Addressed points raised in review.

src/librustc/hir/intravisit.rs

@@ -626,9 +626,6 @@ pub fn walk_ty<'v, V: Visitor<'v>>(visitor: &mut V, typ: &'v Ty) {
         TyKind::CVarArgs(ref lt) => {
             visitor.visit_lifetime(lt)
         }
-        TyKind::AssocTyExistential(ref bounds) => {
-            walk_list!(visitor, visit_param_bound, bounds);
-        }
         TyKind::Infer | TyKind::Err => {}
     }
 }
@@ -677,7 +674,14 @@ pub fn walk_assoc_type_binding<'v, V: Visitor<'v>>(visitor: &mut V,
                                                    type_binding: &'v TypeBinding) {
     visitor.visit_id(type_binding.hir_id);
     visitor.visit_ident(type_binding.ident);
-    visitor.visit_ty(&type_binding.ty);
+    match type_binding.kind {
+        TypeBindingKind::Equality { ref ty } => {
+            visitor.visit_ty(ty);
+        }
+        TypeBindingKind::Constraint { ref bounds } => {
+            walk_list!(visitor, visit_param_bound, bounds);
+        }
+    }
 }
 
 pub fn walk_pat<'v, V: Visitor<'v>>(visitor: &mut V, pattern: &'v Pat) {

src/librustc/hir/lowering.rs

@@ -1377,9 +1377,10 @@ fn lower_assoc_ty_constraint(&mut self,
                                  -> hir::TypeBinding {
         debug!("lower_assoc_ty_constraint(constraint={:?}, itctx={:?})", c, itctx);
 
-        // Convert to a type representing the `T::Item` value.
-        let ty = match c.kind {
-            AssocTyConstraintKind::Equality { ref ty } => self.lower_ty(ty, itctx),
+        let kind = match c.kind {
+            AssocTyConstraintKind::Equality { ref ty } => hir::TypeBindingKind::Equality {
+                ty: self.lower_ty(ty, itctx)
+            },
             AssocTyConstraintKind::Bound { ref bounds } => {
                 // Piggy-back on the `impl Trait` context to figure out the correct behavior.
                 let (desugar_to_impl_trait, itctx) = match itctx {
@@ -1422,7 +1423,7 @@ fn lower_assoc_ty_constraint(&mut self,
 
                 if desugar_to_impl_trait {
                     // Desugar `AssocTy: Bounds` into `AssocTy = impl Bounds`. We do this by
-                    // constructing the HIR for "impl bounds" and then lowering that.
+                    // constructing the HIR for `impl bounds...` and then lowering that.
 
                     let impl_trait_node_id = self.sess.next_node_id();
                     let parent_def_index = self.current_hir_id_owner.last().unwrap().0;
@@ -1436,27 +1437,27 @@ fn lower_assoc_ty_constraint(&mut self,
                     );
 
                     self.with_dyn_type_scope(false, |this| {
-                        this.lower_ty(
+                        let ty = this.lower_ty(
                             &Ty {
                                 id: this.sess.next_node_id(),
                                 node: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
                                 span: DUMMY_SP,
                             },
                             itctx,
-                        )
+                        );
+
+                        hir::TypeBindingKind::Equality {
+                            ty
+                        }
                     })
                 } else {
-                    // Desugar `AssocTy: Bounds` into `AssocTy = ∃ T (T: Bounds)`, where the
-                    // "false existential" later desugars into a trait predicate.
-
+                    // Desugar `AssocTy: Bounds` into a type binding where the
+                    // later desugars into a trait predicate.
                     let bounds = self.lower_param_bounds(bounds, itctx);
 
