Auto merge of #51895 - nikomatsakis:move-self-trait-predicate-to-items, r=scalexm

Move self trait predicate to items

This is a "reimagination" of @tmandry's PR #50183. The main effect is described in this comment from one of the commits:

---

Before we had the following results for `predicates_of`:

```rust
trait Foo { // predicates_of: Self: Foo
  fn bar(); // predicates_of: Self: Foo (inherited from trait)
}
```

Now we have removed the `Self: Foo` from the trait. However, we still
add it to the trait ITEM. This is because when people do things like
`<T as Foo>::bar()`, they still need to prove that `T: Foo`, and
having it in the `predicates_of` seems to be the cleanest way to
ensure that happens right now (otherwise, we'd need special case code
in various places):

```rust
trait Foo { // predicates_of: []
  fn bar(); // predicates_of: Self: Foo
}
```

However, we sometimes want to get the list of *just* the predicates
truly defined on a trait item (e.g., for chalk, but also for a few
other bits of code). For that, we define `predicates_defined_on`,
which does not contain the `Self: Foo` predicate yet, and we plumb
that through metadata and so forth.

---

I'm assigning @eddyb as the main reviewer, but I thought I might delegate to scalexm for this one in any case. I also want to post an alternative that I'll leave in the comments; it occurred to me as I was writing. =)

r? @eddyb
cc @scalexm @tmandry @leodasvacas

17 files changed, 205 insertions, 67 deletions

diff --git a/src/librustc/dep_graph/dep_node.rs b/src/librustc/dep_graph/dep_node.rs
index 6cc61d74800..3c4472aef6b 100644
--- a/src/librustc/dep_graph/dep_node.rs
+++ b/src/librustc/dep_graph/dep_node.rs
@@ -504,6 +504,7 @@ define_dep_nodes!( <'tcx>
     [] GenericsOfItem(DefId),
     [] PredicatesOfItem(DefId),
     [] ExplicitPredicatesOfItem(DefId),
+    [] PredicatesDefinedOnItem(DefId),
     [] InferredOutlivesOf(DefId),
     [] InferredOutlivesCrate(CrateNum),
     [] SuperPredicatesOfItem(DefId),
diff --git a/src/librustc/traits/object_safety.rs b/src/librustc/traits/object_safety.rs
index fe2965146cb..83128ba75d5 100644
--- a/src/librustc/traits/object_safety.rs
+++ b/src/librustc/traits/object_safety.rs
@@ -23,7 +23,6 @@ use hir::def_id::DefId;
 use lint;
 use traits;
 use ty::{self, Ty, TyCtxt, TypeFoldable};
-use ty::subst::Substs;
 use ty::util::ExplicitSelf;
 use std::borrow::Cow;
 use syntax::ast;
@@ -173,10 +172,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         trait_def_id: DefId,
         supertraits_only: bool) -> bool
     {
-        let trait_ref = ty::Binder::dummy(ty::TraitRef {
-            def_id: trait_def_id,
-            substs: Substs::identity_for_item(self, trait_def_id)
-        });
+        let trait_ref = ty::Binder::dummy(ty::TraitRef::identity(self, trait_def_id));
         let predicates = if supertraits_only {
             self.super_predicates_of(trait_def_id)
         } else {
@@ -391,10 +387,9 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
 
                     // Compute supertraits of current trait lazily.
                     if supertraits.is_none() {
-                        let trait_ref = ty::Binder::bind(ty::TraitRef {
-                            def_id: trait_def_id,
-                            substs: Substs::identity_for_item(self, trait_def_id)
-                        });
+                        let trait_ref = ty::Binder::bind(
+                            ty::TraitRef::identity(self, trait_def_id),
+                        );
                         supertraits = Some(traits::supertraits(self, trait_ref).collect());
                     }
 
diff --git a/src/librustc/traits/select.rs b/src/librustc/traits/select.rs
index fe30b6a2f88..40171345f55 100644
--- a/src/librustc/traits/select.rs
+++ b/src/librustc/traits/select.rs
@@ -1237,6 +1237,11 @@ impl<'cx, 'gcx, 'tcx> SelectionContext<'cx, 'gcx, 'tcx> {
         let mut candidates: Vec<EvaluatedCandidate> =
             candidates?.into_iter().filter_map(|c| c).collect();
 
+        debug!("winnowed to {} candidates for {:?}: {:?}",
+               candidates.len(),
+               stack,
+               candidates);
+
         // If there are STILL multiple candidate, we can further
         // reduce the list by dropping duplicates -- including
         // resolving specializations.
diff --git a/src/librustc/ty/mod.rs b/src/librustc/ty/mod.rs
index 54afd795fc0..98042f6389d 100644
--- a/src/librustc/ty/mod.rs
+++ b/src/librustc/ty/mod.rs
@@ -2862,8 +2862,8 @@ fn param_env<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
     // Compute the bounds on Self and the type parameters.
 
