rust-lang · fmease · May 27, 2024
diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
@@ -167,7 +167,7 @@ impl PathSegment {
     }
 }
 
-/// The arguments of a path segment.
+/// The generic arguments and associated item constraints of a path segment.
 ///
 /// E.g., `<A, B>` as in `Foo<A, B>` or `(A, B)` as in `Foo(A, B)`.
 #[derive(Clone, Encodable, Decodable, Debug)]
@@ -221,14 +221,13 @@ pub struct AngleBracketedArgs {
     pub args: ThinVec<AngleBracketedArg>,
 }
 
-/// Either an argument for a parameter e.g., `'a`, `Vec<u8>`, `0`,
-/// or a constraint on an associated item, e.g., `Item = String` or `Item: Bound`.
+/// Either an argument for a generic parameter or a constraint on an associated item.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum AngleBracketedArg {
-    /// Argument for a generic parameter.
+    /// A generic argument for a generic parameter.
     Arg(GenericArg),
-    /// Constraint for an associated item.
-    Constraint(AssocConstraint),
+    /// A constraint on an associated item.
+    Constraint(AssocItemConstraint),
 }
 
 impl AngleBracketedArg {
@@ -418,7 +417,7 @@ impl Default for WhereClause {
 /// A single predicate in a where-clause.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum WherePredicate {
-    /// A type binding (e.g., `for<'c> Foo: Send + Clone + 'c`).
+    /// A type bound (e.g., `for<'c> Foo: Send + Clone + 'c`).
     BoundPredicate(WhereBoundPredicate),
     /// A lifetime predicate (e.g., `'a: 'b + 'c`).
     RegionPredicate(WhereRegionPredicate),
@@ -2034,18 +2033,25 @@ impl UintTy {
     }
 }
 
-/// A constraint on an associated type (e.g., `A = Bar` in `Foo<A = Bar>` or
-/// `A: TraitA + TraitB` in `Foo<A: TraitA + TraitB>`).
-#[derive(Clone, Encodable, Decodable, Debug)]
-pub struct AssocConstraint {
+/// A constraint on an associated item.
+///
+/// ### Examples
+///
+/// * the `A = Ty` and `B = Ty` in `Trait<A = Ty, B = Ty>`
+/// * the `G<Ty> = Ty` in `Trait<G<Ty> = Ty>`
+/// * the `A: Bound` in `Trait<A: Bound>`
+/// * the `RetTy` in `Trait(ArgTy, ArgTy) -> RetTy`
+/// * the `C = { Ct }` in `Trait<C = { Ct }>` (feature `associated_const_equality`)
+/// * the `f(): Bound` in `Trait<f(): Bound>` (feature `return_type_notation`)
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct AssocItemConstraint {
     pub id: NodeId,
     pub ident: Ident,
     pub gen_args: Option<GenericArgs>,
-    pub kind: AssocConstraintKind,
+    pub kind: AssocItemConstraintKind,
     pub span: Span,
 }
 
-/// The kinds of an `AssocConstraint`.
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum Term {
     Ty(P<Ty>),
@@ -2064,12 +2070,17 @@ impl From<AnonConst> for Term {
     }
 }
 
-/// The kinds of an `AssocConstraint`.
+/// The kind of [associated item constraint][AssocItemConstraint].
 #[derive(Clone, Encodable, Decodable, Debug)]
-pub enum AssocConstraintKind {
-    /// E.g., `A = Bar`, `A = 3` in `Foo<A = Bar>` where A is an associated type.
+pub enum AssocItemConstraintKind {
+    /// An equality constraint for an associated item (e.g., `AssocTy = Ty` in `Trait<AssocTy = Ty>`).
+    ///
+    /// Also known as an *associated item binding* (we *bind* an associated item to a term).
+    ///
+    /// Furthermore, associated type equality constraints can also be referred to as *associated type
+    /// bindings*. Similarly with associated const equality constraints and *associated const bindings*.
     Equality { term: Term },
-    /// E.g. `A: TraitA + TraitB` in `Foo<A: TraitA + TraitB>`.
+    /// A bound on an associated type (e.g., `AssocTy: Bound` in `Trait<AssocTy: Bound>`).
     Bound { bounds: GenericBounds },
 }
 

diff --git a/compiler/rustc_ast/src/mut_visit.rs b/compiler/rustc_ast/src/mut_visit.rs
@@ -175,8 +175,8 @@ pub trait MutVisitor: Sized {
         noop_visit_lifetime(l, self);
     }
 
-    fn visit_constraint(&mut self, t: &mut AssocConstraint) {
-        noop_visit_constraint(t, self);
+    fn visit_assoc_item_constraint(&mut self, c: &mut AssocItemConstraint) {
+        noop_visit_assoc_item_constraint(c, self);
     }
 
     fn visit_foreign_mod(&mut self, nm: &mut ForeignMod) {
@@ -463,8 +463,8 @@ pub fn noop_flat_map_arm<T: MutVisitor>(mut arm: Arm, vis: &mut T) -> SmallVec<[
     smallvec![arm]
 }
 
