Ban multi-trait objects via trait aliases.

13 files changed, 215 insertions, 133 deletions

diff --git a/src/liballoc/slice.rs b/src/liballoc/slice.rs
index 6eac8487401..8768f1ff081 100644
--- a/src/liballoc/slice.rs
+++ b/src/liballoc/slice.rs
@@ -123,12 +123,12 @@ pub use core::slice::{RChunks, RChunksMut, RChunksExact, RChunksExactMut};
 ////////////////////////////////////////////////////////////////////////////////
 
 // HACK(japaric) needed for the implementation of `vec!` macro during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::into_vec;
 
 // HACK(japaric) needed for the implementation of `Vec::clone` during testing
-// NB see the hack module in this file for more details
+// N.B., see the `hack` module in this file for more details.
 #[cfg(test)]
 pub use hack::to_vec;
 
@@ -376,7 +376,7 @@ impl<T> [T] {
     pub fn to_vec(&self) -> Vec<T>
         where T: Clone
     {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::to_vec(self)
     }
 
@@ -397,7 +397,7 @@ impl<T> [T] {
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
     pub fn into_vec(self: Box<Self>) -> Vec<T> {
-        // NB see hack module in this file
+        // N.B., see the `hack` module in this file for more details.
         hack::into_vec(self)
     }
 
diff --git a/src/librustc/hir/map/mod.rs b/src/librustc/hir/map/mod.rs
index 4b94f772554..23f4d208571 100644
--- a/src/librustc/hir/map/mod.rs
+++ b/src/librustc/hir/map/mod.rs
@@ -957,7 +957,7 @@ impl<'hir> Map<'hir> {
         }
     }
 
-    /// Returns the name associated with the given NodeId's AST.
+    /// Returns the name associated with the given `NodeId`'s AST.
     pub fn name(&self, id: NodeId) -> Name {
         let hir_id = self.node_to_hir_id(id);
         self.name_by_hir_id(hir_id)
diff --git a/src/librustc/hir/mod.rs b/src/librustc/hir/mod.rs
index 22312e7459b..fafe1cf85a3 100644
--- a/src/librustc/hir/mod.rs
+++ b/src/librustc/hir/mod.rs
@@ -2143,11 +2143,11 @@ pub enum UseKind {
     ListStem,
 }
 
-/// TraitRef's appear in impls.
+/// `TraitRef` are references to traits in impls.
 ///
-/// resolve maps each TraitRef's ref_id to its defining trait; that's all
-/// that the ref_id is for. Note that ref_id's value is not the NodeId of the
-/// trait being referred to but just a unique NodeId that serves as a key
+/// `resolve` maps each `TraitRef`'s `ref_id` to its defining trait; that's all
+/// that the `ref_id` is for. Note that `ref_id`'s value is not the `NodeId` of the
+/// trait being referred to but just a unique `NodeId` that serves as a key
 /// within the resolution map.
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub struct TraitRef {
diff --git a/src/librustc/infer/outlives/verify.rs b/src/librustc/infer/outlives/verify.rs
index e1ad5aeea19..5b8d89c865a 100644
--- a/src/librustc/infer/outlives/verify.rs
+++ b/src/librustc/infer/outlives/verify.rs
@@ -141,9 +141,9 @@ impl<'cx, 'gcx, 'tcx> VerifyBoundCx<'cx, 'gcx, 'tcx> {
     }
 
     fn recursive_type_bound(&self, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
-        let mut bounds = ty.walk_shallow()
+        let mut bounds: Vec<_> = ty.walk_shallow()
             .map(|subty| self.type_bound(subty))
-            .collect::<Vec<_>>();
+            .collect();
 
         let mut regions = smallvec![];
         ty.push_regions(&mut regions);
diff --git a/src/librustc/traits/mod.rs b/src/librustc/traits/mod.rs
index 846924fb591..627f923399b 100644
--- a/src/librustc/traits/mod.rs
+++ b/src/librustc/traits/mod.rs
@@ -62,6 +62,7 @@ pub use self::engine::{TraitEngine, TraitEngineExt};
 pub use self::util::{elaborate_predicates, elaborate_trait_ref, elaborate_trait_refs};
 pub use self::util::{supertraits, supertrait_def_ids, transitive_bounds,
                      Supertraits, SupertraitDefIds};
+pub use self::util::{expand_trait_refs, TraitRefExpander};
 
 pub use self::chalk_fulfill::{
     CanonicalGoal as ChalkCanonicalGoal,
@@ -1043,7 +1044,7 @@ fn vtable_methods<'a, 'tcx>(
     )
 }
 
