trait, these issues do not actually apply in most cases, but sometimes you have a type alias impl trait like type Foo<T: Debug> = (impl Debug, Bar<T>);, which only really checks it for impl Debug, but by accident prevents Bar<T> from only being instantiated after proving T: Debug. This PR makes sure that we always check these bounds explicitly and don't rely on an implementation accident.

To not break all the type aliases out there, we only use it when the type alias contains an opaque type. We can decide to do this for all type aliases over an edition.

Or we can later extend this to more types if we figure out the back-compat concerns with suddenly checking such bounds.

As a side effect, easily allows fixing #108617, which I did.

fixes #108617

rustbot · 2023-03-07T12:09:01Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @spastorino (or someone else) soon.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

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compiler/rustc_trait_selection/src/traits/project.rs

rustbot · 2023-03-08T14:48:10Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

oli-obk · 2023-03-08T14:48:23Z

@bors try @rust-timer queue

bors · 2023-03-08T14:48:33Z

⌛ Trying commit 5df0ff9b38e2ec1ac95f2e5da14052323e27da54 with merge 10ea3ca86b40e3a352e4b76cc61eb09d6e531b17...

compiler-errors · 2023-03-08T16:49:32Z

r? @compiler-errors

bors · 2023-03-08T17:29:17Z

☀️ Try build successful - checks-actions
Build commit: 10ea3ca86b40e3a352e4b76cc61eb09d6e531b17 (10ea3ca86b40e3a352e4b76cc61eb09d6e531b17)

bors · 2023-03-08T17:29:24Z

☀️ Try build successful - checks-actions
Build commit: 10ea3ca86b40e3a352e4b76cc61eb09d6e531b17 (10ea3ca86b40e3a352e4b76cc61eb09d6e531b17)

rust-timer · 2023-03-08T18:47:17Z

Finished benchmarking commit (10ea3ca86b40e3a352e4b76cc61eb09d6e531b17): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Benchmarking 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.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.4%, 0.5%]	3
Regressions ❌ (secondary)	0.4%	[0.3%, 0.5%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.4%, 0.5%]	3

Max RSS (memory usage)

Results

This 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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.9%, -1.3%]	5
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

compiler/rustc_privacy/src/lib.rs

compiler/rustc_trait_selection/src/solve/trait_goals/structural_traits.rs

compiler/rustc_trait_selection/src/traits/coherence.rs

compiler/rustc_trait_selection/src/traits/project.rs

compiler/rustc_trait_selection/src/traits/query/normalize.rs

bors · 2023-03-10T23:59:40Z

☔ The latest upstream changes (presumably #108974) made this pull request unmergeable. Please resolve the merge conflicts.

lcnr

a few more nits, and then r=me

As that will be more difficult to figure out later, it would like to have a doc somewhere which explains the behavior of weak projections. How they differ from ordinary projections and how they're equal.

compiler/rustc_hir_analysis/src/coherence/orphan.rs

compiler/rustc_hir_analysis/src/collect/item_bounds.rs

compiler/rustc_privacy/src/lib.rs

lcnr · 2023-03-13T09:30:40Z

compiler/rustc_trait_selection/src/traits/coherence.rs

@@ -649,7 +649,7 @@ impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for OrphanChecker<'tcx> {
            | ty::RawPtr(..)
            | ty::Never
            | ty::Tuple(..)
-            | ty::Alias(ty::Projection, ..) => self.found_non_local_ty(ty),
+            | ty::Alias(ty::Projection | ty::Weak, ..) => self.found_non_local_ty(ty),


we still have to fix #99554 for all ty::Alias

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

compiler/rustc_trait_selection/src/traits/query/normalize.rs

lcnr · 2023-03-13T09:34:54Z

tests/ui/type-alias-impl-trait/issue-53398-cyclic-types.rs

@@ -1,9 +1,10 @@
 #![feature(type_alias_impl_trait)]

+// check-pass
+
 type Foo = impl Fn() -> Foo;


how is that fixed? Though I guess this should work, so it's fine

it's fixed by not normalizing Foo eagerly. So normalize the weak alias to the opaque type, and if we get the item_bounds, we get an unnormalized Foo.

