diff --git a/src/librustc_middle/ty/trait_def.rs b/src/librustc_middle/ty/trait_def.rs
index 912f8be1d3342..3546e5f429ef5 100644
--- a/src/librustc_middle/ty/trait_def.rs
+++ b/src/librustc_middle/ty/trait_def.rs
@@ -168,15 +168,13 @@ impl<'tcx> TyCtxt<'tcx> {
     }
 
     /// Returns a vector containing all impls
-    pub fn all_impls(self, def_id: DefId) -> Vec<DefId> {
-        let impls = self.trait_impls_of(def_id);
+    pub fn all_impls(self, def_id: DefId) -> impl Iterator<Item = DefId> + 'tcx {
+        let TraitImpls { blanket_impls, non_blanket_impls } = self.trait_impls_of(def_id);
 
-        impls
-            .blanket_impls
-            .iter()
-            .chain(impls.non_blanket_impls.values().flatten())
+        blanket_impls
+            .into_iter()
+            .chain(non_blanket_impls.into_iter().map(|(_, v)| v).flatten())
             .cloned()
-            .collect()
     }
 }
 
diff --git a/src/librustc_trait_selection/traits/error_reporting/mod.rs b/src/librustc_trait_selection/traits/error_reporting/mod.rs
index 927f7c294f068..74cf06d54687d 100644
--- a/src/librustc_trait_selection/traits/error_reporting/mod.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/mod.rs
@@ -1208,8 +1208,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
 
         match simp {
             Some(simp) => all_impls
-                .iter()
-                .filter_map(|&def_id| {
+                .filter_map(|def_id| {
                     let imp = self.tcx.impl_trait_ref(def_id).unwrap();
                     let imp_simp = fast_reject::simplify_type(self.tcx, imp.self_ty(), true);
                     if let Some(imp_simp) = imp_simp {
@@ -1217,13 +1216,10 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
                             return None;
                         }
                     }
-
                     Some(imp)
                 })
                 .collect(),
-            None => {
-                all_impls.iter().map(|&def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect()
-            }
+            None => all_impls.map(|def_id| self.tcx.impl_trait_ref(def_id).unwrap()).collect(),
         }
     }
 
diff --git a/src/librustc_trait_selection/traits/mod.rs b/src/librustc_trait_selection/traits/mod.rs
index 2227477a4635b..c5dbe81629550 100644
--- a/src/librustc_trait_selection/traits/mod.rs
+++ b/src/librustc_trait_selection/traits/mod.rs
@@ -110,8 +110,8 @@ pub enum TraitQueryMode {
 pub fn predicates_for_generics<'tcx>(
     cause: ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
-    generic_bounds: &ty::InstantiatedPredicates<'tcx>,
-) -> PredicateObligations<'tcx> {
+    generic_bounds: ty::InstantiatedPredicates<'tcx>,
+) -> impl Iterator<Item = PredicateObligation<'tcx>> {
     util::predicates_for_generics(cause, 0, param_env, generic_bounds)
 }
 
diff --git a/src/librustc_trait_selection/traits/specialize/mod.rs b/src/librustc_trait_selection/traits/specialize/mod.rs
index fabd8c89b72af..a69c8acef5b16 100644
--- a/src/librustc_trait_selection/traits/specialize/mod.rs
+++ b/src/librustc_trait_selection/traits/specialize/mod.rs
@@ -189,26 +189,22 @@ fn fulfill_implication<'a, 'tcx>(
 
     let selcx = &mut SelectionContext::new(&infcx);
     let target_substs = infcx.fresh_substs_for_item(DUMMY_SP, target_impl);
-    let (target_trait_ref, mut obligations) =
+    let (target_trait_ref, obligations) =
         impl_trait_ref_and_oblig(selcx, param_env, target_impl, target_substs);
-    debug!(
-        "fulfill_implication: target_trait_ref={:?}, obligations={:?}",
-        target_trait_ref, obligations
-    );
 
     // do the impls unify? If not, no specialization.
-    match infcx.at(&ObligationCause::dummy(), param_env).eq(source_trait_ref, target_trait_ref) {
-        Ok(InferOk { obligations: o, .. }) => {
-            obligations.extend(o);
-        }
-        Err(_) => {
-            debug!(
-                "fulfill_implication: {:?} does not unify with {:?}",
-                source_trait_ref, target_trait_ref
-            );
-            return Err(());
-        }
-    }
+    let more_obligations =
+        match infcx.at(&ObligationCause::dummy(), param_env).eq(source_trait_ref, target_trait_ref)
+        {
+            Ok(InferOk { obligations, .. }) => obligations,
+            Err(_) => {
+                debug!(
+                    "fulfill_implication: {:?} does not unify with {:?}",
+                    source_trait_ref, target_trait_ref
+                );
+                return Err(());
+            }
+        };
 