-    let bounds = tcx.predicates_of(def_id).instantiate_identity(tcx);
-    let predicates = bounds.predicates;
+    let InstantiatedPredicates { predicates } =
+        tcx.predicates_of(def_id).instantiate_identity(tcx);
 
     // Finally, we have to normalize the bounds in the environment, in
     // case they contain any associated type projections. This process
diff --git a/src/librustc/ty/query/mod.rs b/src/librustc/ty/query/mod.rs
index 77644cdf02b..9ad93b4d5e2 100644
--- a/src/librustc/ty/query/mod.rs
+++ b/src/librustc/ty/query/mod.rs
@@ -101,11 +101,39 @@ define_queries! { <'tcx>
     [] fn type_of: TypeOfItem(DefId) -> Ty<'tcx>,
 
     /// Maps from the def-id of an item (trait/struct/enum/fn) to its
-    /// associated generics and predicates.
+    /// associated generics.
     [] fn generics_of: GenericsOfItem(DefId) -> &'tcx ty::Generics,
+
+    /// Maps from the def-id of an item (trait/struct/enum/fn) to the
+    /// predicates (where clauses) that must be proven true in order
+    /// to reference it. This is almost always the "predicates query"
+    /// that you want.
+    ///
+    /// `predicates_of` builds on `predicates_defined_on` -- in fact,
+    /// it is almost always the same as that query, except for the
+    /// case of traits. For traits, `predicates_of` contains
+    /// an additional `Self: Trait<...>` predicate that users don't
+    /// actually write. This reflects the fact that to invoke the
+    /// trait (e.g., via `Default::default`) you must supply types
+    /// that actually implement the trait. (However, this extra
+    /// predicate gets in the way of some checks, which are intended
+    /// to operate over only the actual where-clauses written by the
+    /// user.)
     [] fn predicates_of: PredicatesOfItem(DefId) -> ty::GenericPredicates<'tcx>,
+
+    /// Maps from the def-id of an item (trait/struct/enum/fn) to the
+    /// predicates (where clauses) directly defined on it. This is
+    /// equal to the `explicit_predicates_of` predicates plus the
+    /// `inferred_outlives_of` predicates.
+    [] fn predicates_defined_on: PredicatesDefinedOnItem(DefId) -> ty::GenericPredicates<'tcx>,
+
+    /// Returns the predicates written explicit by the user.
     [] fn explicit_predicates_of: ExplicitPredicatesOfItem(DefId) -> ty::GenericPredicates<'tcx>,
 
+    /// Returns the inferred outlives predicates (e.g., for `struct
+    /// Foo<'a, T> { x: &'a T }`, this would return `T: 'a`).
+    [] fn inferred_outlives_of: InferredOutlivesOf(DefId) -> Lrc<Vec<ty::Predicate<'tcx>>>,
+
     /// Maps from the def-id of a trait to the list of
     /// super-predicates. This is a subset of the full list of
     /// predicates. We store these in a separate map because we must
@@ -142,9 +170,6 @@ define_queries! { <'tcx>
     [] fn variances_of: ItemVariances(DefId) -> Lrc<Vec<ty::Variance>>,
 
     /// Maps from def-id of a type to its (inferred) outlives.
-    [] fn inferred_outlives_of: InferredOutlivesOf(DefId) -> Lrc<Vec<ty::Predicate<'tcx>>>,
-
-    /// Maps from def-id of a type to its (inferred) outlives.
     [] fn inferred_outlives_crate: InferredOutlivesCrate(CrateNum)
         -> Lrc<ty::CratePredicatesMap<'tcx>>,
 