-fn noop_visit_constraint<T: MutVisitor>(
-    AssocConstraint { id, ident, gen_args, kind, span }: &mut AssocConstraint,
+fn noop_visit_assoc_item_constraint<T: MutVisitor>(
+    AssocItemConstraint { id, ident, gen_args, kind, span }: &mut AssocItemConstraint,
     vis: &mut T,
 ) {
     vis.visit_id(id);
@@ -473,11 +473,11 @@ fn noop_visit_constraint<T: MutVisitor>(
         vis.visit_generic_args(gen_args);
     }
     match kind {
-        AssocConstraintKind::Equality { term } => match term {
+        AssocItemConstraintKind::Equality { term } => match term {
             Term::Ty(ty) => vis.visit_ty(ty),
             Term::Const(c) => vis.visit_anon_const(c),
         },
-        AssocConstraintKind::Bound { bounds } => visit_bounds(bounds, vis),
+        AssocItemConstraintKind::Bound { bounds } => visit_bounds(bounds, vis),
     }
     vis.visit_span(span);
 }
@@ -607,7 +607,7 @@ fn noop_visit_angle_bracketed_parameter_data<T: MutVisitor>(
     let AngleBracketedArgs { args, span } = data;
     visit_thin_vec(args, |arg| match arg {
         AngleBracketedArg::Arg(arg) => vis.visit_generic_arg(arg),
-        AngleBracketedArg::Constraint(constraint) => vis.visit_constraint(constraint),
+        AngleBracketedArg::Constraint(constraint) => vis.visit_assoc_item_constraint(constraint),
     });
     vis.visit_span(span);
 }

diff --git a/compiler/rustc_ast/src/visit.rs b/compiler/rustc_ast/src/visit.rs
@@ -246,8 +246,11 @@ pub trait Visitor<'ast>: Sized {
     fn visit_generic_arg(&mut self, generic_arg: &'ast GenericArg) -> Self::Result {
         walk_generic_arg(self, generic_arg)
     }
-    fn visit_assoc_constraint(&mut self, constraint: &'ast AssocConstraint) -> Self::Result {
-        walk_assoc_constraint(self, constraint)
+    fn visit_assoc_item_constraint(
+        &mut self,
+        constraint: &'ast AssocItemConstraint,
+    ) -> Self::Result {
+        walk_assoc_item_constraint(self, constraint)
     }
     fn visit_attribute(&mut self, attr: &'ast Attribute) -> Self::Result {
         walk_attribute(self, attr)
@@ -558,7 +561,7 @@ where
                 match arg {
                     AngleBracketedArg::Arg(a) => try_visit!(visitor.visit_generic_arg(a)),
                     AngleBracketedArg::Constraint(c) => {
-                        try_visit!(visitor.visit_assoc_constraint(c))
+                        try_visit!(visitor.visit_assoc_item_constraint(c))
                     }
                 }
             }
@@ -582,18 +585,18 @@ where
     }
 }
 
-pub fn walk_assoc_constraint<'a, V: Visitor<'a>>(
+pub fn walk_assoc_item_constraint<'a, V: Visitor<'a>>(
     visitor: &mut V,
-    constraint: &'a AssocConstraint,
+    constraint: &'a AssocItemConstraint,
 ) -> V::Result {
     try_visit!(visitor.visit_ident(constraint.ident));
     visit_opt!(visitor, visit_generic_args, &constraint.gen_args);
     match &constraint.kind {
-        AssocConstraintKind::Equality { term } => match term {
+        AssocItemConstraintKind::Equality { term } => match term {
             Term::Ty(ty) => try_visit!(visitor.visit_ty(ty)),
             Term::Const(c) => try_visit!(visitor.visit_anon_const(c)),
         },
-        AssocConstraintKind::Bound { bounds } => {
+        AssocItemConstraintKind::Bound { bounds } => {
             walk_list!(visitor, visit_param_bound, bounds, BoundKind::Bound);
         }
     }

diff --git a/compiler/rustc_ast_lowering/src/index.rs b/compiler/rustc_ast_lowering/src/index.rs
@@ -333,10 +333,10 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    fn visit_assoc_type_binding(&mut self, type_binding: &'hir TypeBinding<'hir>) {
-        self.insert(type_binding.span, type_binding.hir_id, Node::TypeBinding(type_binding));
-        self.with_parent(type_binding.hir_id, |this| {
-            intravisit::walk_assoc_type_binding(this, type_binding)
+    fn visit_assoc_item_constraint(&mut self, constraint: &'hir AssocItemConstraint<'hir>) {
+        self.insert(constraint.span, constraint.hir_id, Node::AssocItemConstraint(constraint));
+        self.with_parent(constraint.hir_id, |this| {
+            intravisit::walk_assoc_item_constraint(this, constraint)
         })
     }
 

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
@@ -961,24 +961,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         DelimArgs { dspan: args.dspan, delim: args.delim, tokens: args.tokens.flattened() }
     }
 
