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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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| .. | ||
| impl-trait | ||
| rfc-0000-never_patterns | ||
| rfc-0107-bind-by-move-pattern-guards | ||
| rfc-1014-stdout-existential-crisis | ||
| rfc-1445-restrict-constants-in-patterns | ||
| rfc-1623-static | ||
| rfc-1717-dllimport | ||
| rfc-1789-as-cell | ||
| rfc-1857-stabilize-drop-order | ||
| rfc-1937-termination-trait | ||
| rfc-2005-default-binding-mode | ||
| rfc-2008-non-exhaustive | ||
| rfc-2027-object-safe-for-dispatch | ||
| rfc-2091-track-caller | ||
| rfc-2093-infer-outlives | ||
| rfc-2126-crate-paths | ||
| rfc-2126-extern-absolute-paths | ||
| rfc-2151-raw-identifiers | ||
| rfc-2175-or-if-while-let | ||
| rfc-2294-if-let-guard | ||
| rfc-2302-self-struct-ctor | ||
| rfc-2306-convert-id | ||
| rfc-2361-dbg-macro | ||
| rfc-2396-target_feature-11 | ||
| rfc-2397-do-not-recommend | ||
| rfc-2421-unreserve-pure-offsetof-sizeof-alignof | ||
| rfc-2457-non-ascii-idents | ||
| rfc-2497-if-let-chains | ||
| rfc-2528-type-changing-struct-update | ||
| rfc-2565-param-attrs | ||
| rfc-2627-raw-dylib | ||
| rfc-2632-const-trait-impl | ||
| rfc-3348-c-string-literals | ||
| type-alias-impl-trait | ||