-                    let id = self.sess.next_node_id();
-                    P(hir::Ty {
-                        hir_id: self.lower_node_id(id),
-                        node: hir::TyKind::AssocTyExistential(bounds),
-                        span: DUMMY_SP,
-                    })
+                    hir::TypeBindingKind::Constraint {
+                        bounds
+                    }
                 }
             }
         };
@@ -1464,7 +1465,7 @@ fn lower_assoc_ty_constraint(&mut self,
         hir::TypeBinding {
             hir_id: self.lower_node_id(c.id),
             ident: c.ident,
-            ty,
+            kind,
             span: c.span,
         }
     }
@@ -2359,10 +2360,17 @@ fn lower_parenthesized_parameter_data(
                             hir::TypeBinding {
                                 hir_id: this.next_id(),
                                 ident: Ident::with_empty_ctxt(FN_OUTPUT_NAME),
-                                ty: output
-                                    .as_ref()
-                                    .map(|ty| this.lower_ty(&ty, ImplTraitContext::disallowed()))
-                                    .unwrap_or_else(|| P(mk_tup(this, hir::HirVec::new(), span))),
+                                kind: hir::TypeBindingKind::Equality {
+                                    ty: output
+                                        .as_ref()
+                                        .map(|ty| this.lower_ty(
+                                            &ty,
+                                            ImplTraitContext::disallowed()
+                                        ))
+                                        .unwrap_or_else(||
+                                            P(mk_tup(this, hir::HirVec::new(), span))
+                                        ),
+                                },
                                 span: output.as_ref().map_or(span, |ty| ty.span),
                             }
                         ],
@@ -2666,7 +2674,9 @@ fn lower_async_fn_output_type_to_future_bound(
             args: hir_vec![],
             bindings: hir_vec![hir::TypeBinding {
                 ident: Ident::with_empty_ctxt(FN_OUTPUT_NAME),
-                ty: output_ty,
+                kind: hir::TypeBindingKind::Equality {
+                    ty: output_ty,
+                },
                 hir_id: self.next_id(),
                 span,
             }],

src/librustc/hir/mod.rs

@@ -1780,7 +1780,7 @@ pub struct ImplItem {
     pub span: Span,
 }
 
-/// Represents different contents within `impl`s.
+/// Represents various kinds of content within an `impl`.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub enum ImplItemKind {
     /// An associated constant of the given type, set to the constant result
@@ -1794,16 +1794,51 @@ pub enum ImplItemKind {
     Existential(GenericBounds),
 }
 
-// Bind a type to an associated type (`A = Foo`).
+/// Bind a type to an associated type (i.e., `A = Foo`).
+///
+/// Bindings like `A: Debug` are represented as a special type `A =
+/// $::Debug` that is understood by the astconv code.
+///
+/// FIXME(alexreg) -- why have a separate type for the binding case,
+/// wouldn't it be better to make the `ty` field an enum like:
+///
+/// ```
+/// enum TypeBindingKind {
+///    Equals(...),
+///    Binding(...),
+/// }
+/// ```
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct TypeBinding {
     pub hir_id: HirId,
     #[stable_hasher(project(name))]
     pub ident: Ident,
-    pub ty: P<Ty>,
+    pub kind: TypeBindingKind,
     pub span: Span,
 }
 
+// Represents the two kinds of type bindings.
+#[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
+pub enum TypeBindingKind {
+    /// E.g., `Foo<Bar: Send>`.
+    Constraint {
+        bounds: HirVec<GenericBound>,
+    },
+    /// E.g., `Foo<Bar = ()>`.
+    Equality {
+        ty: P<Ty>,
+    },
+}
+
+impl TypeBinding {
+    pub fn ty(&self) -> &Ty {
+        match self.kind {
+            TypeBindingKind::Equality { ref ty } => ty,
+            _ => bug!("expected equality type binding for parenthesized generic args"),
+        }
+    }
+}
+
 #[derive(Clone, RustcEncodable, RustcDecodable)]
 pub struct Ty {
     pub hir_id: HirId,
@@ -1898,8 +1933,6 @@ pub enum TyKind {
     /// Placeholder for C-variadic arguments. We "spoof" the `VaList` created
     /// from the variadic arguments. This type is only valid up to typeck.
     CVarArgs(Lifetime),
-    /// The existential type (i.e., `impl Trait`) that constrains an associated type.
-    AssocTyExistential(HirVec<GenericBound>),
 }
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
@@ -2236,18 +2269,18 @@ pub fn is_positional(&self) -> bool {
     }
 }
 