     // attempt to prove all of the predicates for impl2 given those for impl1
     // (which are packed up in penv)
@@ -226,7 +222,7 @@ fn fulfill_implication<'a, 'tcx>(
         // we already make a mockery out of the region system, so
         // why not ignore them a bit earlier?
         let mut fulfill_cx = FulfillmentContext::new_ignoring_regions();
-        for oblig in obligations.into_iter() {
+        for oblig in obligations.chain(more_obligations) {
             fulfill_cx.register_predicate_obligation(&infcx, oblig);
         }
         match fulfill_cx.select_all_or_error(infcx) {
@@ -261,7 +257,7 @@ pub(super) fn specialization_graph_provider(
 ) -> &specialization_graph::Graph {
     let mut sg = specialization_graph::Graph::new();
 
-    let mut trait_impls = tcx.all_impls(trait_id);
+    let mut trait_impls: Vec<_> = tcx.all_impls(trait_id).collect();
 
     // The coherence checking implementation seems to rely on impls being
     // iterated over (roughly) in definition order, so we are sorting by
diff --git a/src/librustc_trait_selection/traits/util.rs b/src/librustc_trait_selection/traits/util.rs
index ffece42ec306c..4aceccf64ce65 100644
--- a/src/librustc_trait_selection/traits/util.rs
+++ b/src/librustc_trait_selection/traits/util.rs
@@ -81,12 +81,10 @@ impl<'tcx> TraitAliasExpansionInfo<'tcx> {
 
 pub fn expand_trait_aliases<'tcx>(
     tcx: TyCtxt<'tcx>,
-    trait_refs: impl IntoIterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
+    trait_refs: impl Iterator<Item = (ty::PolyTraitRef<'tcx>, Span)>,
 ) -> TraitAliasExpander<'tcx> {
-    let items: Vec<_> = trait_refs
-        .into_iter()
-        .map(|(trait_ref, span)| TraitAliasExpansionInfo::new(trait_ref, span))
-        .collect();
+    let items: Vec<_> =
+        trait_refs.map(|(trait_ref, span)| TraitAliasExpansionInfo::new(trait_ref, span)).collect();
     TraitAliasExpander { tcx, stack: items }
 }
 
@@ -199,7 +197,7 @@ pub fn impl_trait_ref_and_oblig<'a, 'tcx>(
     param_env: ty::ParamEnv<'tcx>,
     impl_def_id: DefId,
     impl_substs: SubstsRef<'tcx>,
-) -> (ty::TraitRef<'tcx>, Vec<PredicateObligation<'tcx>>) {
+) -> (ty::TraitRef<'tcx>, impl Iterator<Item = PredicateObligation<'tcx>>) {
     let impl_trait_ref = selcx.tcx().impl_trait_ref(impl_def_id).unwrap();
     let impl_trait_ref = impl_trait_ref.subst(selcx.tcx(), impl_substs);
     let Normalized { value: impl_trait_ref, obligations: normalization_obligations1 } =
@@ -210,13 +208,11 @@ pub fn impl_trait_ref_and_oblig<'a, 'tcx>(
     let Normalized { value: predicates, obligations: normalization_obligations2 } =
         super::normalize(selcx, param_env, ObligationCause::dummy(), &predicates);
     let impl_obligations =
-        predicates_for_generics(ObligationCause::dummy(), 0, param_env, &predicates);
+        predicates_for_generics(ObligationCause::dummy(), 0, param_env, predicates);
 
-    let impl_obligations: Vec<_> = impl_obligations
-        .into_iter()
-        .chain(normalization_obligations1)
-        .chain(normalization_obligations2)
-        .collect();
+    let impl_obligations = impl_obligations
+        .chain(normalization_obligations1.into_iter())
+        .chain(normalization_obligations2.into_iter());
 
     (impl_trait_ref, impl_obligations)
 }
@@ -226,20 +222,16 @@ pub fn predicates_for_generics<'tcx>(
     cause: ObligationCause<'tcx>,
     recursion_depth: usize,
     param_env: ty::ParamEnv<'tcx>,
-    generic_bounds: &ty::InstantiatedPredicates<'tcx>,
-) -> Vec<PredicateObligation<'tcx>> {
+    generic_bounds: ty::InstantiatedPredicates<'tcx>,
+) -> impl Iterator<Item = PredicateObligation<'tcx>> {
     debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
 
-    generic_bounds
-        .predicates
-        .iter()
-        .map(|&predicate| Obligation {
-            cause: cause.clone(),
-            recursion_depth,
-            param_env,
-            predicate,
-        })
-        .collect()
+    generic_bounds.predicates.into_iter().map(move |predicate| Obligation {
+        cause: cause.clone(),
+        recursion_depth,
+        param_env,
+        predicate,
+    })
 }
 
 pub fn predicate_for_trait_ref<'tcx>(
diff --git a/src/librustc_typeck/check/method/confirm.rs b/src/librustc_typeck/check/method/confirm.rs
index 5c5f3f89aba42..22c81ece39181 100644
--- a/src/librustc_typeck/check/method/confirm.rs
+++ b/src/librustc_typeck/check/method/confirm.rs
@@ -114,7 +114,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         // a custom error in that case.
         if illegal_sized_bound.is_none() {
             let method_ty = self.tcx.mk_fn_ptr(ty::Binder::bind(method_sig));
-            self.add_obligations(method_ty, all_substs, &method_predicates);
+            self.add_obligations(method_ty, all_substs, method_predicates);
         }
 
