Make associated type bounds in supertrait position implied #112629

compiler-errors · 2023-06-14T18:43:47Z

trait A: B<Assoc: C> {} should be able to imply both Self: B and <Self as B>::Assoc: C. Adjust the way that we collect implied predicates to do so.

Fixes #112573
Fixes #112568

rustbot · 2023-06-14T18:43:52Z

r? @spastorino

(rustbot has picked a reviewer for you, use r? to override)

jackh726 · 2023-06-14T22:52:13Z

I took a look at this briefly at one point. Trying to remember why trait Trait: Super<Assoc = Bound> {} is fine but trait Trait: Super<Assoc: Bound> {} isn't.

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

compiler-errors · 2023-06-15T01:26:07Z

(had to move some code around in the last commit, since astconv was hitting the size limit for a single file)

bors · 2023-06-18T13:40:19Z

☔ The latest upstream changes (presumably #112755) made this pull request unmergeable. Please resolve the merge conflicts.

spastorino · 2023-06-19T18:49:36Z

Looks good to me but given that @jackh726 reviewed it and left comments already, I'd give it up to him.

r? @jackh726

bors · 2023-06-21T11:47:01Z

☔ The latest upstream changes (presumably #112877) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2023-06-27T06:56:50Z

☔ The latest upstream changes (presumably #112938) made this pull request unmergeable. Please resolve the merge conflicts.

jackh726 · 2023-06-28T22:26:43Z

@bors r+ rollup=never

bors · 2023-06-28T22:26:45Z

📌 Commit de0e7d3 has been approved by jackh726

It is now in the queue for this repository.

bors · 2023-06-28T23:58:31Z

⌛ Testing commit de0e7d3 with merge 75726ca...

bors · 2023-06-29T02:42:29Z

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing 75726ca to master...

rust-timer · 2023-06-29T03:59:21Z

Finished benchmarking commit (75726ca): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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	mean	range	count
Regressions ❌ (primary)	1.7%	[0.4%, 4.9%]	5
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.7%	[-1.0%, -0.4%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[-1.0%, 4.9%]	7

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Bootstrap: 661.172s -> 659.556s (-0.24%)

…li-obk Stabilize associated type bounds (RFC 2289) This PR stabilizes associated type bounds, which were laid out in [RFC 2289]. This gives us a shorthand to express nested type bounds that would otherwise need to be expressed with nested `impl Trait` or broken into several `where` clauses. ### What are we stabilizing? We're stabilizing the associated item bounds syntax, which allows us to put bounds in associated type position within other bounds, i.e. `T: Trait<Assoc: Bounds...>`. See [RFC 2289] for motivation. In all position, the associated type bound syntax expands into a set of two (or more) bounds, and never anything else (see "How does this differ[...]" section for more info). Associated type bounds are stabilized in four positions: * **`where` clauses (and APIT)** - This is equivalent to breaking up the bound into two (or more) `where` clauses. For example, `where T: Trait<Assoc: Bound>` is equivalent to `where T: Trait, <T as Trait>::Assoc: Bound`. * **Supertraits** - Similar to above, `trait CopyIterator: Iterator<Item: Copy> {}`. This is almost equivalent to breaking up the bound into two (or more) `where` clauses; however, the bound on the associated item is implied whenever the trait is used. See rust-lang#112573/rust-lang#112629. * **Associated type item bounds** - This allows constraining the *nested* rigid projections that are associated with a trait's associated types. e.g. `trait Trait { type Assoc: Trait2<Assoc2: Copy>; }`. * **opaque item bounds (RPIT, TAIT)** - This allows constraining associated types that are associated with the opaque without having to *name* the opaque. For example, `impl Iterator<Item: Copy>` defines an iterator whose item is `Copy` without having to actually name that item bound. The latter three are not expressible in surface Rust (though for associated type item bounds, this will change in rust-lang#120752, which I don't believe should block this PR), so this does represent a slight expansion of what can be expressed in trait bounds. ### How does this differ from the RFC? Compared to the RFC, the current implementation *always* desugars associated type bounds to sets of `ty::Clause`s internally. Specifically, it does *not* introduce a position-dependent desugaring as laid out in [RFC 2289], and in particular: * It does *not* desugar to anonymous associated items in associated type item bounds. * It does *not* desugar to nested RPITs in RPIT bounds, nor nested TAITs in TAIT bounds. This position-dependent desugaring laid out in the RFC existed simply to side-step limitations of the trait solver, which have mostly been fixed in rust-lang#120584. The desugaring laid out in the RFC also added unnecessary complication to the design of the feature, and introduces its own limitations to, for example: * Conditionally lowering to nested `impl Trait` in certain positions such as RPIT and TAIT means that we inherit the limitations of RPIT/TAIT, namely lack of support for higher-ranked opaque inference. See this code example: rust-lang#120752 (comment). * Introducing anonymous associated types makes traits no longer object safe, since anonymous associated types are not nameable, and all associated types must be named in `dyn` types. This last point motivates why this PR is *not* stabilizing support for associated type bounds in `dyn` types, e.g, `dyn Assoc<Item: Bound>`. Why? Because `dyn` types need to have *concrete* types for all associated items, this would necessitate a distinct lowering for associated type bounds, which seems both complicated and unnecessary compared to just requiring the user to write `impl Trait` themselves. See rust-lang#120719. ### Implementation history: Limited to the significant behavioral changes and fixes and relevant PRs, ping me if I left something out-- * rust-lang#57428 * rust-lang#108063 * rust-lang#110512 * rust-lang#112629 * rust-lang#120719 * rust-lang#120584 Closes rust-lang#52662 [RFC 2289]: https://rust-lang.github.io/rfcs/2289-associated-type-bounds.html