-impl<'tcx,O> Obligation<'tcx,O> {
+impl<'tcx, O> Obligation<'tcx,O> {
     pub fn new(cause: ObligationCause<'tcx>,
                param_env: ty::ParamEnv<'tcx>,
                predicate: O)
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index c4be85050db..ec5e127a5ec 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -1772,7 +1772,8 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
             bounds
         );
 
-        let matching_bound = util::elaborate_predicates(self.tcx(), bounds.predicates)
+        let elaborated_predicates = util::elaborate_predicates(self.tcx(), bounds.predicates);
+        let matching_bound = elaborated_predicates
             .filter_to_traits()
             .find(|bound| {
                 self.infcx.probe(|_| {
diff --git a/src/librustc/traits/util.rs b/src/librustc/traits/util.rs
index 578d8bbedf3..897681e538e 100644
--- a/src/librustc/traits/util.rs
+++ b/src/librustc/traits/util.rs
@@ -1,3 +1,5 @@
+use syntax_pos::Span;
+
 use crate::hir;
 use crate::hir::def_id::DefId;
 use crate::traits::specialize::specialization_graph::NodeItem;
@@ -41,7 +43,6 @@ fn anonymize_predicate<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
     }
 }
 
-
 struct PredicateSet<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
     set: FxHashSet<ty::Predicate<'tcx>>,
@@ -73,12 +74,11 @@ impl<'a, 'gcx, 'tcx> PredicateSet<'a, 'gcx, 'tcx> {
 
 /// "Elaboration" is the process of identifying all the predicates that
 /// are implied by a source predicate. Currently this basically means
-/// walking the "supertraits" and other similar assumptions. For
-/// example, if we know that `T : Ord`, the elaborator would deduce
-/// that `T : PartialOrd` holds as well. Similarly, if we have `trait
-/// Foo : 'static`, and we know that `T : Foo`, then we know that `T :
-/// 'static`.
-pub struct Elaborator<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
+/// walking the "supertraits" and other similar assumptions. For example,
+/// if we know that `T: Ord`, the elaborator would deduce that `T: PartialOrd`
+/// holds as well. Similarly, if we have `trait Foo: 'static`, and we know that
+/// `T: Foo`, then we know that `T: 'static`.
+pub struct Elaborator<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
     stack: Vec<ty::Predicate<'tcx>>,
     visited: PredicateSet<'a, 'gcx, 'tcx>,
 }
@@ -96,8 +96,7 @@ pub fn elaborate_trait_refs<'cx, 'gcx, 'tcx>(
     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>)
     -> Elaborator<'cx, 'gcx, 'tcx>
 {
-    let predicates = trait_refs.map(|trait_ref| trait_ref.to_predicate())
-                               .collect();
+    let predicates = trait_refs.map(|trait_ref| trait_ref.to_predicate()).collect();
     elaborate_predicates(tcx, predicates)
 }
 
@@ -120,7 +119,7 @@ impl<'cx, 'gcx, 'tcx> Elaborator<'cx, 'gcx, 'tcx> {
         let tcx = self.visited.tcx;
         match *predicate {
             ty::Predicate::Trait(ref data) => {
-                // Predicates declared on the trait.
+                // Get predicates declared on the trait.
                 let predicates = tcx.super_predicates_of(data.def_id());
 
                 let mut predicates: Vec<_> = predicates.predicates
@@ -130,12 +129,11 @@ impl<'cx, 'gcx, 'tcx> Elaborator<'cx, 'gcx, 'tcx> {
                 debug!("super_predicates: data={:?} predicates={:?}",
                        data, predicates);
 
