diff --git a/src/librustc_infer/infer/outlives/verify.rs b/src/librustc_infer/infer/outlives/verify.rs
index 1a601d723cb..5020dc4132c 100644
--- a/src/librustc_infer/infer/outlives/verify.rs
+++ b/src/librustc_infer/infer/outlives/verify.rs
@@ -223,7 +223,7 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         // like `T` and `T::Item`. It may not work as well for things
         // like `<T as Foo<'a>>::Item`.
         let c_b = self.param_env.caller_bounds;
-        let param_bounds = self.collect_outlives_from_predicate_list(&compare_ty, c_b);
+        let param_bounds = self.collect_outlives_from_predicate_list(&compare_ty, c_b.into_iter());
 
         // Next, collect regions we scraped from the well-formedness
         // constraints in the fn signature. To do that, we walk the list
@@ -315,15 +315,12 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
         let tcx = self.tcx;
         let assoc_item = tcx.associated_item(assoc_item_def_id);
         let trait_def_id = assoc_item.container.assert_trait();
-        let trait_predicates =
-            tcx.predicates_of(trait_def_id).predicates.iter().map(|(p, _)| *p).collect();
+        let trait_predicates = tcx.predicates_of(trait_def_id).predicates.iter().map(|(p, _)| *p);
         let identity_substs = InternalSubsts::identity_for_item(tcx, assoc_item_def_id);
         let identity_proj = tcx.mk_projection(assoc_item_def_id, identity_substs);
         self.collect_outlives_from_predicate_list(
             move |ty| ty == identity_proj,
-            traits::elaborate_predicates(tcx, trait_predicates)
-                .map(|o| o.predicate)
-                .collect::<Vec<_>>(),
+            traits::elaborate_predicates(tcx, trait_predicates).map(|o| o.predicate),
         )
         .map(|b| b.1)
     }
@@ -337,10 +334,9 @@ impl<'cx, 'tcx> VerifyBoundCx<'cx, 'tcx> {
     fn collect_outlives_from_predicate_list(
         &self,
         compare_ty: impl Fn(Ty<'tcx>) -> bool,
-        predicates: impl IntoIterator<Item = impl AsRef<ty::Predicate<'tcx>>>,
+        predicates: impl Iterator<Item = impl AsRef<ty::Predicate<'tcx>>>,
     ) -> impl Iterator<Item = ty::OutlivesPredicate<Ty<'tcx>, ty::Region<'tcx>>> {
         predicates
-            .into_iter()
             .filter_map(|p| p.as_ref().to_opt_type_outlives())
             .filter_map(|p| p.no_bound_vars())
             .filter(move |p| compare_ty(p.0))
diff --git a/src/librustc_infer/traits/util.rs b/src/librustc_infer/traits/util.rs
index 3f63d25fb47..8aacea735f4 100644
--- a/src/librustc_infer/traits/util.rs
+++ b/src/librustc_infer/traits/util.rs
@@ -97,24 +97,22 @@ pub fn elaborate_trait_ref<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_ref: ty::PolyTraitRef<'tcx>,
 ) -> Elaborator<'tcx> {
-    elaborate_predicates(tcx, vec![trait_ref.without_const().to_predicate()])
+    elaborate_predicates(tcx, std::iter::once(trait_ref.without_const().to_predicate()))
 }
 
 pub fn elaborate_trait_refs<'tcx>(
     tcx: TyCtxt<'tcx>,
     trait_refs: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
 ) -> Elaborator<'tcx> {
-    let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate()).collect();
+    let predicates = trait_refs.map(|trait_ref| trait_ref.without_const().to_predicate());
     elaborate_predicates(tcx, predicates)
 }
 