Previously, when registering a hidden type, we used to instantiate the opaque type, which then contained itself, which we tried to instantiate, too, and thus cycled.

lcnr · 2023-03-13T09:35:54Z

tests/ui/type-alias-impl-trait/self-referential-3.rs

 #![feature(type_alias_impl_trait)]

 type Bar<'a, 'b> = impl PartialEq<Bar<'a, 'b>> + std::fmt::Debug;

 fn bar<'a, 'b>(i: &'a i32) -> Bar<'a, 'b> {
+    //~^ ERROR can't compare `&i32` with `Bar<'a, 'b>`


why does this break? 🤔 is Bar<'a, 'b> a ty::Opaque here or a weak projection?

We're not normalizing the obligations before matching them against candidates. Doing so will fix this case and other existing known-bug cases. I'll have a PR open for just that soon that we can then analyze on its own

I opened #109115

We could also just run normalize_with_depth on the nested obligations in the assoc type normalizer, like the rest of the selection/etc. code does

lcnr · 2023-03-13T09:37:30Z

r? @lcnr

compiler-errors

r=me fixing tidy, addressing my one nit

compiler/rustc_trait_selection/src/solve/weak_types.rs

Only use it when the type alias contains an opaque type. Also does wf-checking on such type aliases.

oli-obk · 2023-06-16T19:44:35Z

@bors r=compiler-errors

bors · 2023-06-16T19:44:37Z

📌 Commit 79c2c98 has been approved by compiler-errors

It is now in the queue for this repository.

bors · 2023-06-17T00:33:33Z

⌛ Testing commit 79c2c98 with merge 0cc541e...

bors · 2023-06-17T03:21:27Z

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 0cc541e to master...

rust-timer · 2023-06-17T04:37:11Z

Finished benchmarking commit (0cc541e): comparison URL.

Overall result: ❌ regressions - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.2%, 0.5%]	8
Regressions ❌ (secondary)	0.4%	[0.2%, 0.8%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.2%, 0.5%]	8

Max RSS (memory usage)

Results

This 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.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-3.7%, -3.7%]	1
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 655.939s -> 656.882s (0.14%)

lcnr · 2023-06-19T07:23:08Z

have we documented the behavior of weak type aliases somewhere? I want to avoid us getting stuck with an unclear implementation and weird surprises/bugs later on.

oli-obk · 2023-06-19T07:55:21Z

Do we have documentation for projections anywhere? XD i'll happily add it nearby.

If not, where should the docs for all four aliases live?

lcnr · 2023-06-19T08:10:24Z

rustc-dev-guide 🤔 there's also #60471 (comment)

I feel like we may want to restructure the dev guide to have a chapter "type system info dump" where we can put info about parts of the type system without having to figure out how to sensibly integrate them with the rest of the dev-guide. And then I would put "type aliases" as a subchapter there 🤔

rylev · 2023-06-21T08:18:55Z

@oli-obk @lcnr This seems to have caused a tiny bit of perf regression in some doc cases. Seems maybe get_auto_trait_impls was impacted - though I'm unsure why that would be. Thoughts?

oli-obk · 2023-06-21T08:27:26Z

It's hard to tell. As none of our perf tests use TAITs, this is likely some inlining changes in the type folders:

-30,081,414  ???:<&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_infer::infer::freshen::TypeFreshener>
 19,793,318  ???:<rustc_infer::infer::freshen::TypeFreshener as rustc_type_ir::fold::FallibleTypeFolder<rustc_middle::ty::context::TyCtxt>>::try_fold_ty
 18,941,215  ???:<rustc_middle::ty::sty::TraitRef as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_infer::infer::freshen::TypeFreshener>
 12,763,035  ???:<rustc_infer::infer::freshen::TypeFreshener as rustc_type_ir::fold::FallibleTypeFolder<rustc_middle::ty::context::TyCtxt>>::try_fold_binder::<rustc_middle::ty::PredicateKind>
-12,290,330  ???:<rustc_middle::ty::sty::Binder<rustc_middle::ty::PredicateKind> as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::super_fold_with::<rustc_infer::infer::freshen::TypeFreshener>
-11,001,070  ???:rustc_trait_selection::traits::do_normalize_predicates
 10,896,326  ???:rustc_trait_selection::traits::normalize_param_env_or_error
  5,304,007  ???:<rustc_middle::ty::context::TyCtxt>::lang_items
  5,248,049  ???:<smallvec::SmallVec<[rustc_ast::ast::Attribute; 8]> as core::iter::traits::collect::Extend<rustc_ast::ast::Attribute>>::extend::<rustc_metadata::rmeta::decoder::DecodeIterator<rustc_ast::ast::Attribute>>
  5,206,509  ???:<rustc_trait_selection::traits::select::SelectionContext>::select
 -5,186,609  ???:<rustc_span::symbol::Symbol>::intern
  2,826,798  ???:<rustc_middle::ty::PredicateKind as rustc_type_ir::fold::TypeFoldable<rustc_middle::ty::context::TyCtxt>>::try_fold_with::<rustc_infer::infer::freshen::TypeFreshener>
 -2,384,760  ???:<rustc_trait_selection::traits::select::SelectionContext>::impl_or_trait_obligations
 -2,309,840  ???:<rustc_middle::ty::context::TyCtxt>::hygienic_eq
  2,140,440  ???:<rustc_infer::traits::Obligation<rustc_middle::ty::sty::Binder<rustc_middle::ty::TraitPredicate>>>::with_depth::<rustc_middle::ty::sty::Binder<rustc_middle::ty::TraitPredicate>>
 -2,126,043  ???:<&rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg> as rustc_serialize::serialize::Decodable<rustc_metadata::rmeta::decoder::DecodeContext>>::decode
 -1,962,828  ???:<rustc_middle::ty::Ty as rustc_type_ir::fold::TypeSuperFoldable<rustc_middle::ty::context::TyCtxt>>::try_super_fold_with::<rustc_infer::infer::freshen::TypeFreshener>
 -1,639,261  ???:<hashbrown::raw::RawTable<(rustc_middle::ty::context::InternedInSet<rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg>>, ())>>::reserve_rehash::<hashbrown::map::make_hasher<rustc_middle::ty::context::InternedInSet<rustc_middle::ty::list::List<rustc_middle::ty::subst::GenericArg>>, (), core::hash::BuildHasherDefault<rustc_hash::FxHasher>>::{closure
  1,610,331  ???:<hashbrown::raw::RawTable<(rustc_middle::ty::context::InternedInSet<rustc_middle::ty::list::List<rustc_middle::ty::Ty>>, ())>>::reserve_rehash::<hashbrown::map::make_hasher<rustc_middle::ty::context::InternedInSet<rustc_middle::ty::list::List<rustc_middle::ty::Ty>>, (), core::hash::BuildHasherDefault<rustc_hash::FxHasher>>::{closure
  1,562,392  ???:rustc_query_impl::plumbing::__rust_begin_short_backtrace::<rustc_query_impl::query_impl::impl_trait_ref::dynamic_query::{closure
  1,292,944  ???:<rustc_middle::ty::assoc::AssocItem>::ident
  1,059,942  ???:<rustc_middle::ty::assoc::AssocItems>::find_by_name_and_kind

rustbot assigned spastorino Mar 7, 2023

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oli-obk added 2 commits June 16, 2023 19:39

Add AliasKind::Weak for type aliases.

f3b7dd6

Only use it when the type alias contains an opaque type. Also does wf-checking on such type aliases.

Pacify tidy

79c2c98

oli-obk force-pushed the tait_alias branch from 7140194 to 79c2c98 Compare June 16, 2023 19:40

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 16, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 17, 2023

bors merged commit 0cc541e into rust-lang:master Jun 17, 2023

rustbot added this to the 1.72.0 milestone Jun 17, 2023

This was referenced Jun 17, 2023

Do not substitute nonexistent lifetime in RPIT in recursive function #110844

Closed

Don't require associated types with Self: Sized bounds in dyn Trait objects #112319

Merged

split NormalizesTo out of Projection #112658

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oli-obk deleted the tait_alias branch June 17, 2023 06:42

xFrednet mentioned this pull request Jun 3, 2023

Bump nightly -> 2023-07-13 rust-marker/marker#134

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GuillaumeGomez mentioned this pull request Jun 19, 2023

Tracking issue for lazy type aliases #112792

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6 tasks

xxchan mentioned this pull request Aug 16, 2023

Regression of TAIT with generics #114909

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This was referenced Oct 5, 2023

Add TyAlias into rustc_type_ir TyKind enum #97974

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finish up type alias PRs #55222

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fmease mentioned this pull request Nov 15, 2023

Add Identity lang item #103985

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Add `AliasKind::Weak` for type aliases. #108860

Add `AliasKind::Weak` for type aliases. #108860

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Add AliasKind::Weak for type aliases. #108860

Add AliasKind::Weak for type aliases. #108860

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Overall result: ❌ regressions - ACTION NEEDED

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Overall result: ❌ regressions - ACTION NEEDED

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Add `AliasKind::Weak` for type aliases. #108860

Add `AliasKind::Weak` for type aliases. #108860

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