-                // Only keep those bounds that we haven't already
-                // seen.  This is necessary to prevent infinite
-                // recursion in some cases.  One common case is when
-                // people define `trait Sized: Sized { }` rather than `trait
-                // Sized { }`.
-                predicates.retain(|r| self.visited.insert(r));
+                // Only keep those bounds that we haven't already seen.
+                // This is necessary to prevent infinite recursion in some
+                // cases. One common case is when people define
+                // `trait Sized: Sized { }` rather than `trait Sized { }`.
+                predicates.retain(|p| self.visited.insert(p));
 
                 self.stack.extend(predicates);
             }
@@ -161,11 +159,9 @@ impl<'cx, 'gcx, 'tcx> Elaborator<'cx, 'gcx, 'tcx> {
                 // Currently, we do not elaborate const-evaluatable
                 // predicates.
             }
-
             ty::Predicate::RegionOutlives(..) => {
                 // Nothing to elaborate from `'a: 'b`.
             }
-
             ty::Predicate::TypeOutlives(ref data) => {
                 // We know that `T: 'a` for some type `T`. We can
                 // often elaborate this. For example, if we know that
@@ -192,34 +188,35 @@ impl<'cx, 'gcx, 'tcx> Elaborator<'cx, 'gcx, 'tcx> {
                 tcx.push_outlives_components(ty_max, &mut components);
                 self.stack.extend(
                     components
-                       .into_iter()
-                       .filter_map(|component| match component {
-                           Component::Region(r) => if r.is_late_bound() {
-                               None
-                           } else {
-                               Some(ty::Predicate::RegionOutlives(
-                                   ty::Binder::dummy(ty::OutlivesPredicate(r, r_min))))
-                           },
-
-                           Component::Param(p) => {
-                               let ty = tcx.mk_ty_param(p.index, p.name);
-                               Some(ty::Predicate::TypeOutlives(
-                                   ty::Binder::dummy(ty::OutlivesPredicate(ty, r_min))))
-                           },
-
-                           Component::UnresolvedInferenceVariable(_) => {
-                               None
-                           },
-
-                           Component::Projection(_) |
-                           Component::EscapingProjection(_) => {
-                               // We can probably do more here. This
-                               // corresponds to a case like `<T as
-                               // Foo<'a>>::U: 'b`.
-                               None
-                           },
-                       })
-                       .filter(|p| visited.insert(p)));
+                        .into_iter()
+                        .filter_map(|component| match component {
+                            Component::Region(r) => if r.is_late_bound() {
+                                None
+                            } else {
+                                Some(ty::Predicate::RegionOutlives(
+                                    ty::Binder::dummy(ty::OutlivesPredicate(r, r_min))))
+                            }
+
+                            Component::Param(p) => {
+                                let ty = tcx.mk_ty_param(p.index, p.name);
+                                Some(ty::Predicate::TypeOutlives(
+                                    ty::Binder::dummy(ty::OutlivesPredicate(ty, r_min))))
+                            }
+
+                            Component::UnresolvedInferenceVariable(_) => {
+                                None
+                            }
+
+                            Component::Projection(_) |
+                            Component::EscapingProjection(_) => {
+                                // We can probably do more here. This
+                                // corresponds to a case like `<T as
+                                // Foo<'a>>::U: 'b`.
+                                None
+                            }
+                        })
+                        .filter(|p| visited.insert(p))
+                );
             }
         }
     }
@@ -233,16 +230,10 @@ impl<'cx, 'gcx, 'tcx> Iterator for Elaborator<'cx, 'gcx, 'tcx> {
     }
 
     fn next(&mut self) -> Option<ty::Predicate<'tcx>> {
-        // Extract next item from top-most stack frame, if any.
-        let next_predicate = match self.stack.pop() {
-            Some(predicate) => predicate,
-            None => {
-                // No more stack frames. Done.
-                return None;
-            }
-        };
-        self.push(&next_predicate);
-        return Some(next_predicate);
+        self.stack.pop().map(|item| {
+            self.push(&item);
+            item
+        })
     }
 }
 