-/// Fields and constructor ids of enum variants and structs
+/// Fields and constructor IDs of enum variants and structs.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub enum VariantData {
-    /// Struct variant.
+    /// A struct variant.
     ///
-    /// e.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
+    /// E.g., `Bar { .. }` as in `enum Foo { Bar { .. } }`.
     Struct(HirVec<StructField>, /* recovered */ bool),
-    /// Tuple variant.
+    /// A tuple variant.
     ///
     /// E.g., `Bar(..)` as in `enum Foo { Bar(..) }`.
     Tuple(HirVec<StructField>, HirId),
-    /// Unit variant.
+    /// A unit variant.
     ///
     /// E.g., `Bar = ..` as in `enum Foo { Bar = .. }`.
     Unit(HirId),

src/librustc/hir/print.rs

@@ -409,9 +409,6 @@ pub fn print_type(&mut self, ty: &hir::Ty) -> io::Result<()> {
             hir::TyKind::CVarArgs(_) => {
                 self.s.word("...")?;
             }
-            hir::TyKind::AssocTyExistential(ref bounds) => {
-                self.print_bounds(":", bounds)?;
-            }
         }
         self.end()
     }
@@ -1648,7 +1645,7 @@ fn print_generic_args(&mut self,
 
             self.space_if_not_bol()?;
             self.word_space("->")?;
-            self.print_type(&generic_args.bindings[0].ty)?;
+            self.print_type(generic_args.bindings[0].ty())?;
         } else {
             let start = if colons_before_params { "::<" } else { "<" };
             let empty = Cell::new(true);
@@ -1693,8 +1690,15 @@ fn print_generic_args(&mut self,
                 start_or_comma(self)?;
                 self.print_ident(binding.ident)?;
                 self.s.space()?;
-                self.word_space("=")?;
-                self.print_type(&binding.ty)?;
+                match generic_args.bindings[0].kind {
+                    hir::TypeBindingKind::Equality { ref ty } => {
+                        self.word_space("=")?;
+                        self.print_type(ty)?;
+                    }
+                    hir::TypeBindingKind::Constraint { ref bounds } => {
+                        self.print_bounds(":", bounds)?;
+                    }
+                }
             }
 
             if !empty.get() {

src/librustc/middle/resolve_lifetime.rs

@@ -923,7 +923,7 @@ fn visit_path(&mut self, path: &'tcx hir::Path, _: hir::HirId) {
     fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl) {
         let output = match fd.output {
             hir::DefaultReturn(_) => None,
-            hir::Return(ref ty) => Some(ty),
+            hir::Return(ref ty) => Some(&**ty),
         };
         self.visit_fn_like_elision(&fd.inputs, output);
     }
@@ -1884,7 +1884,7 @@ fn visit_segment_args(&mut self, res: Res, depth: usize, generic_args: &'tcx hir
         if generic_args.parenthesized {
             let was_in_fn_syntax = self.is_in_fn_syntax;
             self.is_in_fn_syntax = true;
-            self.visit_fn_like_elision(generic_args.inputs(), Some(&generic_args.bindings[0].ty));
+            self.visit_fn_like_elision(generic_args.inputs(), Some(generic_args.bindings[0].ty()));
             self.is_in_fn_syntax = was_in_fn_syntax;
             return;
         }
@@ -2020,7 +2020,7 @@ fn visit_segment_args(&mut self, res: Res, depth: usize, generic_args: &'tcx hir
         }
     }
 
-    fn visit_fn_like_elision(&mut self, inputs: &'tcx [hir::Ty], output: Option<&'tcx P<hir::Ty>>) {
+    fn visit_fn_like_elision(&mut self, inputs: &'tcx [hir::Ty], output: Option<&'tcx hir::Ty>) {
         debug!("visit_fn_like_elision: enter");
         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
         let arg_scope = Scope::Elision {

