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

compiler/rustc_ast/src/lib.rs

@@ -11,7 +11,7 @@
 #![doc(rust_logo)]
 #![allow(internal_features)]
 #![feature(rustdoc_internals)]
-#![feature(associated_type_bounds)]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(associated_type_defaults)]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -463,13 +463,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
                     constraint.span,
                     "return type notation is experimental"
                 );
-            } else {
-                gate!(
-                    &self,
-                    associated_type_bounds,
-                    constraint.span,
-                    "associated type bounds are unstable"
-                );
             }
         }
         visit::walk_assoc_constraint(self, constraint)
@@ -615,7 +608,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     }
 
     gate_all_legacy_dont_use!(trait_alias, "trait aliases are experimental");
-    gate_all_legacy_dont_use!(associated_type_bounds, "associated type bounds are unstable");
     // Despite being a new feature, `where T: Trait<Assoc(): Sized>`, which is RTN syntax now,
     // used to be gated under associated_type_bounds, which are right above, so RTN needs to
     // be too.

compiler/rustc_borrowck/src/lib.rs

@@ -4,7 +4,7 @@
 #![feature(rustdoc_internals)]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
-#![feature(associated_type_bounds)]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(let_chains)]

compiler/rustc_codegen_gcc/src/lib.rs

@@ -19,11 +19,11 @@
 #![feature(
     rustc_private,
     decl_macro,
-    associated_type_bounds,
     never_type,
     trusted_len,
     hash_raw_entry
 )]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![allow(broken_intra_doc_links)]
 #![recursion_limit = "256"]
 #![warn(rust_2018_idioms)]

compiler/rustc_codegen_ssa/src/lib.rs

@@ -4,7 +4,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![feature(associated_type_bounds)]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(box_patterns)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]

compiler/rustc_error_codes/src/error_codes/E0719.md

@@ -3,8 +3,6 @@ An associated type value was specified more than once.
 Erroneous code example:
 
 ```compile_fail,E0719
-#![feature(associated_type_bounds)]
-
 trait FooTrait {}
 trait BarTrait {}
 
@@ -19,8 +17,6 @@ specify the associated type with the new trait.
 Corrected example:
 
 ```
-#![feature(associated_type_bounds)]
-
 trait FooTrait {}
 trait BarTrait {}
 trait FooBarTrait: FooTrait + BarTrait {}

compiler/rustc_expand/src/lib.rs

@@ -1,7 +1,7 @@
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
 #![feature(array_windows)]
-#![feature(associated_type_bounds)]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(associated_type_defaults)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]

compiler/rustc_feature/src/accepted.rs

@@ -59,6 +59,8 @@ declare_features! (
     (accepted, asm_sym, "1.66.0", Some(93333)),
     /// Allows the definition of associated constants in `trait` or `impl` blocks.
     (accepted, associated_consts, "1.20.0", Some(29646)),
+    /// Allows the user of associated type bounds.
+    (accepted, associated_type_bounds, "CURRENT_RUSTC_VERSION", Some(52662)),
     /// Allows using associated `type`s in `trait`s.
     (accepted, associated_types, "1.0.0", None),
     /// Allows free and inherent `async fn`s, `async` blocks, and `<expr>.await` expressions.

compiler/rustc_feature/src/unstable.rs

@@ -351,8 +351,6 @@ declare_features! (
     (unstable, asm_unwind, "1.58.0", Some(93334)),
     /// Allows users to enforce equality of associated constants `TraitImpl<AssocConst=3>`.
     (unstable, associated_const_equality, "1.58.0", Some(92827)),
-    /// Allows the user of associated type bounds.
-    (unstable, associated_type_bounds, "1.34.0", Some(52662)),
     /// Allows associated type defaults.
     (unstable, associated_type_defaults, "1.2.0", Some(29661)),
     /// Allows `async || body` closures.

compiler/rustc_infer/src/lib.rs

@@ -18,7 +18,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![feature(associated_type_bounds)]
+#![cfg_attr(bootstrap, feature(associated_type_bounds))]
 #![feature(box_patterns)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]