@@ -254,20 +245,124 @@ pub type Supertraits<'cx, 'gcx, 'tcx> = FilterToTraits<Elaborator<'cx, 'gcx, 'tc
 
 pub fn supertraits<'cx, 'gcx, 'tcx>(tcx: TyCtxt<'cx, 'gcx, 'tcx>,
                                     trait_ref: ty::PolyTraitRef<'tcx>)
-                                    -> Supertraits<'cx, 'gcx, 'tcx>
-{
+                                    -> Supertraits<'cx, 'gcx, 'tcx> {
     elaborate_trait_ref(tcx, trait_ref).filter_to_traits()
 }
 
 pub fn transitive_bounds<'cx, 'gcx, 'tcx>(tcx: TyCtxt<'cx, 'gcx, 'tcx>,
                                           bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>)
-                                          -> Supertraits<'cx, 'gcx, 'tcx>
-{
+                                          -> Supertraits<'cx, 'gcx, 'tcx> {
     elaborate_trait_refs(tcx, bounds).filter_to_traits()
 }
 
 ///////////////////////////////////////////////////////////////////////////
+// `TraitRefExpander` iterator
+///////////////////////////////////////////////////////////////////////////
+
+/// "Trait reference expansion" is the process of expanding a sequence of trait
+/// references into another sequence by transitively following all trait
+/// aliases. e.g. If you have bounds like `Foo + Send`, a trait alias
+/// `trait Foo = Bar + Sync;`, and another trait alias
+/// `trait Bar = Read + Write`, then the bounds would expand to
+/// `Read + Write + Sync + Send`.
+pub struct TraitRefExpander<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
+    stack: Vec<TraitRefExpansionInfo<'tcx>>,
+    visited: PredicateSet<'a, 'gcx, 'tcx>,
+}
+
+#[derive(Debug, Clone)]
+pub struct TraitRefExpansionInfo<'tcx> {
+    pub top_level_trait_ref: ty::PolyTraitRef<'tcx>,
+    pub top_level_span: Span,
+    pub trait_ref: ty::PolyTraitRef<'tcx>,
+    pub span: Span,
+}
+
+pub fn expand_trait_refs<'cx, 'gcx, 'tcx>(
+    tcx: TyCtxt<'cx, 'gcx, 'tcx>,
+    trait_refs: impl IntoIterator<Item = (ty::PolyTraitRef<'tcx>, Span)>
+) -> TraitRefExpander<'cx, 'gcx, 'tcx> {
+    let mut visited = PredicateSet::new(tcx);
+    let mut items: Vec<_> =
+        trait_refs
+            .into_iter()
+            .map(|(tr, sp)| TraitRefExpansionInfo {
+                top_level_trait_ref: tr.clone(),
+                top_level_span: sp,
+                trait_ref: tr,
+                span: sp,
+            })
+            .collect();
+    items.retain(|item| visited.insert(&item.trait_ref.to_predicate()));
+    TraitRefExpander { stack: items, visited: visited, }
+}
+
+impl<'cx, 'gcx, 'tcx> TraitRefExpander<'cx, 'gcx, 'tcx> {
+    // Returns `true` if `item` refers to a trait.
+    fn push(&mut self, item: &TraitRefExpansionInfo<'tcx>) -> bool {
+        let tcx = self.visited.tcx;
+
+        if !tcx.is_trait_alias(item.trait_ref.def_id()) {
+            return true;
+        }
+
+        // Get predicates declared on the trait.
+        let predicates = tcx.super_predicates_of(item.trait_ref.def_id());
+
+        let mut items: Vec<_> = predicates.predicates
+            .iter()
+            .rev()
+            .filter_map(|(pred, sp)| {
+                pred.subst_supertrait(tcx, &item.trait_ref)
+                    .to_opt_poly_trait_ref()
+                    .map(|trait_ref|
+                        TraitRefExpansionInfo {
+                            trait_ref,
+                            span: *sp,
+                            ..*item
+                        }
+                    )
+            })
+            .collect();
+
+        debug!("expand_trait_refs: trait_ref={:?} items={:?}",
+                item.trait_ref, items);
+
+        // Only keep those items that we haven't already seen.
+        items.retain(|i| self.visited.insert(&i.trait_ref.to_predicate()));
+
+        self.stack.extend(items);
+        false
+    }
+}
+
+impl<'cx, 'gcx, 'tcx> Iterator for TraitRefExpander<'cx, 'gcx, 'tcx> {
+    type Item = TraitRefExpansionInfo<'tcx>;
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.stack.len(), None)
+    }
+
+    fn next(&mut self) -> Option<TraitRefExpansionInfo<'tcx>> {
+        loop {
+            let item = self.stack.pop();
+            match item {
+                Some(item) => {
+                    if self.push(&item) {
+                        return Some(item);
+                    }
+                }
+                None => {
+                    return None;
+                }
+            }
+        }
+    }
+}
+
+///////////////////////////////////////////////////////////////////////////
 // Iterator over def-ids of supertraits
+///////////////////////////////////////////////////////////////////////////
 