 pub fn elaborate_predicates<'tcx>(
     tcx: TyCtxt<'tcx>,
-    mut predicates: Vec<ty::Predicate<'tcx>>,
+    predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 ) -> Elaborator<'tcx> {
-    let mut visited = PredicateSet::new(tcx);
-    predicates.retain(|pred| visited.insert(pred));
-    let obligations: Vec<_> =
+    let obligations =
         predicates.into_iter().map(|predicate| predicate_obligation(predicate, None)).collect();
     elaborate_obligations(tcx, obligations)
 }
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index 79dba2c5db8..8e983a00ada 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -122,8 +122,7 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
             .predicates_of(source.def_id())
             .predicates
             .iter()
-            .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None })
-            .collect();
+            .filter_map(|(p, _)| if p.is_global() { Some(*p) } else { None });
         if !traits::normalize_and_test_predicates(
             tcx,
             traits::elaborate_predicates(tcx, predicates).map(|o| o.predicate).collect(),
diff --git a/src/librustc_trait_selection/opaque_types.rs b/src/librustc_trait_selection/opaque_types.rs
index f67b8b87ced..7647775eff4 100644
--- a/src/librustc_trait_selection/opaque_types.rs
+++ b/src/librustc_trait_selection/opaque_types.rs
@@ -418,7 +418,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             let opaque_type = tcx.mk_opaque(def_id, opaque_defn.substs);
 
             let required_region_bounds =
-                required_region_bounds(tcx, opaque_type, bounds.predicates);
+                required_region_bounds(tcx, opaque_type, bounds.predicates.into_iter());
             debug_assert!(!required_region_bounds.is_empty());
 
             for required_region in required_region_bounds {
@@ -1127,7 +1127,8 @@ impl<'a, 'tcx> Instantiator<'a, 'tcx> {
 
         debug!("instantiate_opaque_types: bounds={:?}", bounds);
 
-        let required_region_bounds = required_region_bounds(tcx, ty, bounds.predicates.clone());
+        let required_region_bounds =
+            required_region_bounds(tcx, ty, bounds.predicates.iter().cloned());
         debug!("instantiate_opaque_types: required_region_bounds={:?}", required_region_bounds);
 
         // Make sure that we are in fact defining the *entire* type
@@ -1245,17 +1246,15 @@ pub fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: DefId, opaque_hir_id: hir
 crate fn required_region_bounds(
     tcx: TyCtxt<'tcx>,
     erased_self_ty: Ty<'tcx>,
-    predicates: Vec<ty::Predicate<'tcx>>,
+    predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 ) -> Vec<ty::Region<'tcx>> {
-    debug!(
-        "required_region_bounds(erased_self_ty={:?}, predicates={:?})",
-        erased_self_ty, predicates
-    );
+    debug!("required_region_bounds(erased_self_ty={:?})", erased_self_ty);
 
     assert!(!erased_self_ty.has_escaping_bound_vars());
 
     traits::elaborate_predicates(tcx, predicates)
         .filter_map(|obligation| {
+            debug!("required_region_bounds(obligation={:?})", obligation);
             match obligation.predicate {
                 ty::Predicate::Projection(..)
                 | ty::Predicate::Trait(..)
diff --git a/src/librustc_trait_selection/traits/auto_trait.rs b/src/librustc_trait_selection/traits/auto_trait.rs
index 052de4a4e5b..b28658e0f1e 100644
--- a/src/librustc_trait_selection/traits/auto_trait.rs
+++ b/src/librustc_trait_selection/traits/auto_trait.rs
@@ -360,8 +360,7 @@ impl AutoTraitFinder<'tcx> {
 
             computed_preds.extend(user_computed_preds.iter().cloned());
             let normalized_preds =
-                elaborate_predicates(tcx, computed_preds.iter().cloned().collect())
-                    .map(|o| o.predicate);
+                elaborate_predicates(tcx, computed_preds.iter().cloned()).map(|o| o.predicate);
             new_env =
                 ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None);
         }
diff --git a/src/librustc_trait_selection/traits/error_reporting/mod.rs b/src/librustc_trait_selection/traits/error_reporting/mod.rs
index 8a9017960fb..927f7c294f0 100644
--- a/src/librustc_trait_selection/traits/error_reporting/mod.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/mod.rs
@@ -1007,7 +1007,7 @@ impl<'a, 'tcx> InferCtxtPrivExt<'tcx> for InferCtxt<'a, 'tcx> {
             }
         };
 