src/librustc_typeck/astconv.rs

@@ -710,10 +710,11 @@ fn create_substs_for_ast_path<'a>(&self,
         // back separately.
         let assoc_bindings = generic_args.bindings.iter()
             .map(|binding| {
-                let kind = if let hir::TyKind::AssocTyExistential(ref bounds) = binding.ty.node {
-                    ConvertedBindingKind::Constraint(bounds.clone())
-                } else {
-                    ConvertedBindingKind::Equality(self.ast_ty_to_ty(&binding.ty))
+                let kind = match binding.kind {
+                    hir::TypeBindingKind::Equality { ref ty } =>
+                        ConvertedBindingKind::Equality(self.ast_ty_to_ty(ty)),
+                    hir::TypeBindingKind::Constraint { ref bounds } =>
+                        ConvertedBindingKind::Constraint(bounds.clone()),
                 };
                 ConvertedBinding {
                     item_name: binding.ident,
@@ -2060,10 +2061,6 @@ pub fn ast_ty_to_ty(&self, ast_ty: &hir::Ty) -> Ty<'tcx> {
                 let region = self.ast_region_to_region(&lt, None);
                 tcx.type_of(va_list_did).subst(tcx, &[region.into()])
             }
-            hir::TyKind::AssocTyExistential(..) => {
-                // Type is never actually used.
-                tcx.types.err
-            }
             hir::TyKind::Err => {
                 tcx.types.err
             }

src/librustdoc/clean/auto_trait.rs

@@ -626,7 +626,9 @@ fn param_env_to_generics<'b, 'c, 'cx>(
                                             } => {
                                                 bindings.push(TypeBinding {
                                                     name: left_name.clone(),
-                                                    ty: rhs,
+                                                    kind: TypeBindingKind::Equality {
+                                                        ty: rhs,
+                                                    },
                                                 });
                                             }
                                             &mut GenericArgs::Parenthesized { .. } => {

src/librustdoc/clean/mod.rs

@@ -1979,7 +1979,7 @@ pub fn sugared_async_return_type(&self) -> FunctionRetTy {
                 match &bounds[0] {
                     GenericBound::TraitBound(PolyTrait { trait_, .. }, ..) => {
                         let bindings = trait_.bindings().unwrap();
-                        FunctionRetTy::Return(bindings[0].ty.clone())
+                        FunctionRetTy::Return(bindings[0].ty().clone())
                     }
                     _ => panic!("unexpected desugaring of async function"),
                 }
@@ -2937,11 +2937,8 @@ fn clean(&self, cx: &DocContext<'_>) -> Type {
             }
             TyKind::BareFn(ref barefn) => BareFunction(box barefn.clean(cx)),
             TyKind::Infer | TyKind::Err => Infer,
-            TyKind::Typeof(..) => panic!("Unimplemented type {:?}", self.node),
+            TyKind::Typeof(..) => panic!("unimplemented type {:?}", self.node),
             TyKind::CVarArgs(_) => CVarArgs,
-            TyKind::AssocTyExistential(ref bounds) => {
-                ImplTrait(bounds.into_iter().map(|b| b.clean(cx)).collect())
-            }
         }
     }
 }
@@ -3057,7 +3054,9 @@ fn clean(&self, cx: &DocContext<'_>) -> Type {
                 for pb in obj.projection_bounds() {
                     bindings.push(TypeBinding {
                         name: cx.tcx.associated_item(pb.item_def_id()).ident.name.clean(cx),
-                        ty: pb.skip_binder().ty.clean(cx)
+                        kind: TypeBindingKind::Equality {
+                            ty: pb.skip_binder().ty.clean(cx)
+                        },
                     });
                 }
 
@@ -3113,7 +3112,9 @@ fn clean(&self, cx: &DocContext<'_>) -> Type {
                                 Some(TypeBinding {
                                     name: cx.tcx.associated_item(proj.projection_ty.item_def_id)
                                                 .ident.name.clean(cx),
-                                    ty: proj.ty.clean(cx),
+                                    kind: TypeBindingKind::Equality {
+                                        ty: proj.ty.clean(cx),
+                                    },
                                 })
                             } else {
                                 None
@@ -3498,7 +3499,7 @@ pub enum GenericArgs {
 impl Clean<GenericArgs> for hir::GenericArgs {
     fn clean(&self, cx: &DocContext<'_>) -> GenericArgs {
         if self.parenthesized {
-            let output = self.bindings[0].ty.clean(cx);
+            let output = self.bindings[0].ty().clean(cx);
             GenericArgs::Parenthesized {
                 inputs: self.inputs().clean(cx),
                 output: if output != Type::Tuple(Vec::new()) { Some(output) } else { None }
@@ -4346,18 +4347,53 @@ fn clean(&self, _: &DocContext<'_>) -> Deprecation {
     }
 }
 