 pub struct SupertraitDefIds<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'gcx, 'tcx>,
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index c55e508c8b5..6020a737853 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -1234,7 +1234,7 @@ impl<'tcx> TraitPredicate<'tcx> {
         self.trait_ref.def_id
     }
 
-    pub fn input_types<'a>(&'a self) -> impl DoubleEndedIterator<Item=Ty<'tcx>> + 'a {
+    pub fn input_types<'a>(&'a self) -> impl DoubleEndedIterator<Item = Ty<'tcx>> + 'a {
         self.trait_ref.input_types()
     }
 
@@ -2400,7 +2400,7 @@ impl<'a, 'gcx, 'tcx> AdtDef {
     pub fn discriminants(
         &'a self,
         tcx: TyCtxt<'a, 'gcx, 'tcx>,
-    ) -> impl Iterator<Item=(VariantIdx, Discr<'tcx>)> + Captures<'gcx> + 'a {
+    ) -> impl Iterator<Item = (VariantIdx, Discr<'tcx>)> + Captures<'gcx> + 'a {
         let repr_type = self.repr.discr_type();
         let initial = repr_type.initial_discriminant(tcx.global_tcx());
         let mut prev_discr = None::<Discr<'tcx>>;
diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index 1e65f868eba..5f184bf4a7d 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1177,13 +1177,13 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
                 );
             }
             ast::ImplItemKind::Type(ref ty) => {
-                // FIXME uses of the assoc type should ideally point to this
+                // FIXME: uses of the assoc type should ideally point to this
                 // 'def' and the name here should be a ref to the def in the
                 // trait.
                 self.visit_ty(ty)
             }
             ast::ImplItemKind::Existential(ref bounds) => {
-                // FIXME uses of the assoc type should ideally point to this
+                // FIXME: uses of the assoc type should ideally point to this
                 // 'def' and the name here should be a ref to the def in the
                 // trait.
                 for bound in bounds.iter() {
@@ -1216,7 +1216,7 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
         let hir_id = self.tcx.hir().node_to_hir_id(id);
         let access = access_from!(self.save_ctxt, root_item, hir_id);
 
-        // The parent def id of a given use tree is always the enclosing item.
+        // The parent def-ID of a given use tree is always the enclosing item.
         let parent = self.save_ctxt.tcx.hir().opt_local_def_id(id)
             .and_then(|id| self.save_ctxt.tcx.parent(id))
             .map(id_from_def_id);
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 06605695630..fc6edc2627b 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -975,12 +975,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
 
         if trait_bounds.is_empty() {
             span_err!(tcx.sess, span, E0224,
-                      "at least one non-builtin trait is required for an object type");
+                "at least one non-builtin trait is required for an object type");
             return tcx.types.err;
         }
 
         let mut projection_bounds = Vec::new();
         let dummy_self = self.tcx().types.trait_object_dummy_self;
+        let mut bound_trait_refs = Vec::with_capacity(trait_bounds.len());
         let (principal, potential_assoc_types) = self.instantiate_poly_trait_ref(
             &trait_bounds[0],
             dummy_self,
@@ -988,22 +989,29 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
         );
         debug!("principal: {:?}", principal);
 
