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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 #112573/#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 #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 #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: https://github.com/rust-lang/rust/pull/120752#issuecomment-1979412531.
* 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 #120719.
### Implementation history:
Limited to the significant behavioral changes and fixes and relevant PRs, ping me if I left something out--
* #57428
* #108063
* #110512
* #112629
* #120719
* #120584
Closes #52662
[RFC 2289]: https://rust-lang.github.io/rfcs/2289-associated-type-bounds.html
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| .. | ||
| auxiliary | ||
| assoc-const-eq-ambiguity.rs | ||
| assoc-const-eq-ambiguity.stderr | ||
| assoc-const-eq-bound-var-in-ty-not-wf.rs | ||
| assoc-const-eq-bound-var-in-ty-not-wf.stderr | ||
| assoc-const-eq-bound-var-in-ty.rs | ||
| assoc-const-eq-esc-bound-var-in-ty.rs | ||
| assoc-const-eq-esc-bound-var-in-ty.stderr | ||
| assoc-const-eq-missing.rs | ||
| assoc-const-eq-missing.stderr | ||
| assoc-const-eq-param-in-ty.rs | ||
| assoc-const-eq-param-in-ty.stderr | ||
| assoc-const-eq-supertraits.rs | ||
| assoc-const-eq-ty-alias-noninteracting.rs | ||
| assoc-const-ty-mismatch.rs | ||
| assoc-const-ty-mismatch.stderr | ||
| assoc-const.rs | ||
| associated-const-ambiguity-report.rs | ||
| associated-const-ambiguity-report.stderr | ||
| associated-const-array-len.rs | ||
| associated-const-array-len.stderr | ||
| associated-const-const-eval.rs | ||
| associated-const-cross-crate-const-eval.rs | ||
| associated-const-cross-crate-defaults.rs | ||
| associated-const-cross-crate.rs | ||
| associated-const-dead-code.rs | ||
| associated-const-dead-code.stderr | ||
| associated-const-generic-obligations.rs | ||
| associated-const-generic-obligations.stderr | ||
| associated-const-impl-wrong-lifetime.rs | ||
| associated-const-impl-wrong-lifetime.stderr | ||
| associated-const-impl-wrong-type.rs | ||
| associated-const-impl-wrong-type.stderr | ||
| associated-const-in-global-const.rs | ||
| associated-const-in-trait.rs | ||
| associated-const-in-trait.stderr | ||
| associated-const-inherent-impl.rs | ||
| associated-const-marks-live-code.rs | ||
| associated-const-match-patterns.rs | ||
| associated-const-no-item.rs | ||
| associated-const-no-item.stderr | ||
| associated-const-outer-ty-refs.rs | ||
| associated-const-overwrite-default.rs | ||
| associated-const-private-impl.rs | ||
| associated-const-private-impl.stderr | ||
| associated-const-public-impl.rs | ||
| associated-const-range-match-patterns.rs | ||
| associated-const-resolution-order.rs | ||
| associated-const-self-type.rs | ||
| associated-const-trait-bound.rs | ||
| associated-const-type-parameter-arms.rs | ||
| associated-const-type-parameter-arms.stderr | ||
| associated-const-type-parameter-arrays-2.rs | ||
| associated-const-type-parameter-arrays-2.stderr | ||
| associated-const-type-parameter-arrays.rs | ||
| associated-const-type-parameter-arrays.stderr | ||
| associated-const-type-parameters.rs | ||
| associated-const-type-parameters.stderr | ||
| associated-const-ufcs-infer-trait.rs | ||
| associated-const-use-default.rs | ||
| associated-const-use-impl-of-same-trait.rs | ||
| associated-const.rs | ||
| defaults-cyclic-fail.rs | ||
| defaults-cyclic-fail.stderr | ||
| defaults-cyclic-pass.rs | ||
| defaults-not-assumed-fail.rs | ||
| defaults-not-assumed-fail.stderr | ||
| defaults-not-assumed-pass.rs | ||
| double-elided.rs | ||
| double-elided.stderr | ||
| freeze.rs | ||
| infer-placeholder-in-non-suggestable-pos.rs | ||
| infer-placeholder-in-non-suggestable-pos.stderr | ||
| issue-24949-assoc-const-static-recursion-impl.rs | ||
| issue-24949-assoc-const-static-recursion-impl.stderr | ||
| issue-24949-assoc-const-static-recursion-trait-default.rs | ||
| issue-24949-assoc-const-static-recursion-trait-default.stderr | ||
| issue-24949-assoc-const-static-recursion-trait.rs | ||
| issue-24949-assoc-const-static-recursion-trait.stderr | ||
| issue-47814.rs | ||
| issue-47814.stderr | ||
| issue-58022.rs | ||
| issue-58022.stderr | ||
| issue-63496.rs | ||
| issue-63496.stderr | ||
| issue-69020-assoc-const-arith-overflow.noopt.stderr | ||
| issue-69020-assoc-const-arith-overflow.opt_with_overflow_checks.stderr | ||
| issue-69020-assoc-const-arith-overflow.opt.stderr | ||
| issue-69020-assoc-const-arith-overflow.rs | ||
| issue-88599-ref-self.rs | ||
| issue-93775.rs | ||
| issue-93835.rs | ||
| issue-93835.stderr | ||
| issue-102335-const.rs | ||
| issue-102335-const.stderr | ||
| issue-105330.rs | ||
| issue-105330.stderr | ||
| issue-110933.rs | ||
| mismatched_impl_ty_1.rs | ||
| mismatched_impl_ty_2.rs | ||
| mismatched_impl_ty_3.rs | ||
| projection-unspecified-but-bounded.rs | ||
| projection-unspecified-but-bounded.stderr | ||
| shadowed-const.rs | ||
| shadowed-const.stderr | ||