diff --git a/src/librustc/ty/query/plumbing.rs b/src/librustc/ty/query/plumbing.rs
index d783b9574ef..9f802f7fdcd 100644
--- a/src/librustc/ty/query/plumbing.rs
+++ b/src/librustc/ty/query/plumbing.rs
@@ -1074,6 +1074,7 @@ pub fn force_from_dep_node<'a, 'gcx, 'lcx>(tcx: TyCtxt<'a, 'gcx, 'lcx>,
         DepKind::TypeOfItem => { force!(type_of, def_id!()); }
         DepKind::GenericsOfItem => { force!(generics_of, def_id!()); }
         DepKind::PredicatesOfItem => { force!(predicates_of, def_id!()); }
+        DepKind::PredicatesDefinedOnItem => { force!(predicates_defined_on, def_id!()); }
         DepKind::ExplicitPredicatesOfItem => { force!(explicit_predicates_of, def_id!()); }
         DepKind::InferredOutlivesOf => { force!(inferred_outlives_of, def_id!()); }
         DepKind::InferredOutlivesCrate => { force!(inferred_outlives_crate, LOCAL_CRATE); }
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index 996ebd722fd..f93da53e043 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -614,6 +614,15 @@ impl<'tcx> TraitRef<'tcx> {
         TraitRef { def_id: def_id, substs: substs }
     }
 
+    /// Returns a TraitRef of the form `P0: Foo<P1..Pn>` where `Pi`
+    /// are the parameters defined on trait.
+    pub fn identity<'a, 'gcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>, def_id: DefId) -> TraitRef<'tcx> {
+        TraitRef {
+            def_id,
+            substs: Substs::identity_for_item(tcx, def_id),
+        }
+    }
+
     pub fn self_ty(&self) -> Ty<'tcx> {
         self.substs.type_at(0)
     }
diff --git a/src/librustc/ty/util.rs b/src/librustc/ty/util.rs
index 4e281231a41..f118d22c54d 100644
--- a/src/librustc/ty/util.rs
+++ b/src/librustc/ty/util.rs
@@ -518,10 +518,25 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         result
     }
 
+    /// True if `def_id` refers to a closure (e.g., `|x| x * 2`). Note
+    /// that closures have a def-id, but the closure *expression* also
+    /// has a `HirId` that is located within the context where the
+    /// closure appears (and, sadly, a corresponding `NodeId`, since
+    /// those are not yet phased out). The parent of the closure's
+    /// def-id will also be the context where it appears.
     pub fn is_closure(self, def_id: DefId) -> bool {
         self.def_key(def_id).disambiguated_data.data == DefPathData::ClosureExpr
     }
 
+    /// True if `def_id` refers to a trait (e.g., `trait Foo { ... }`).
+    pub fn is_trait(self, def_id: DefId) -> bool {
+        if let DefPathData::Trait(_) = self.def_key(def_id).disambiguated_data.data {
+            true
+        } else {
+            false
+        }
+    }
+
     /// True if this def-id refers to the implicit constructor for
     /// a tuple struct like `struct Foo(u32)`.
     pub fn is_struct_constructor(self, def_id: DefId) -> bool {
diff --git a/src/librustc_metadata/cstore_impl.rs b/src/librustc_metadata/cstore_impl.rs
index 23da82f5a45..20d9121668b 100644
--- a/src/librustc_metadata/cstore_impl.rs
+++ b/src/librustc_metadata/cstore_impl.rs
@@ -107,6 +107,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
         tcx.alloc_generics(cdata.get_generics(def_id.index, tcx.sess))
     }
     predicates_of => { cdata.get_predicates(def_id.index, tcx) }
+    predicates_defined_on => { cdata.get_predicates_defined_on(def_id.index, tcx) }
     super_predicates_of => { cdata.get_super_predicates(def_id.index, tcx) }
     trait_def => {
         tcx.alloc_trait_def(cdata.get_trait_def(def_id.index, tcx.sess))
diff --git a/src/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index 1985900b3e1..d604ac819b8 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -563,6 +563,13 @@ impl<'a, 'tcx> CrateMetadata {
         self.entry(item_id).predicates.unwrap().decode((self, tcx))
     }
 