-        for trait_bound in trait_bounds[1..].iter() {
-            // sanity check for non-principal trait bounds
-            self.instantiate_poly_trait_ref(trait_bound,
-                                            dummy_self,
-                                            &mut vec![]);
+        for trait_bound in trait_bounds[1..].iter().rev() {
+            // Sanity check for non-principal trait bounds.
+            let (tr, _) = self.instantiate_poly_trait_ref(
+                trait_bound,
+                dummy_self,
+                &mut Vec::new()
+            );
+            bound_trait_refs.push((tr, trait_bound.span));
         }
-
-        let (mut auto_traits, trait_bounds) = split_auto_traits(tcx, &trait_bounds[1..]);
-
-        if !trait_bounds.is_empty() {
-            let b = &trait_bounds[0];
-            let span = b.trait_ref.path.span;
-            struct_span_err!(self.tcx().sess, span, E0225,
-                "only auto traits can be used as additional traits in a trait object")
-                .span_label(span, "non-auto additional trait")
-                .emit();
+        bound_trait_refs.push((principal, trait_bounds[0].span));
+
+        let expanded_traits = traits::expand_trait_refs(tcx, bound_trait_refs);
+        let (auto_traits, regular_traits): (Vec<_>, Vec<_>) =
+            expanded_traits.partition(|i| tcx.trait_is_auto(i.trait_ref.def_id()));
+        if regular_traits.len() > 1 {
+            let extra_trait = &regular_traits[1];
+            let mut err = struct_span_err!(tcx.sess, extra_trait.top_level_span, E0225,
+                "only auto traits can be used as additional traits in a trait object");
+            err.span_label(extra_trait.span, "non-auto additional trait");
+            if extra_trait.span != extra_trait.top_level_span {
+                err.span_label(extra_trait.top_level_span, "expanded from this alias");
+            }
+            err.emit();
         }
 
         // Check that there are no gross object safety violations;
@@ -1024,9 +1032,10 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
             debug!("conv_object_ty_poly_trait_ref: observing object predicate `{:?}`", tr);
             match tr {
                 ty::Predicate::Trait(pred) => {
-                    associated_types.extend(tcx.associated_items(pred.def_id())
-                                    .filter(|item| item.kind == ty::AssociatedKind::Type)
-                                    .map(|item| item.def_id));
+                    associated_types
+                        .extend(tcx.associated_items(pred.def_id())
+                        .filter(|item| item.kind == ty::AssociatedKind::Type)
+                        .map(|item| item.def_id));
                 }
                 ty::Predicate::Projection(pred) => {
                     // A `Self` within the original bound will be substituted with a
@@ -1145,11 +1154,15 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
             })
         });
 
-        // Dedup auto traits so that `dyn Trait + Send + Send` is the same as `dyn Trait + Send`.
+        // De-duplicate auto traits so that, e.g., `dyn Trait + Send + Send` is the same as
+        // `dyn Trait + Send`.
+        let mut auto_traits: Vec<_> =
+            auto_traits.into_iter().map(|i| i.trait_ref.def_id()).collect();
         auto_traits.sort();
         auto_traits.dedup();
+        debug!("auto_traits: {:?}", auto_traits);
 