-        for obligation in super::elaborate_predicates(self.tcx, vec![*cond]) {
+        for obligation in super::elaborate_predicates(self.tcx, std::iter::once(*cond)) {
             if let ty::Predicate::Trait(implication, _) = obligation.predicate {
                 let error = error.to_poly_trait_ref();
                 let implication = implication.to_poly_trait_ref();
diff --git a/src/librustc_trait_selection/traits/mod.rs b/src/librustc_trait_selection/traits/mod.rs
index f8fc155f582..2227477a463 100644
--- a/src/librustc_trait_selection/traits/mod.rs
+++ b/src/librustc_trait_selection/traits/mod.rs
@@ -297,7 +297,7 @@ pub fn normalize_param_env_or_error<'tcx>(
     );
 
     let mut predicates: Vec<_> =
-        util::elaborate_predicates(tcx, unnormalized_env.caller_bounds.to_vec())
+        util::elaborate_predicates(tcx, unnormalized_env.caller_bounds.into_iter().cloned())
             .map(|obligation| obligation.predicate)
             .collect();
 
diff --git a/src/librustc_trait_selection/traits/object_safety.rs b/src/librustc_trait_selection/traits/object_safety.rs
index d9fba1fd783..96b2b904e65 100644
--- a/src/librustc_trait_selection/traits/object_safety.rs
+++ b/src/librustc_trait_selection/traits/object_safety.rs
@@ -302,7 +302,7 @@ fn generics_require_sized_self(tcx: TyCtxt<'_>, def_id: DefId) -> bool {
     // Search for a predicate like `Self : Sized` amongst the trait bounds.
     let predicates = tcx.predicates_of(def_id);
     let predicates = predicates.instantiate_identity(tcx).predicates;
-    elaborate_predicates(tcx, predicates).any(|obligation| match obligation.predicate {
+    elaborate_predicates(tcx, predicates.into_iter()).any(|obligation| match obligation.predicate {
         ty::Predicate::Trait(ref trait_pred, _) => {
             trait_pred.def_id() == sized_def_id && trait_pred.skip_binder().self_ty().is_param(0)
         }
diff --git a/src/librustc_trait_selection/traits/project.rs b/src/librustc_trait_selection/traits/project.rs
index 4d02c5eb230..2b4a0409fd1 100644
--- a/src/librustc_trait_selection/traits/project.rs
+++ b/src/librustc_trait_selection/traits/project.rs
@@ -900,7 +900,7 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
     // If so, extract what we know from the trait and try to come up with a good answer.
     let trait_predicates = tcx.predicates_of(def_id);
     let bounds = trait_predicates.instantiate(tcx, substs);
-    let bounds = elaborate_predicates(tcx, bounds.predicates).map(|o| o.predicate);
+    let bounds = elaborate_predicates(tcx, bounds.predicates.into_iter()).map(|o| o.predicate);
     assemble_candidates_from_predicates(
         selcx,
         obligation,
@@ -911,16 +911,14 @@ fn assemble_candidates_from_trait_def<'cx, 'tcx>(
     )
 }
 
-fn assemble_candidates_from_predicates<'cx, 'tcx, I>(
+fn assemble_candidates_from_predicates<'cx, 'tcx>(
     selcx: &mut SelectionContext<'cx, 'tcx>,
     obligation: &ProjectionTyObligation<'tcx>,
     obligation_trait_ref: &ty::TraitRef<'tcx>,
     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
-    env_predicates: I,
-) where
-    I: IntoIterator<Item = ty::Predicate<'tcx>>,
-{
+    env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
+) {
     debug!("assemble_candidates_from_predicates(obligation={:?})", obligation);
     let infcx = selcx.infcx();
     for predicate in env_predicates {
@@ -1153,10 +1151,8 @@ fn confirm_object_candidate<'cx, 'tcx>(
             object_ty
         ),
     };
-    let env_predicates = data
-        .projection_bounds()
-        .map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate())
-        .collect();
+    let env_predicates =
+        data.projection_bounds().map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate());
     let env_predicate = {
         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
 
diff --git a/src/librustc_trait_selection/traits/select.rs b/src/librustc_trait_selection/traits/select.rs
index 45fda3004b0..dfbb0742448 100644
--- a/src/librustc_trait_selection/traits/select.rs
+++ b/src/librustc_trait_selection/traits/select.rs
@@ -1443,7 +1443,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             bounds
         );
 
