diff --git a/src/librustc/middle/kind.rs b/src/librustc/middle/kind.rs
index ba5d1dcd7fd..5886eedc48b 100644
--- a/src/librustc/middle/kind.rs
+++ b/src/librustc/middle/kind.rs
@@ -119,8 +119,13 @@ fn check_impl_of_trait(cx: &mut Context, it: @item, trait_ref: &trait_ref, self_
     let ast_trait_def = cx.tcx.def_map.find(&trait_ref.ref_id)
                             .expect("trait ref not in def map!");
     let trait_def_id = ast_util::def_id_of_def(*ast_trait_def);
-    let trait_def = cx.tcx.trait_defs.find(&trait_def_id)
-                        .expect("trait def not in trait-defs map!");
+    let trait_def;
+    {
+        let trait_defs = cx.tcx.trait_defs.borrow();
+        trait_def = *trait_defs.get()
+                               .find(&trait_def_id)
+                               .expect("trait def not in trait-defs map!");
+    }
 
     // If this trait has builtin-kind supertraits, meet them.
     let self_ty: ty::t = ty::node_id_to_type(cx.tcx, it.id);
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index c0a66ee81fd..5633be1d83c 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -298,7 +298,7 @@ struct ctxt_ {
     impl_trait_cache: RefCell<HashMap<ast::DefId, Option<@ty::TraitRef>>>,
 
     trait_refs: RefCell<HashMap<NodeId, @TraitRef>>,
-    trait_defs: @mut HashMap<DefId, @TraitDef>,
+    trait_defs: RefCell<HashMap<DefId, @TraitDef>>,
 
     /// Despite its name, `items` does not only map NodeId to an item but
     /// also to expr/stmt/local/arg/etc
@@ -987,7 +987,7 @@ pub fn mk_ctxt(s: session::Session,
         node_types: @mut HashMap::new(),
         node_type_substs: RefCell::new(HashMap::new()),
         trait_refs: RefCell::new(HashMap::new()),
-        trait_defs: @mut HashMap::new(),
+        trait_defs: RefCell::new(HashMap::new()),
         items: amap,
         intrinsic_defs: @mut HashMap::new(),
         freevars: freevars,
@@ -3963,7 +3963,8 @@ pub fn lookup_impl_vtables(cx: ctxt,
 
 /// Given the did of a trait, returns its canonical trait ref.
 pub fn lookup_trait_def(cx: ctxt, did: ast::DefId) -> @ty::TraitDef {
-    match cx.trait_defs.find(&did) {
+    let mut trait_defs = cx.trait_defs.borrow_mut();
+    match trait_defs.get().find(&did) {
         Some(&trait_def) => {
             // The item is in this crate. The caller should have added it to the
             // type cache already
@@ -3972,7 +3973,7 @@ pub fn lookup_trait_def(cx: ctxt, did: ast::DefId) -> @ty::TraitDef {
         None => {
             assert!(did.crate != ast::LOCAL_CRATE);
             let trait_def = @csearch::get_trait_def(cx, did);
-            cx.trait_defs.insert(did, trait_def);
+            trait_defs.get().insert(did, trait_def);
             return trait_def;
         }
     }
diff --git a/src/librustc/middle/typeck/collect.rs b/src/librustc/middle/typeck/collect.rs
index 2391742ea9a..8e37745e621 100644
--- a/src/librustc/middle/typeck/collect.rs
+++ b/src/librustc/middle/typeck/collect.rs
@@ -740,10 +740,14 @@ fn get_trait_def(ccx: &CrateCtxt, trait_id: ast::DefId) -> @ty::TraitDef {
 pub fn trait_def_of_item(ccx: &CrateCtxt, it: &ast::item) -> @ty::TraitDef {
     let def_id = local_def(it.id);
     let tcx = ccx.tcx;
-    match tcx.trait_defs.find(&def_id) {
-      Some(&def) => return def,
-      _ => {}
+    {
+        let trait_defs = tcx.trait_defs.borrow();
+        match trait_defs.get().find(&def_id) {
+          Some(&def) => return def,
+          _ => {}
+        }
     }
+
     match it.node {
         ast::item_trait(ref generics, ref supertraits, _) => {
             let self_ty = ty::mk_self(tcx, def_id);
@@ -755,7 +759,8 @@ pub fn trait_def_of_item(ccx: &CrateCtxt, it: &ast::item) -> @ty::TraitDef {
             let trait_def = @ty::TraitDef {generics: ty_generics,
                                            bounds: bounds,
                                            trait_ref: trait_ref};
-            tcx.trait_defs.insert(def_id, trait_def);
+            let mut trait_defs = tcx.trait_defs.borrow_mut();
+            trait_defs.get().insert(def_id, trait_def);
             return trait_def;
         }
         ref s => {