-        // Calling `skip_binder` is okay, because the predicates are re-bound.
+        // Calling `skip_binder` is okay because the predicates are re-bound.
         let principal = if tcx.trait_is_auto(existential_principal.def_id()) {
             ty::ExistentialPredicate::AutoTrait(existential_principal.def_id())
         } else {
@@ -1175,14 +1188,13 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
                 } else {
                     self.re_infer(span, None).unwrap_or_else(|| {
                         span_err!(tcx.sess, span, E0228,
-                                  "the lifetime bound for this object type cannot be deduced \
-                                   from context; please supply an explicit bound");
+                            "the lifetime bound for this object type cannot be deduced \
+                             from context; please supply an explicit bound");
                         tcx.lifetimes.re_static
                     })
                 }
             })
         };
-
         debug!("region_bound: {:?}", region_bound);
 
         let ty = tcx.mk_dynamic(existential_predicates, region_bound);
@@ -2097,33 +2109,6 @@ impl<'o, 'gcx: 'tcx, 'tcx> dyn AstConv<'gcx, 'tcx> + 'o {
     }
 }
 
-/// Divides a list of general trait bounds into two groups: auto traits (e.g., Sync and Send) and
-/// the remaining general trait bounds.
-fn split_auto_traits<'a, 'b, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
-                                         trait_bounds: &'b [hir::PolyTraitRef])
-    -> (Vec<DefId>, Vec<&'b hir::PolyTraitRef>)
-{
-    let (auto_traits, trait_bounds): (Vec<_>, _) = trait_bounds.iter().partition(|bound| {
-        // Checks whether `trait_did` is an auto trait and adds it to `auto_traits` if so.
-        match bound.trait_ref.path.res {
-            Res::Def(DefKind::Trait, trait_did) if tcx.trait_is_auto(trait_did) => {
-                true
-            }
-            _ => false
-        }
-    });
-
-    let auto_traits = auto_traits.into_iter().map(|tr| {
-        if let Res::Def(DefKind::Trait, trait_did) = tr.trait_ref.path.res {
-            trait_did
-        } else {
-            unreachable!()
-        }
-    }).collect::<Vec<_>>();
-
-    (auto_traits, trait_bounds)
-}
-
 // A helper struct for conveniently grouping a set of bounds which we pass to
 // and return from functions in multiple places.
 #[derive(PartialEq, Eq, Clone, Debug)]
diff --git a/src/librustc_typeck/check/method/probe.rs b/src/librustc_typeck/check/method/probe.rs
index 251400e65f3..590ae9d46e8 100644
--- a/src/librustc_typeck/check/method/probe.rs
+++ b/src/librustc_typeck/check/method/probe.rs
@@ -758,7 +758,7 @@ impl<'a, 'gcx, 'tcx> ProbeContext<'a, 'gcx, 'tcx> {
     }
 
     fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
-        // FIXME -- Do we want to commit to this behavior for param bounds?
+        // FIXME: do we want to commit to this behavior for param bounds?
 
         let bounds = self.param_env
             .caller_bounds
diff --git a/src/librustc_typeck/check/wfcheck.rs b/src/librustc_typeck/check/wfcheck.rs
index b009c8ea6dc..7e7a8d59266 100644
--- a/src/librustc_typeck/check/wfcheck.rs
+++ b/src/librustc_typeck/check/wfcheck.rs
@@ -1019,7 +1019,7 @@ fn check_false_global_bounds<'a, 'gcx, 'tcx>(
         .iter()
         .map(|(p, _)| *p)
         .collect();
-    // Check elaborated bounds
+    // Check elaborated bounds.
     let implied_obligations = traits::elaborate_predicates(fcx.tcx, predicates);
 
     for pred in implied_obligations {
diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index d12240655e6..e299518af0b 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -2133,10 +2133,10 @@ pub struct TraitRef {
 
 #[derive(Clone, RustcEncodable, RustcDecodable, Debug)]
 pub struct PolyTraitRef {
-    /// The `'a` in `<'a> Foo<&'a T>`
+    /// The `'a` in `<'a> Foo<&'a T>`.
     pub bound_generic_params: Vec<GenericParam>,
 
-    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`
+    /// The `Foo<&'a T>` in `<'a> Foo<&'a T>`.
     pub trait_ref: TraitRef,
 
     pub span: Span,