Ban associated type bounds in bad positions #108063

compiler-errors · 2023-02-14T20:36:27Z

We should not try to lower associated type bounds into TAITs in positions where impl Trait is not allowed (except for in where clauses, like where T: Trait<Assoc: Bound>).

This is achieved by using the same rustc_ast_lowering machinery as impl-trait does to characterize positions as universal/existential/disallowed.

Fixes #106077

Split out the first commit into #108066, since it's not really related.

rustbot · 2023-02-14T20:36:32Z

r? @eholk

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rustbot · 2023-02-14T20:36:36Z

rustc_error_messages was changed

cc @davidtwco, @compiler-errors, @JohnTitor, @TaKO8Ki

bors · 2023-02-16T14:56:31Z

☔ The latest upstream changes (presumably #101841) made this pull request unmergeable. Please resolve the merge conflicts.

eholk

Looks pretty good! Just a few questions and wording suggestions.

compiler/rustc_ast_lowering/src/lib.rs

tests/ui/associated-type-bounds/bad-universal-in-dyn-in-where-clause.stderr

tests/ui/associated-type-bounds/bad-universal-in-impl-sig.stderr

tests/ui/associated-type-bounds/inside-adt.stderr

compiler/rustc_ast_lowering/src/lib.rs

compiler/rustc_hir_analysis/src/astconv/mod.rs

bors · 2023-02-22T19:14:23Z

☔ The latest upstream changes (presumably #103042) made this pull request unmergeable. Please resolve the merge conflicts.

This makes the check for when associated type bounds more accurate

cjgillot

@bors r+

cjgillot · 2023-02-22T22:17:09Z

@bors r+

bors · 2023-02-22T22:17:12Z

📌 Commit b14eb0c has been approved by cjgillot

It is now in the queue for this repository.

…iaskrgr Rollup of 7 pull requests Successful merges: - rust-lang#108063 (Ban associated type bounds in bad positions) - rust-lang#108208 (Correctly handle aggregates in DataflowConstProp) - rust-lang#108218 (Windows: Quote more batch file arguments) - rust-lang#108349 (rustdoc: Prevent duplicated imports) - rust-lang#108350 (Use associated type bounds in some places in the compiler) - rust-lang#108358 (Add git config command to `.git-blame-ignore-revs`) - rust-lang#108373 (hir-analysis: make where-clause-on-main diagnostic translatable) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

…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

rustbot assigned eholk Feb 14, 2023

compiler-errors force-pushed the associated-type-bounds-in-bad-position branch from ff71676 to ec3b723 Compare February 14, 2023 23:03

compiler-errors force-pushed the associated-type-bounds-in-bad-position branch 2 times, most recently from 4b16922 to 13ed5df Compare February 18, 2023 20:19

eholk reviewed Feb 21, 2023

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compiler-errors commented Feb 22, 2023

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cjgillot reviewed Feb 22, 2023

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compiler-errors added 3 commits February 22, 2023 20:18

Move associated type bounds check to ast lowering

e7c4908

This makes the check for when associated type bounds more accurate

Suppress duplicated errors for associated type bounds in object types

deb1357

pluralize stuff

b14eb0c

compiler-errors force-pushed the associated-type-bounds-in-bad-position branch from 08add30 to b14eb0c Compare February 22, 2023 21:52

cjgillot reviewed Feb 22, 2023

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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 Feb 22, 2023

matthiaskrgr mentioned this pull request Feb 23, 2023

Rollup of 7 pull requests #108386

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bors merged commit ef27e43 into rust-lang:master Feb 23, 2023

rustbot added this to the 1.69.0 milestone Feb 23, 2023

compiler-errors deleted the associated-type-bounds-in-bad-position branch August 11, 2023 19:55

compiler-errors mentioned this pull request Mar 6, 2024

Stabilize associated type bounds (RFC 2289) #122055

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Ban associated type bounds in bad positions #108063

Ban associated type bounds in bad positions #108063

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Ban associated type bounds in bad positions #108063

Ban associated type bounds in bad positions #108063

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