+    pub fn get_predicates_defined_on(&self,
+                                   item_id: DefIndex,
+                                   tcx: TyCtxt<'a, 'tcx, 'tcx>)
+                                   -> ty::GenericPredicates<'tcx> {
+        self.entry(item_id).predicates_defined_on.unwrap().decode((self, tcx))
+    }
+
     pub fn get_super_predicates(&self,
                                 item_id: DefIndex,
                                 tcx: TyCtxt<'a, 'tcx, 'tcx>)
diff --git a/src/librustc_metadata/encoder.rs b/src/librustc_metadata/encoder.rs
index 3cfde7a8297..36f053e5aa9 100644
--- a/src/librustc_metadata/encoder.rs
+++ b/src/librustc_metadata/encoder.rs
@@ -629,6 +629,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: self.encode_optimized_mir(def_id),
         }
@@ -666,6 +667,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: None,
             predicates: None,
+            predicates_defined_on: None,
 
             mir: None
         }
@@ -706,6 +708,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: None,
         }
@@ -763,6 +766,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: self.encode_optimized_mir(def_id),
         }
@@ -780,6 +784,12 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
         self.lazy(&tcx.predicates_of(def_id))
     }
 
+    fn encode_predicates_defined_on(&mut self, def_id: DefId) -> Lazy<ty::GenericPredicates<'tcx>> {
+        debug!("IsolatedEncoder::encode_predicates_defined_on({:?})", def_id);
+        let tcx = self.tcx;
+        self.lazy(&tcx.predicates_defined_on(def_id))
+    }
+
     fn encode_info_for_trait_item(&mut self, def_id: DefId) -> Entry<'tcx> {
         debug!("IsolatedEncoder::encode_info_for_trait_item({:?})", def_id);
         let tcx = self.tcx;
@@ -869,6 +879,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: self.encode_optimized_mir(def_id),
         }
@@ -965,6 +976,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: if mir { self.encode_optimized_mir(def_id) } else { None },
         }
@@ -1228,6 +1240,16 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
                 _ => None,
             },
 
+            // The only time that `predicates_defined_on` is used (on
+            // an external item) is for traits, during chalk lowering,
+            // so only encode it in that case as an efficiency
+            // hack. (No reason not to expand it in the future if
+            // necessary.)
+            predicates_defined_on: match item.node {
+                hir::ItemTrait(..) => Some(self.encode_predicates_defined_on(def_id)),
+                _ => None, // not *wrong* for other kinds of items, but not needed
+            },
+
             mir: match item.node {
                 hir::ItemStatic(..) => {
                     self.encode_optimized_mir(def_id)
@@ -1278,6 +1300,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: None,
             predicates: None,
+            predicates_defined_on: None,
             mir: None,
         }
     }
@@ -1305,6 +1328,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: None,
             predicates: None,
+            predicates_defined_on: None,
 
             mir: None,
         }
@@ -1349,6 +1373,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: None,
+            predicates_defined_on: None,
 
             mir: self.encode_optimized_mir(def_id),
         }
@@ -1376,6 +1401,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             variances: LazySeq::empty(),
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: self.encode_optimized_mir(def_id),
         }
@@ -1577,6 +1603,7 @@ impl<'a, 'b: 'a, 'tcx: 'b> IsolatedEncoder<'a, 'b, 'tcx> {
             },
             generics: Some(self.encode_generics(def_id)),
             predicates: Some(self.encode_predicates(def_id)),
+            predicates_defined_on: None,
 
             mir: None,
         }
diff --git a/src/librustc_metadata/schema.rs b/src/librustc_metadata/schema.rs
index 21d6d15457a..a0b21e63ac5 100644
--- a/src/librustc_metadata/schema.rs
+++ b/src/librustc_metadata/schema.rs
@@ -273,6 +273,7 @@ pub struct Entry<'tcx> {
     pub variances: LazySeq<ty::Variance>,
     pub generics: Option<Lazy<ty::Generics>>,
     pub predicates: Option<Lazy<ty::GenericPredicates<'tcx>>>,
+    pub predicates_defined_on: Option<Lazy<ty::GenericPredicates<'tcx>>>,
 
     pub mir: Option<Lazy<mir::Mir<'tcx>>>,
 }
@@ -290,6 +291,7 @@ impl_stable_hash_for!(struct Entry<'tcx> {
     variances,
     generics,
     predicates,
+    predicates_defined_on,
     mir
 });
 
diff --git a/src/librustc_traits/lowering.rs b/src/librustc_traits/lowering.rs
index 214376b2e53..27daebbf8c1 100644
--- a/src/librustc_traits/lowering.rs
+++ b/src/librustc_traits/lowering.rs
@@ -15,7 +15,6 @@ use rustc::hir::{self, ImplPolarity};
 use rustc::traits::{Clause, Clauses, DomainGoal, Goal, PolyDomainGoal, ProgramClause,
                     WhereClause, FromEnv, WellFormed};
 use rustc::ty::query::Providers;
-use rustc::ty::subst::Substs;
 use rustc::ty::{self, Slice, TyCtxt};
 use rustc_data_structures::fx::FxHashSet;
 use std::mem;
@@ -225,10 +224,7 @@ fn program_clauses_for_trait<'a, 'tcx>(
 
