Implemented for traits (associated type definitions).

src/librustc/hir/intravisit.rs

@@ -626,6 +626,9 @@ 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 => {}
     }
 }

src/librustc/hir/lowering.rs

@@ -195,6 +195,12 @@ enum ImplTraitContext<'a> {
     /// (e.g., for consts and statics).
     Existential(Option<DefId> /* fn def-ID */),
 
+    /// Treat `impl Trait` as a bound on the associated type applied to the trait.
+    /// Example: `trait Foo { type Bar: Iterator<Item = impl Debug>; }` is conceptually
+    /// equivalent to `trait Foo where <Self::Bar as Iterator>::Item: Debug
+    /// { type Bar: Iterator; }`.
+    AssociatedTy,
+
     /// `impl Trait` is not accepted in this position.
     Disallowed(ImplTraitPosition),
 }
@@ -217,6 +223,7 @@ impl<'a> ImplTraitContext<'a> {
         match self {
             Universal(params) => Universal(params),
             Existential(fn_def_id) => Existential(*fn_def_id),
+            AssociatedTy => AssociatedTy,
             Disallowed(pos) => Disallowed(*pos),
         }
     }
@@ -1537,6 +1544,16 @@ impl<'a> LoweringContext<'a> {
                             }),
                         ))
                     }
+                    ImplTraitContext::AssociatedTy => {
+                        let hir_bounds = self.lower_param_bounds(
+                            bounds,
+                            ImplTraitContext::AssociatedTy,
+                        );
+
+                        hir::TyKind::AssocTyExistential(
+                            hir_bounds,
+                        )
+                    }
                     ImplTraitContext::Disallowed(pos) => {
                         let allowed_in = if self.sess.features_untracked()
                                                 .impl_trait_in_bindings {
@@ -1640,8 +1657,8 @@ impl<'a> LoweringContext<'a> {
         })
     }
 
-    /// Registers a new existential type with the proper `NodeId`ss and
-    /// returns the lowered node ID for the existential type.
+    /// Registers a new existential type with the proper `NodeId`s and
+    /// returns the lowered node-ID for the existential type.
     fn generate_existential_type(
         &mut self,
         exist_ty_node_id: NodeId,
@@ -2226,8 +2243,9 @@ impl<'a> LoweringContext<'a> {
         (
             hir::GenericArgs {
                 args: args.iter().map(|a| self.lower_generic_arg(a, itctx.reborrow())).collect(),
-                bindings: constraints.iter().map(
-                    |b| self.lower_assoc_ty_constraint(b, itctx.reborrow())).collect(),
+                bindings: constraints.iter()
+                    .map(|b| self.lower_assoc_ty_constraint(b, itctx.reborrow()))
+                    .collect(),
                 parenthesized: false,
             },
             !has_types && param_mode == ParamMode::Optional
@@ -3257,13 +3275,13 @@ impl<'a> LoweringContext<'a> {
             ItemKind::ForeignMod(ref nm) => hir::ItemKind::ForeignMod(self.lower_foreign_mod(nm)),
             ItemKind::GlobalAsm(ref ga) => hir::ItemKind::GlobalAsm(self.lower_global_asm(ga)),
             ItemKind::Ty(ref t, ref generics) => hir::ItemKind::Ty(
-                self.lower_ty(t, ImplTraitContext::disallowed()),
-                self.lower_generics(generics, ImplTraitContext::disallowed()),
+                self.lower_ty(t, ImplTraitContext::AssociatedTy),
+                self.lower_generics(generics, ImplTraitContext::AssociatedTy),
             ),
             ItemKind::Existential(ref b, ref generics) => hir::ItemKind::Existential(
                 hir::ExistTy {
-                    generics: self.lower_generics(generics, ImplTraitContext::disallowed()),
-                    bounds: self.lower_param_bounds(b, ImplTraitContext::Existential(None)),
+                    generics: self.lower_generics(generics, ImplTraitContext::AssociatedTy),
+                    bounds: self.lower_param_bounds(b, ImplTraitContext::AssociatedTy),
                     impl_trait_fn: None,
                     origin: hir::ExistTyOrigin::ExistentialType,
                 },
@@ -3276,20 +3294,20 @@ impl<'a> LoweringContext<'a> {
                         .map(|x| self.lower_variant(x))
                         .collect(),
                 },
-                self.lower_generics(generics, ImplTraitContext::disallowed()),
+                self.lower_generics(generics, ImplTraitContext::AssociatedTy),
             ),
             ItemKind::Struct(ref struct_def, ref generics) => {
                 let struct_def = self.lower_variant_data(struct_def);
                 hir::ItemKind::Struct(
                     struct_def,
-                    self.lower_generics(generics, ImplTraitContext::disallowed()),
+                    self.lower_generics(generics, ImplTraitContext::AssociatedTy),
                 )
             }
             ItemKind::Union(ref vdata, ref generics) => {
                 let vdata = self.lower_variant_data(vdata);
                 hir::ItemKind::Union(
                     vdata,
-                    self.lower_generics(generics, ImplTraitContext::disallowed()),
+                    self.lower_generics(generics, ImplTraitContext::AssociatedTy),
                 )
             }
             ItemKind::Impl(
@@ -3656,15 +3674,17 @@ impl<'a> LoweringContext<'a> {
                 );
                 (generics, hir::TraitItemKind::Method(sig, hir::TraitMethod::Provided(body_id)))
             }
-            TraitItemKind::Type(ref bounds, ref default) => (
-                self.lower_generics(&i.generics, ImplTraitContext::disallowed()),
-                hir::TraitItemKind::Type(
-                    self.lower_param_bounds(bounds, ImplTraitContext::disallowed()),
+            TraitItemKind::Type(ref bounds, ref default) => {
+                let generics = self.lower_generics(&i.generics, ImplTraitContext::AssociatedTy);
+                let node = hir::TraitItemKind::Type(
+                    self.lower_param_bounds(bounds, ImplTraitContext::AssociatedTy),
                     default
                         .as_ref()
                         .map(|x| self.lower_ty(x, ImplTraitContext::disallowed())),
-                ),
-            ),
+                );
+
+                (generics, node)
+            },
             TraitItemKind::Macro(..) => bug!("macro item shouldn't exist at this point"),
         };
 

