diff --git a/src/librustc/dep_graph/dep_tracking_map.rs b/src/librustc/dep_graph/dep_tracking_map.rs
index 96607582203..103f8ef653c 100644
--- a/src/librustc/dep_graph/dep_tracking_map.rs
+++ b/src/librustc/dep_graph/dep_tracking_map.rs
@@ -61,15 +61,10 @@ pub fn get(&self, k: &M::Key) -> Option<&M::Value> {
         self.map.get(k)
     }
 
-    pub fn get_mut(&mut self, k: &M::Key) -> Option<&mut M::Value> {
-        self.read(k);
-        self.write(k);
-        self.map.get_mut(k)
-    }
-
-    pub fn insert(&mut self, k: M::Key, v: M::Value) -> Option<M::Value> {
+    pub fn insert(&mut self, k: M::Key, v: M::Value) {
         self.write(&k);
-        self.map.insert(k, v)
+        let _old_value = self.map.insert(k, v);
+        // assert!(old_value.is_none());
     }
 
     pub fn contains_key(&self, k: &M::Key) -> bool {
@@ -80,17 +75,6 @@ pub fn contains_key(&self, k: &M::Key) -> bool {
     pub fn keys(&self) -> Vec<M::Key> {
         self.map.keys().cloned().collect()
     }
-
-    /// Append `elem` to the vector stored for `k`, creating a new vector if needed.
-    /// This is considered a write to `k`.
-    pub fn push<E: Clone>(&mut self, k: M::Key, elem: E)
-        where M: DepTrackingMapConfig<Value=Vec<E>>
-    {
-        self.write(&k);
-        self.map.entry(k)
-                .or_insert(Vec::new())
-                .push(elem);
-    }
 }
 
 impl<M: DepTrackingMapConfig> MemoizationMap for RefCell<DepTrackingMap<M>> {
diff --git a/src/librustc_mir/mir_map.rs b/src/librustc_mir/mir_map.rs
index c71255dcc7c..b7f90682c7c 100644
--- a/src/librustc_mir/mir_map.rs
+++ b/src/librustc_mir/mir_map.rs
@@ -139,7 +139,7 @@ fn visit_nested_body(&mut self, body_id: hir::BodyId) {
 
             let mir = tcx.alloc_mir(mir);
             let def_id = tcx.hir.local_def_id(src.item_id());
-            assert!(tcx.mir_map.borrow_mut().insert(def_id, mir).is_none());
+            tcx.mir_map.borrow_mut().insert(def_id, mir);
         });
 
         let body = self.tcx.hir.body(body_id);
diff --git a/src/librustc_typeck/coherence/mod.rs b/src/librustc_typeck/coherence/mod.rs
index e9c710d2fec..4a96a5e960d 100644
--- a/src/librustc_typeck/coherence/mod.rs
+++ b/src/librustc_typeck/coherence/mod.rs
@@ -29,17 +29,19 @@
 use rustc::hir::itemlikevisit::ItemLikeVisitor;
 use rustc::hir::{Item, ItemImpl};
 use rustc::hir;
+use rustc::util::nodemap::DefIdMap;
 
 mod builtin;
 mod orphan;
 mod overlap;
 mod unsafety;
 
-struct CoherenceChecker<'a, 'tcx: 'a> {
+struct CoherenceCollect<'a, 'tcx: 'a> {
     tcx: TyCtxt<'a, 'tcx, 'tcx>,
+    inherent_impls: DefIdMap<Vec<DefId>>,
 }
 
-impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for CoherenceChecker<'a, 'tcx> {
+impl<'a, 'tcx, 'v> ItemLikeVisitor<'v> for CoherenceCollect<'a, 'tcx> {
     fn visit_item(&mut self, item: &Item) {
         if let ItemImpl(..) = item.node {
             self.check_implementation(item)
@@ -53,7 +55,7 @@ fn visit_impl_item(&mut self, _impl_item: &hir::ImplItem) {
     }
 }
 
-impl<'a, 'tcx> CoherenceChecker<'a, 'tcx> {
+impl<'a, 'tcx> CoherenceCollect<'a, 'tcx> {
     // Returns the def ID of the base type, if there is one.
     fn get_base_type_def_id(&self, span: Span, ty: Ty<'tcx>) -> Option<DefId> {
         match ty.sty {
@@ -80,9 +82,14 @@ fn check(&mut self) {
         // containing the inherent methods and extension methods. It also
         // builds up the trait inheritance table.
         self.tcx.visit_all_item_likes_in_krate(DepNode::CoherenceCheckImpl, self);
+
+        // Transfer the inherent impl lists, not that they are known, into the tcx
+        for (ty_def_id, impl_def_ids) in self.inherent_impls.drain() {
+            self.tcx.inherent_impls.borrow_mut().insert(ty_def_id, impl_def_ids);
+        }
     }
 