-        let elaborated_predicates = util::elaborate_predicates(self.tcx(), bounds.predicates);
+        let elaborated_predicates =
+            util::elaborate_predicates(self.tcx(), bounds.predicates.into_iter());
         let matching_bound = elaborated_predicates.filter_to_traits().find(|bound| {
             self.infcx.probe(|_| {
                 self.match_projection(
diff --git a/src/librustc_trait_selection/traits/wf.rs b/src/librustc_trait_selection/traits/wf.rs
index f6953971ef5..cf8c12eae0a 100644
--- a/src/librustc_trait_selection/traits/wf.rs
+++ b/src/librustc_trait_selection/traits/wf.rs
@@ -627,16 +627,13 @@ pub fn object_region_bounds<'tcx>(
     // a placeholder type.
     let open_ty = tcx.mk_ty_infer(ty::FreshTy(0));
 
-    let predicates = existential_predicates
-        .iter()
-        .filter_map(|predicate| {
-            if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
-                None
-            } else {
-                Some(predicate.with_self_ty(tcx, open_ty))
-            }
-        })
-        .collect();
+    let predicates = existential_predicates.iter().filter_map(|predicate| {
+        if let ty::ExistentialPredicate::Projection(_) = *predicate.skip_binder() {
+            None
+        } else {
+            Some(predicate.with_self_ty(tcx, open_ty))
+        }
+    });
 
     required_region_bounds(tcx, open_ty, predicates)
 }
diff --git a/src/librustc_typeck/check/method/confirm.rs b/src/librustc_typeck/check/method/confirm.rs
index 3f81689cdc9..5c5f3f89aba 100644
--- a/src/librustc_typeck/check/method/confirm.rs
+++ b/src/librustc_typeck/check/method/confirm.rs
@@ -572,7 +572,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
             None => return None,
         };
 
-        traits::elaborate_predicates(self.tcx, predicates.predicates.clone())
+        traits::elaborate_predicates(self.tcx, predicates.predicates.iter().copied())
             .filter_map(|obligation| match obligation.predicate {
                 ty::Predicate::Trait(trait_pred, _) if trait_pred.def_id() == sized_def_id => {
                     let span = predicates
diff --git a/src/librustc_typeck/check/wfcheck.rs b/src/librustc_typeck/check/wfcheck.rs
index 32004744ff9..d537891c0d7 100644
--- a/src/librustc_typeck/check/wfcheck.rs
+++ b/src/librustc_typeck/check/wfcheck.rs
@@ -1225,7 +1225,7 @@ fn check_false_global_bounds(fcx: &FnCtxt<'_, '_>, span: Span, id: hir::HirId) {
     let empty_env = ty::ParamEnv::empty();
 
     let def_id = fcx.tcx.hir().local_def_id(id);
-    let predicates = fcx.tcx.predicates_of(def_id).predicates.iter().map(|(p, _)| *p).collect();
+    let predicates = fcx.tcx.predicates_of(def_id).predicates.iter().map(|(p, _)| *p);
     // Check elaborated bounds.
     let implied_obligations = traits::elaborate_predicates(fcx.tcx, predicates);
 
diff --git a/src/librustc_typeck/impl_wf_check/min_specialization.rs b/src/librustc_typeck/impl_wf_check/min_specialization.rs
index ebfb3684eb0..076c13d1065 100644
--- a/src/librustc_typeck/impl_wf_check/min_specialization.rs
+++ b/src/librustc_typeck/impl_wf_check/min_specialization.rs
@@ -322,7 +322,7 @@ fn check_predicates<'tcx>(
     // which is sound because we forbid impls like the following
     //
     // impl<D: Debug> AlwaysApplicable for D { }
-    let always_applicable_traits: Vec<_> = impl1_predicates
+    let always_applicable_traits = impl1_predicates
         .predicates
         .iter()
         .filter(|predicate| {
@@ -331,8 +331,7 @@ fn check_predicates<'tcx>(
                 Some(TraitSpecializationKind::AlwaysApplicable)
             )
         })
-        .copied()
-        .collect();
+        .copied();
 
     // Include the well-formed predicates of the type parameters of the impl.
     for ty in tcx.impl_trait_ref(impl1_def_id).unwrap().substs.types() {