-/// An equality constraint on an associated type, e.g., `A = Bar` in `Foo<A = Bar>`
+/// An type binding on an associated type (e.g., `A = Bar` in `Foo<A = Bar>` or
+/// `A: Send + Sync` in `Foo<A: Send + Sync>`).
 #[derive(Clone, PartialEq, Eq, RustcDecodable, RustcEncodable, Debug, Hash)]
 pub struct TypeBinding {
     pub name: String,
-    pub ty: Type
+    pub kind: TypeBindingKind,
+}
+
+#[derive(Clone, PartialEq, Eq, RustcDecodable, RustcEncodable, Debug, Hash)]
+pub enum TypeBindingKind {
+    Equality {
+        ty: Type,
+    },
+    Constraint {
+        bounds: Vec<GenericBound>,
+    },
+}
+
+impl TypeBinding {
+    pub fn ty(&self) -> &Type {
+        match self.kind {
+            TypeBindingKind::Equality { ref ty } => ty,
+            _ => panic!("expected equality type binding for parenthesized generic args"),
+        }
+    }
 }
 
 impl Clean<TypeBinding> for hir::TypeBinding {
     fn clean(&self, cx: &DocContext<'_>) -> TypeBinding {
         TypeBinding {
             name: self.ident.name.clean(cx),
-            ty: self.ty.clean(cx)
+            kind: self.kind.clean(cx),
+        }
+    }
+}
+
+impl Clean<TypeBindingKind> for hir::TypeBindingKind {
+    fn clean(&self, cx: &DocContext<'_>) -> TypeBindingKind {
+        match *self {
+            hir::TypeBindingKind::Equality { ref ty } =>
+                TypeBindingKind::Equality {
+                    ty: ty.clean(cx),
+                },
+            hir::TypeBindingKind::Constraint { ref bounds } =>
+                TypeBindingKind::Constraint {
+                    bounds: bounds.into_iter().map(|b| b.clean(cx)).collect(),
+                },
         }
     }
 }

src/librustdoc/clean/simplify.rs

@@ -91,7 +91,9 @@ pub fn where_clauses(cx: &DocContext<'_>, clauses: Vec<WP>) -> Vec<WP> {
                 PP::AngleBracketed { ref mut bindings, .. } => {
                     bindings.push(clean::TypeBinding {
                         name: name.clone(),
-                        ty: rhs.clone(),
+                        kind: clean::TypeBindingKind::Equality {
+                            ty: rhs.clone(),
+                        },
                     });
                 }
                 PP::Parenthesized { ref mut output, .. } => {

src/librustdoc/html/format.rs

@@ -1019,11 +1019,26 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 
 impl fmt::Display for clean::TypeBinding {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        if f.alternate() {
-            write!(f, "{} = {:#}", self.name, self.ty)
-        } else {
-            write!(f, "{} = {}", self.name, self.ty)
+        f.write_str(&self.name)?;
+        match self.kind {
+            clean::TypeBindingKind::Equality { ref ty } => {
+                if f.alternate() {
+                    write!(f, " = {:#}", ty)?;
+                } else {
+                    write!(f, " = {}", ty)?;
+                }
+            }
+            clean::TypeBindingKind::Constraint { ref bounds } => {
+                if !bounds.is_empty() {
+                    if f.alternate() {
+                        write!(f, ": {:#}", GenericBounds(bounds))?;
+                    } else {
+                        write!(f, ":&nbsp;{}", GenericBounds(bounds))?;
+                    }
+                }
+            }
         }
+        Ok(())
     }
 }