…-obk Stabilize associated type bounds (RFC 2289) This PR stabilizes associated type bounds, which were laid out in [RFC 2289]. This gives us a shorthand to express nested type bounds that would otherwise need to be expressed with nested `impl Trait` or broken into several `where` clauses. ### What are we stabilizing? We're stabilizing the associated item bounds syntax, which allows us to put bounds in associated type position within other bounds, i.e. `T: Trait<Assoc: Bounds...>`. See [RFC 2289] for motivation. In all position, the associated type bound syntax expands into a set of two (or more) bounds, and never anything else (see "How does this differ[...]" section for more info). Associated type bounds are stabilized in four positions: * **`where` clauses (and APIT)** - This is equivalent to breaking up the bound into two (or more) `where` clauses. For example, `where T: Trait<Assoc: Bound>` is equivalent to `where T: Trait, <T as Trait>::Assoc: Bound`. * **Supertraits** - Similar to above, `trait CopyIterator: Iterator<Item: Copy> {}`. This is almost equivalent to breaking up the bound into two (or more) `where` clauses; however, the bound on the associated item is implied whenever the trait is used. See rust-lang#112573/rust-lang#112629. * **Associated type item bounds** - This allows constraining the *nested* rigid projections that are associated with a trait's associated types. e.g. `trait Trait { type Assoc: Trait2<Assoc2: Copy>; }`. * **opaque item bounds (RPIT, TAIT)** - This allows constraining associated types that are associated with the opaque without having to *name* the opaque. For example, `impl Iterator<Item: Copy>` defines an iterator whose item is `Copy` without having to actually name that item bound. The latter three are not expressible in surface Rust (though for associated type item bounds, this will change in rust-lang#120752, which I don't believe should block this PR), so this does represent a slight expansion of what can be expressed in trait bounds. ### How does this differ from the RFC? Compared to the RFC, the current implementation *always* desugars associated type bounds to sets of `ty::Clause`s internally. Specifically, it does *not* introduce a position-dependent desugaring as laid out in [RFC 2289], and in particular: * It does *not* desugar to anonymous associated items in associated type item bounds. * It does *not* desugar to nested RPITs in RPIT bounds, nor nested TAITs in TAIT bounds. This position-dependent desugaring laid out in the RFC existed simply to side-step limitations of the trait solver, which have mostly been fixed in rust-lang#120584. The desugaring laid out in the RFC also added unnecessary complication to the design of the feature, and introduces its own limitations to, for example: * Conditionally lowering to nested `impl Trait` in certain positions such as RPIT and TAIT means that we inherit the limitations of RPIT/TAIT, namely lack of support for higher-ranked opaque inference. See this code example: rust-lang#120752 (comment). * Introducing anonymous associated types makes traits no longer object safe, since anonymous associated types are not nameable, and all associated types must be named in `dyn` types. This last point motivates why this PR is *not* stabilizing support for associated type bounds in `dyn` types, e.g, `dyn Assoc<Item: Bound>`. Why? Because `dyn` types need to have *concrete* types for all associated items, this would necessitate a distinct lowering for associated type bounds, which seems both complicated and unnecessary compared to just requiring the user to write `impl Trait` themselves. See rust-lang#120719. ### Implementation history: Limited to the significant behavioral changes and fixes and relevant PRs, ping me if I left something out-- * rust-lang#57428 * rust-lang#108063 * rust-lang#110512 * rust-lang#112629 * rust-lang#120719 * rust-lang#120584 Closes rust-lang#52662 [RFC 2289]: https://rust-lang.github.io/rfcs/2289-associated-type-bounds.html

…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

rustbot assigned spastorino Jun 14, 2023

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compiler-errors added 2 commits June 27, 2023 18:28

Make associated type bounds in supertrait position implied

858a861

pass PredicateFilter to compute_bounds

de0e7d3

rust-cloud-vms bot force-pushed the atb-imply branch from 6469f5c to de0e7d3 Compare June 27, 2023 18:37

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bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Jun 28, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Jun 29, 2023

bors merged commit 75726ca into rust-lang:master Jun 29, 2023
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rustbot added this to the 1.72.0 milestone Jun 29, 2023

compiler-errors deleted the atb-imply branch August 11, 2023 19:55

fmease mentioned this pull request Aug 21, 2023

Cycle detected when using associated type bounds in super trait #65913

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compiler-errors mentioned this pull request Mar 6, 2024

Stabilize associated type bounds (RFC 2289) #122055

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compiler-errors mentioned this pull request Mar 22, 2024

Encode implied predicates for traits #122891

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Make associated type bounds in supertrait position implied #112629

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Make associated type bounds in supertrait position implied #112629

Make associated type bounds in supertrait position implied #112629

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