         // Create the final `MethodCallee`.
@@ -395,7 +395,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
         &mut self,
         fty: Ty<'tcx>,
         all_substs: SubstsRef<'tcx>,
-        method_predicates: &ty::InstantiatedPredicates<'tcx>,
+        method_predicates: ty::InstantiatedPredicates<'tcx>,
     ) {
         debug!(
             "add_obligations: fty={:?} all_substs={:?} method_predicates={:?}",
diff --git a/src/librustc_typeck/check/method/mod.rs b/src/librustc_typeck/check/method/mod.rs
index c4f53332cb673..65e9abcd14644 100644
--- a/src/librustc_typeck/check/method/mod.rs
+++ b/src/librustc_typeck/check/method/mod.rs
@@ -390,7 +390,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         assert!(!bounds.has_escaping_bound_vars());
 
         let cause = traits::ObligationCause::misc(span, self.body_id);
-        obligations.extend(traits::predicates_for_generics(cause.clone(), self.param_env, &bounds));
+        obligations.extend(traits::predicates_for_generics(cause.clone(), self.param_env, bounds));
 
         // Also add an obligation for the method type being well-formed.
         let method_ty = tcx.mk_fn_ptr(ty::Binder::bind(fn_sig));
diff --git a/src/librustc_typeck/check/method/probe.rs b/src/librustc_typeck/check/method/probe.rs
index 3f159fe5e3029..03e32c21a54ac 100644
--- a/src/librustc_typeck/check/method/probe.rs
+++ b/src/librustc_typeck/check/method/probe.rs
@@ -1342,7 +1342,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             // clauses) that must be considered. Make sure that those
             // match as well (or at least may match, sometimes we
             // don't have enough information to fully evaluate).
-            let candidate_obligations: Vec<_> = match probe.kind {
+            match probe.kind {
                 InherentImplCandidate(ref substs, ref ref_obligations) => {
                     // Check whether the impl imposes obligations we have to worry about.
                     let impl_def_id = probe.item.container.id();
@@ -1353,19 +1353,23 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
                     // Convert the bounds into obligations.
                     let impl_obligations =
-                        traits::predicates_for_generics(cause, self.param_env, &impl_bounds);
+                        traits::predicates_for_generics(cause, self.param_env, impl_bounds);
 
-                    debug!("impl_obligations={:?}", impl_obligations);
-                    impl_obligations
-                        .into_iter()
+                    let candidate_obligations = impl_obligations
                         .chain(norm_obligations.into_iter())
-                        .chain(ref_obligations.iter().cloned())
-                        .collect()
+                        .chain(ref_obligations.iter().cloned());
+                    // Evaluate those obligations to see if they might possibly hold.
+                    for o in candidate_obligations {
+                        let o = self.resolve_vars_if_possible(&o);
+                        if !self.predicate_may_hold(&o) {
+                            result = ProbeResult::NoMatch;
+                            possibly_unsatisfied_predicates.push((o.predicate, None));
+                        }
+                    }
                 }
 
                 ObjectCandidate | WhereClauseCandidate(..) => {
                     // These have no additional conditions to check.
-                    vec![]
                 }
 
                 TraitCandidate(trait_ref) => {
@@ -1412,17 +1416,11 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                             return ProbeResult::NoMatch;
                         }
                     }
-                    vec![]
                 }
-            };
-
-            debug!(
-                "consider_probe - candidate_obligations={:?} sub_obligations={:?}",
-                candidate_obligations, sub_obligations
-            );
+            }
 
             // Evaluate those obligations to see if they might possibly hold.
-            for o in candidate_obligations.into_iter().chain(sub_obligations) {
+            for o in sub_obligations {
                 let o = self.resolve_vars_if_possible(&o);
                 if !self.predicate_may_hold(&o) {
                     result = ProbeResult::NoMatch;
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 1be8d258dcb18..ec5d3331956e5 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -3434,7 +3434,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn add_obligations_for_parameters(
         &self,
         cause: traits::ObligationCause<'tcx>,
-        predicates: &ty::InstantiatedPredicates<'tcx>,
+        predicates: ty::InstantiatedPredicates<'tcx>,
     ) {
         assert!(!predicates.has_escaping_bound_vars());
 
@@ -4385,7 +4385,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let (bounds, _) = self.instantiate_bounds(path_span, did, substs);
             let cause =
                 traits::ObligationCause::new(path_span, self.body_id, traits::ItemObligation(did));
-            self.add_obligations_for_parameters(cause, &bounds);
+            self.add_obligations_for_parameters(cause, bounds);
 
             Some((variant, ty))
         } else {
@@ -5654,9 +5654,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         for (i, mut obligation) in traits::predicates_for_generics(
             traits::ObligationCause::new(span, self.body_id, traits::ItemObligation(def_id)),
             self.param_env,
-            &bounds,
+            bounds,
         )
-        .into_iter()
         .enumerate()
         {
             // This makes the error point at the bound, but we want to point at the argument