     // `Self: Trait<P1..Pn>`
     let trait_pred = ty::TraitPredicate {
-        trait_ref: ty::TraitRef {
-            def_id,
-            substs: Substs::identity_for_item(tcx, def_id),
-        },
+        trait_ref: ty::TraitRef::identity(tcx, def_id),
     };
 
     // `Implemented(Self: Trait<P1..Pn>)`
@@ -256,10 +252,8 @@ fn program_clauses_for_trait<'a, 'tcx>(
     // ```
 
     // `FromEnv(WC) :- FromEnv(Self: Trait<P1..Pn>)`, for each where clause WC
-    // FIXME: Remove the [1..] slice; this is a hack because the query
-    // predicates_of currently includes the trait itself (`Self: Trait<P1..Pn>`).
-    let where_clauses = &tcx.predicates_of(def_id).predicates;
-    let implied_bound_clauses = where_clauses[1..]
+    let where_clauses = &tcx.predicates_defined_on(def_id).predicates;
+    let implied_bound_clauses = where_clauses
         .into_iter()
         .map(|wc| wc.lower())
 
diff --git a/src/librustc_typeck/check/wfcheck.rs b/src/librustc_typeck/check/wfcheck.rs
index be42549df74..85fdcd417ff 100644
--- a/src/librustc_typeck/check/wfcheck.rs
+++ b/src/librustc_typeck/check/wfcheck.rs
@@ -364,15 +364,16 @@ fn check_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 }
 
 /// Checks where clauses and inline bounds that are declared on def_id.
-fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
-                                    fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
-                                    span: Span,
-                                    def_id: DefId) {
+fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(
+    tcx: TyCtxt<'a, 'gcx, 'gcx>,
+    fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
+    span: Span,
+    def_id: DefId,
+) {
     use ty::subst::Subst;
     use rustc::ty::TypeFoldable;
 
-    let mut predicates = fcx.tcx.predicates_of(def_id);
-    let mut substituted_predicates = Vec::new();
+    let predicates = fcx.tcx.predicates_of(def_id);
 
     let generics = tcx.generics_of(def_id);
     let is_our_default = |def: &ty::GenericParamDef| {
@@ -433,7 +434,7 @@ fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
         }
     });
     // Now we build the substituted predicates.
-    for &pred in predicates.predicates.iter() {
+    let default_obligations = predicates.predicates.iter().flat_map(|&pred| {
         struct CountParams { params: FxHashSet<u32> }
         impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
             fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
@@ -455,21 +456,37 @@ fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
         let substituted_pred = pred.subst(fcx.tcx, substs);
         // Don't check non-defaulted params, dependent defaults (including lifetimes)
         // or preds with multiple params.
-        if substituted_pred.references_error() || param_count.params.len() > 1
-            || has_region {
-            continue;
-        }
-        // Avoid duplication of predicates that contain no parameters, for example.
-        if !predicates.predicates.contains(&substituted_pred) {
-            substituted_predicates.push(substituted_pred);
+        if {
+            substituted_pred.references_error() || param_count.params.len() > 1
+                || has_region
+        } {
+                None
+        } else if predicates.predicates.contains(&substituted_pred) {
+            // Avoid duplication of predicates that contain no parameters, for example.
+            None
+        } else {
+            Some(substituted_pred)
         }
-    }
+    }).map(|pred| {
+        // convert each of those into an obligation. So if you have
+        // something like `struct Foo<T: Copy = String>`, we would
+        // take that predicate `T: Copy`, substitute to `String: Copy`
+        // (actually that happens in the previous `flat_map` call),
+        // and then try to prove it (in this case, we'll fail).
+        //
+        // Note the subtle difference from how we handle `predicates`
+        // below: there, we are not trying to prove those predicates
+        // to be *true* but merely *well-formed*.
+        let pred = fcx.normalize_associated_types_in(span, &pred);
+        let cause = traits::ObligationCause::new(span, fcx.body_id, traits::ItemObligation(def_id));
+        traits::Obligation::new(cause, fcx.param_env, pred)
+    });
 