-    fn check_implementation(&self, item: &Item) {
+    fn check_implementation(&mut self, item: &Item) {
         let tcx = self.tcx;
         let impl_did = tcx.hir.local_def_id(item.id);
         let self_type = tcx.item_type(impl_did);
@@ -119,8 +126,10 @@ fn check_implementation(&self, item: &Item) {
         }
     }
 
-    fn add_inherent_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
-        self.tcx.inherent_impls.borrow_mut().push(base_def_id, impl_def_id);
+    fn add_inherent_impl(&mut self, base_def_id: DefId, impl_def_id: DefId) {
+        self.inherent_impls.entry(base_def_id)
+                           .or_insert(vec![])
+                           .push(impl_def_id);
     }
 
     fn add_trait_impl(&self, impl_trait_ref: ty::TraitRef<'tcx>, impl_def_id: DefId) {
@@ -161,7 +170,7 @@ fn enforce_trait_manually_implementable(tcx: TyCtxt, sp: Span, trait_def_id: Def
 
 pub fn check_coherence(ccx: &CrateCtxt) {
     let _task = ccx.tcx.dep_graph.in_task(DepNode::Coherence);
-    CoherenceChecker { tcx: ccx.tcx }.check();
+    CoherenceCollect { tcx: ccx.tcx, inherent_impls: DefIdMap() }.check();
     unsafety::check(ccx.tcx);
     orphan::check(ccx.tcx);
     overlap::check(ccx.tcx);
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index bbd0c805815..cb0a6cccb03 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -634,7 +634,7 @@ fn convert_field<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
     ccx.tcx.item_types.borrow_mut().insert(ty_f.did, tt);
 
     let def_id = ccx.tcx.hir.local_def_id(field.id);
-    ccx.tcx.item_types.borrow_mut().insert(def_id, tt);
+    assert_eq!(def_id, ty_f.did);
     ccx.tcx.generics.borrow_mut().insert(def_id, struct_generics);
     ccx.tcx.predicates.borrow_mut().insert(def_id, struct_predicates.clone());
 }
@@ -1283,9 +1283,7 @@ fn convert_trait_predicates<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>, it: &hir::Item)
                                                            items);
     trait_predicates.predicates.extend(assoc_predicates);
 
-    let prev_predicates = tcx.predicates.borrow_mut().insert(def_id, trait_predicates);
-    assert!(prev_predicates.is_none());
-
+    tcx.predicates.borrow_mut().insert(def_id, trait_predicates);
     return;
 
     fn predicates_for_associated_types<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
@@ -1592,9 +1590,7 @@ fn predicates_of_item<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
     };
 
     let predicates = ty_generic_predicates(ccx, generics, None, vec![], false);
-    let prev_predicates = ccx.tcx.predicates.borrow_mut().insert(def_id,
-                                                                 predicates.clone());
-    assert!(prev_predicates.is_none());
+    ccx.tcx.predicates.borrow_mut().insert(def_id, predicates.clone());
 
     predicates
 }
@@ -1617,8 +1613,7 @@ fn convert_foreign_item<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
     };
 
     let predicates = ty_generic_predicates(ccx, generics, None, vec![], false);
-    let prev_predicates = ccx.tcx.predicates.borrow_mut().insert(def_id, predicates);
-    assert!(prev_predicates.is_none());
+    ccx.tcx.predicates.borrow_mut().insert(def_id, predicates);
 }
 
 // Is it marked with ?Sized
diff --git a/src/librustc_typeck/variance/solve.rs b/src/librustc_typeck/variance/solve.rs
index bdf1d0590bc..3ccec97d606 100644
--- a/src/librustc_typeck/variance/solve.rs
+++ b/src/librustc_typeck/variance/solve.rs
@@ -137,11 +137,9 @@ fn write(&self) {
                           item_variances);
             }
 
-            let newly_added = tcx.item_variance_map
-                .borrow_mut()
-                .insert(item_def_id, Rc::new(item_variances))
-                .is_none();
-            assert!(newly_added);
+            tcx.item_variance_map
+               .borrow_mut()
+               .insert(item_def_id, Rc::new(item_variances));
         }
     }
 
diff --git a/src/librustc_typeck/variance/terms.rs b/src/librustc_typeck/variance/terms.rs
index 0e09d34cc9b..253d7a25b63 100644
--- a/src/librustc_typeck/variance/terms.rs
+++ b/src/librustc_typeck/variance/terms.rs
@@ -178,12 +178,10 @@ fn add_inferreds_for_item(&mut self,
         // parameters".
         if self.num_inferred() == inferreds_on_entry {
             let item_def_id = self.tcx.hir.local_def_id(item_id);
-            let newly_added = self.tcx
+            self.tcx
                 .item_variance_map
                 .borrow_mut()
-                .insert(item_def_id, self.empty_variances.clone())
-                .is_none();
-            assert!(newly_added);
+                .insert(item_def_id, self.empty_variances.clone());
         }
     }