-    /// Given an associated type constraint like one of these:
-    ///
-    /// ```ignore (illustrative)
-    /// T: Iterator<Item: Debug>
-    ///             ^^^^^^^^^^^
-    /// T: Iterator<Item = Debug>
-    ///             ^^^^^^^^^^^^
-    /// ```
-    ///
-    /// returns a `hir::TypeBinding` representing `Item`.
-    #[instrument(level = "debug", skip(self))]
-    fn lower_assoc_ty_constraint(
+    /// Lower an associated item constraint.
+    #[instrument(level = "debug", skip_all)]
+    fn lower_assoc_item_constraint(
         &mut self,
-        constraint: &AssocConstraint,
+        constraint: &AssocItemConstraint,
         itctx: ImplTraitContext,
-    ) -> hir::TypeBinding<'hir> {
-        debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", constraint, itctx);
-        // lower generic arguments of identifier in constraint
+    ) -> hir::AssocItemConstraint<'hir> {
+        debug!(?constraint, ?itctx);
+        // Lower the generic arguments for the associated item.
         let gen_args = if let Some(gen_args) = &constraint.gen_args {
             let gen_args_ctor = match gen_args {
                 GenericArgs::AngleBracketed(data) => {
@@ -994,7 +985,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         };
                         GenericArgsCtor {
                             args: Default::default(),
-                            bindings: &[],
+                            constraints: &[],
                             parenthesized,
                             span: data.inputs_span,
                         }
@@ -1024,7 +1015,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         err.emit();
                         GenericArgsCtor {
                             args: Default::default(),
-                            bindings: &[],
+                            constraints: &[],
                             parenthesized: hir::GenericArgsParentheses::ReturnTypeNotation,
                             span: data.span,
                         }
@@ -1046,14 +1037,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             self.arena.alloc(hir::GenericArgs::none())
         };
         let kind = match &constraint.kind {
-            AssocConstraintKind::Equality { term } => {
+            AssocItemConstraintKind::Equality { term } => {
                 let term = match term {
                     Term::Ty(ty) => self.lower_ty(ty, itctx).into(),
                     Term::Const(c) => self.lower_anon_const(c).into(),
                 };
-                hir::TypeBindingKind::Equality { term }
+                hir::AssocItemConstraintKind::Equality { term }
             }
-            AssocConstraintKind::Bound { bounds } => {
+            AssocItemConstraintKind::Bound { bounds } => {
                 // Disallow ATB in dyn types
                 if self.is_in_dyn_type {
                     let suggestion = match itctx {
@@ -1077,18 +1068,18 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     });
                     let err_ty =
                         &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err(guar)));
-                    hir::TypeBindingKind::Equality { term: err_ty.into() }
+                    hir::AssocItemConstraintKind::Equality { term: err_ty.into() }
                 } else {
-                    // Desugar `AssocTy: Bounds` into a type binding where the
+                    // Desugar `AssocTy: Bounds` into an assoc type binding where the
                     // later desugars into a trait predicate.
                     let bounds = self.lower_param_bounds(bounds, itctx);
 
-                    hir::TypeBindingKind::Constraint { bounds }
+                    hir::AssocItemConstraintKind::Bound { bounds }
                 }
             }
         };
 
-        hir::TypeBinding {
+        hir::AssocItemConstraint {
             hir_id: self.lower_node_id(constraint.id),
             ident: self.lower_ident(constraint.ident),
             gen_args,
@@ -2008,7 +1999,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
 
         let bound_args = self.arena.alloc(hir::GenericArgs {
             args: &[],
-            bindings: arena_vec![self; self.assoc_ty_binding(assoc_ty_name, opaque_ty_span, output_ty)],
+            constraints: arena_vec![self; self.assoc_ty_binding(assoc_ty_name, opaque_ty_span, output_ty)],
             parenthesized: hir::GenericArgsParentheses::No,
             span_ext: DUMMY_SP,
         });
@@ -2581,10 +2572,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 }
 
-/// Helper struct for delayed construction of GenericArgs.
+/// Helper struct for the delayed construction of [`hir::GenericArgs`].
 struct GenericArgsCtor<'hir> {
     args: SmallVec<[hir::GenericArg<'hir>; 4]>,
-    bindings: &'hir [hir::TypeBinding<'hir>],
+    constraints: &'hir [hir::AssocItemConstraint<'hir>],
     parenthesized: hir::GenericArgsParentheses,
     span: Span,
 }
@@ -2664,14 +2655,14 @@ impl<'hir> GenericArgsCtor<'hir> {
 
     fn is_empty(&self) -> bool {
         self.args.is_empty()
-            && self.bindings.is_empty()
+            && self.constraints.is_empty()
             && self.parenthesized == hir::GenericArgsParentheses::No
     }
 
     fn into_generic_args(self, this: &LoweringContext<'_, 'hir>) -> &'hir hir::GenericArgs<'hir> {
         let ga = hir::GenericArgs {
             args: this.arena.alloc_from_iter(self.args),
-            bindings: self.bindings,
+            constraints: self.constraints,
             parenthesized: self.parenthesized,
             span_ext: this.lower_span(self.span),
         };