-    predicates.predicates.extend(substituted_predicates);
     let predicates = predicates.instantiate_identity(fcx.tcx);
     let predicates = fcx.normalize_associated_types_in(span, &predicates);
 
-    let obligations =
+    debug!("check_where_clauses: predicates={:?}", predicates.predicates);
+    let wf_obligations =
         predicates.predicates
                     .iter()
                     .flat_map(|p| ty::wf::predicate_obligations(fcx,
@@ -478,7 +495,8 @@ fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
                                                                 p,
                                                                 span));
 
-    for obligation in obligations {
+    for obligation in wf_obligations.chain(default_obligations) {
+        debug!("next obligation cause: {:?}", obligation.cause);
         fcx.register_predicate(obligation);
     }
 }
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index 4931cbfa5ac..94b17532cb7 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -67,6 +67,7 @@ pub fn provide(providers: &mut Providers) {
         type_of,
         generics_of,
         predicates_of,
+        predicates_defined_on,
         explicit_predicates_of,
         super_predicates_of,
         type_param_predicates,
@@ -274,10 +275,9 @@ fn type_param_predicates<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 ItemTrait(_, _, ref generics, ..) => {
                     // Implied `Self: Trait` and supertrait bounds.
                     if param_id == item_node_id {
-                        result.predicates.push(ty::TraitRef {
-                            def_id: item_def_id,
-                            substs: Substs::identity_for_item(tcx, item_def_id)
-                        }.to_predicate());
+                        result.predicates.push(
+                            ty::TraitRef::identity(tcx, item_def_id).to_predicate()
+                        );
                     }
                     generics
                 }
@@ -1306,10 +1306,10 @@ fn early_bound_lifetimes_from_generics<'a, 'tcx>(
     })
 }
 
-fn predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
-                           def_id: DefId)
-                           -> ty::GenericPredicates<'tcx> {
-    let explicit = explicit_predicates_of(tcx, def_id);
+fn predicates_defined_on<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+                                   def_id: DefId)
+                                   -> ty::GenericPredicates<'tcx> {
+    let explicit = tcx.explicit_predicates_of(def_id);
     let predicates = if tcx.sess.features_untracked().infer_outlives_requirements {
         [&explicit.predicates[..], &tcx.inferred_outlives_of(def_id)[..]].concat()
     } else { explicit.predicates };
@@ -1320,9 +1320,35 @@ fn predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     }
 }
 
-pub fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
+fn predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                            def_id: DefId)
                            -> ty::GenericPredicates<'tcx> {
+    let ty::GenericPredicates { parent, mut predicates } =
+        tcx.predicates_defined_on(def_id);
+
+    if tcx.is_trait(def_id) {
+        // For traits, add `Self: Trait` predicate. This is
+        // not part of the predicates that a user writes, but it
+        // is something that one must prove in order to invoke a
+        // method or project an associated type.
+        //
+        // In the chalk setup, this predicate is not part of the
+        // "predicates" for a trait item. But it is useful in
+        // rustc because if you directly (e.g.) invoke a trait
+        // method like `Trait::method(...)`, you must naturally
+        // prove that the trait applies to the types that were
+        // used, and adding the predicate into this list ensures
+        // that this is done.
+        predicates.push(ty::TraitRef::identity(tcx, def_id).to_predicate());
+    }
+
+    ty::GenericPredicates { parent, predicates }
+}
+
+fn explicit_predicates_of<'a, 'tcx>(
+    tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    def_id: DefId,
+) -> ty::GenericPredicates<'tcx> {
     use rustc::hir::map::*;
     use rustc::hir::*;
 
@@ -1337,7 +1363,10 @@ pub fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     let icx = ItemCtxt::new(tcx, def_id);
     let no_generics = hir::Generics::empty();
     let ast_generics = match node {
-        NodeTraitItem(item) => &item.generics,
+        NodeTraitItem(item) => {
+            &item.generics
+        }
+
         NodeImplItem(item) => &item.generics,
 
         NodeItem(item) => {
@@ -1355,10 +1384,7 @@ pub fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 ItemUnion(_, ref generics) => generics,
 
