Auto merge of rust-lang#122055 - compiler-errors:stabilize-atb, r=oli-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

Author: bors

alloc/src/lib.rs

@@ -173,12 +173,12 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![cfg_attr(not(test), feature(coroutine_trait))]
 #![cfg_attr(test, feature(panic_update_hook))]
 #![cfg_attr(test, feature(test))]
 #![feature(allocator_internals)]
 #![feature(allow_internal_unstable)]
-#![feature(associated_type_bounds)]
 #![feature(c_unwind)]
 #![feature(cfg_sanitize)]
 #![feature(const_mut_refs)]

alloc/tests/lib.rs

@@ -1,3 +1,4 @@
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(allocator_api)]
 #![feature(alloc_layout_extra)]
 #![feature(iter_array_chunks)]
@@ -22,7 +23,6 @@
 #![feature(try_reserve_kind)]
 #![feature(try_with_capacity)]
 #![feature(unboxed_closures)]
-#![feature(associated_type_bounds)]
 #![feature(binary_heap_into_iter_sorted)]
 #![feature(binary_heap_drain_sorted)]
 #![feature(slice_ptr_get)]

core/src/lib.rs

@@ -203,6 +203,7 @@
 //
 // Language features:
 // tidy-alphabetical-start
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![cfg_attr(bootstrap, feature(diagnostic_namespace))]
 #![cfg_attr(bootstrap, feature(exhaustive_patterns))]
 #![cfg_attr(bootstrap, feature(platform_intrinsics))]
@@ -213,7 +214,6 @@
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]
 #![feature(asm_const)]
-#![feature(associated_type_bounds)]
 #![feature(auto_traits)]
 #![feature(c_unwind)]
 #![feature(cfg_sanitize)]