src/librustc/hir/mod.rs

@@ -1898,6 +1898,8 @@ 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)]

src/librustc/hir/print.rs

@@ -409,6 +409,9 @@ impl<'a> State<'a> {
             hir::TyKind::CVarArgs(_) => {
                 self.s.word("...")?;
             }
+            hir::TyKind::AssocTyExistential(ref bounds) => {
+                self.print_bounds(":", bounds)?;
+            }
         }
         self.end()
     }

src/librustc/infer/opaque_types/mod.rs

@@ -948,15 +948,17 @@ pub fn may_define_existential_type(
     let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
     trace!(
         "may_define_existential_type(def={:?}, opaque_node={:?})",
-        tcx.hir().get(hir_id),
-        tcx.hir().get(opaque_hir_id)
+        tcx.hir().get_by_hir_id(hir_id),
+        tcx.hir().get_by_hir_id(opaque_hir_id)
     );
 
     // Named existential types can be defined by any siblings or children of siblings.
-    let scope_id = tcx.hir().get_defining_scope(opaque_hir_id)
-                            .expect("could not get defining scope");
+    let scope_node_id = tcx.hir()
+        .get_defining_scope(tcx.hir().hir_to_node_id(opaque_hir_id))
+        .expect("could not get defining scope");
+    let scope_id = tcx.hir().node_to_hir_id(scope_node_id);
     // We walk up the node tree until we hit the root or the scope of the opaque type.
-    while hir_id != scope_id && hir_id != ast::CRATE_hir_ID {
+    while hir_id != scope_id && hir_id != hir::CRATE_HIR_ID {
         hir_id = tcx.hir().get_parent_item(hir_id);
     }
     // Syntactically, we are allowed to define the concrete type if:

src/librustc_interface/util.rs

@@ -726,7 +726,8 @@ impl<'a> ReplaceBodyWithLoop<'a> {
                                 any_assoc_ty_bounds ||
                                 any_involves_impl_trait(types.into_iter()) ||
                                 any_involves_impl_trait(data.constraints.iter().filter_map(|c| {
-                                    if let ast::AssocTyConstraintKind::Equality { ref ty } = c.kind {
+                                    if let ast::AssocTyConstraintKind::Equality { ref ty }
+                                            = c.kind {
                                         Some(ty)
                                     } else {
                                         None

src/librustc_privacy/lib.rs

@@ -1040,12 +1040,11 @@ impl<'a, 'tcx> Visitor<'tcx> for TypePrivacyVisitor<'a, 'tcx> {
         if !self.in_body {
             // Avoid calling `hir_trait_to_predicates` in bodies, it will ICE.
             // The traits' privacy in bodies is already checked as a part of trait object types.
-            let (principal, projections) =
-                rustc_typeck::hir_trait_to_predicates(self.tcx, trait_ref);
+            let (principal, bounds) = rustc_typeck::hir_trait_to_predicates(self.tcx, trait_ref);
             if self.visit_trait(*principal.skip_binder()) {
                 return;
             }
-            for (poly_predicate, _) in projections {
+            for (poly_predicate, _) in bounds.projection_bounds {
                 let tcx = self.tcx;
                 if self.visit(poly_predicate.skip_binder().ty) ||
                    self.visit_trait(poly_predicate.skip_binder().projection_ty.trait_ref(tcx)) {