                 ItemTrait(_, _, ref generics, .., ref items) => {
-                    is_trait = Some((ty::TraitRef {
-                        def_id,
-                        substs: Substs::identity_for_item(tcx, def_id)
-                    }, items));
+                    is_trait = Some((ty::TraitRef::identity(tcx, def_id), items));
                     generics
                 }
                 ItemExistential(ref exist_ty) => {
@@ -1405,12 +1431,8 @@ pub fn explicit_predicates_of<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
     // and the explicit where-clauses, but to get the full set of predicates
     // on a trait we need to add in the supertrait bounds and bounds found on
     // associated types.
-    if let Some((trait_ref, _)) = is_trait {
+    if let Some((_trait_ref, _)) = is_trait {
         predicates = tcx.super_predicates_of(def_id).predicates;
-
-        // Add in a predicate that `Self:Trait` (where `Trait` is the
-        // current trait).  This is needed for builtin bounds.
-        predicates.push(trait_ref.to_poly_trait_ref().to_predicate());
     }
 
     // In default impls, we can assume that the self type implements
diff --git a/src/test/ui/chalkify/lower_env1.stderr b/src/test/ui/chalkify/lower_env1.stderr
index 3a280f85e76..812418a3053 100644
--- a/src/test/ui/chalkify/lower_env1.stderr
+++ b/src/test/ui/chalkify/lower_env1.stderr
@@ -4,7 +4,7 @@ error: program clause dump
 LL | #[rustc_dump_program_clauses] //~ ERROR program clause dump
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: FromEnv(Self: Bar) :- FromEnv(Self: Bar).
+   = note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).
    = note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).
    = note: Implemented(Self: Bar) :- FromEnv(Self: Bar).
 
@@ -14,7 +14,7 @@ error: program clause dump
 LL | #[rustc_dump_env_program_clauses] //~ ERROR program clause dump
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-   = note: FromEnv(Self: Bar) :- FromEnv(Self: Bar).
+   = note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).
    = note: FromEnv(Self: Foo) :- FromEnv(Self: Bar).
    = note: Implemented(Self: Bar) :- FromEnv(Self: Bar).
    = note: Implemented(Self: Foo) :- FromEnv(Self: Foo).
diff --git a/src/test/ui/type-check-defaults.stderr b/src/test/ui/type-check-defaults.stderr
index aa124110243..f6d7086679a 100644
--- a/src/test/ui/type-check-defaults.stderr
+++ b/src/test/ui/type-check-defaults.stderr
@@ -30,7 +30,11 @@ error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not sa
 LL | struct Bounds<T:Copy=String>(T);
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::string::String`
    |
-   = note: required by `std::marker::Copy`
+note: required by `Bounds`
+  --> $DIR/type-check-defaults.rs:21:1
+   |
+LL | struct Bounds<T:Copy=String>(T);
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
   --> $DIR/type-check-defaults.rs:24:1
@@ -38,7 +42,11 @@ error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not sa
 LL | struct WhereClause<T=String>(T) where T: Copy;
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::string::String`
    |
-   = note: required by `std::marker::Copy`
+note: required by `WhereClause`
+  --> $DIR/type-check-defaults.rs:24:1
+   |
+LL | struct WhereClause<T=String>(T) where T: Copy;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not satisfied
   --> $DIR/type-check-defaults.rs:27:1
@@ -46,7 +54,11 @@ error[E0277]: the trait bound `std::string::String: std::marker::Copy` is not sa
 LL | trait TraitBound<T:Copy=String> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `std::marker::Copy` is not implemented for `std::string::String`
    |
-   = note: required by `std::marker::Copy`
+note: required by `TraitBound`
+  --> $DIR/type-check-defaults.rs:27:1
+   |
+LL | trait TraitBound<T:Copy=String> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0277]: the trait bound `T: std::marker::Copy` is not satisfied
   --> $DIR/type-check-defaults.rs:31:1
@@ -68,7 +80,11 @@ LL | trait ProjectionPred<T:Iterator = IntoIter<i32>> where T::Item : Add<u8> {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no implementation for `i32 + u8`
    |
    = help: the trait `std::ops::Add<u8>` is not implemented for `i32`
-   = note: required by `std::ops::Add`
+note: required by `ProjectionPred`
+  --> $DIR/type-check-defaults.rs:34:1
+   |
+LL | trait ProjectionPred<T:Iterator = IntoIter<i32>> where T::Item : Add<u8> {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 7 previous errors