diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 7148b83ccf85..957e94ac6f88 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -321,7 +321,7 @@ struct ctxt_ {
 
     // Maps from def-id of a type or region parameter to its
     // (inferred) variance.
-    item_variance_map: @mut HashMap<ast::DefId, @ItemVariances>,
+    item_variance_map: RefCell<HashMap<ast::DefId, @ItemVariances>>,
 
     // A mapping from the def ID of an enum or struct type to the def ID
     // of the method that implements its destructor. If the type is not
@@ -975,7 +975,7 @@ pub fn mk_ctxt(s: session::Session,
             -> ctxt {
     @ctxt_ {
         named_region_map: named_region_map,
-        item_variance_map: @mut HashMap::new(),
+        item_variance_map: RefCell::new(HashMap::new()),
         diag: s.diagnostic(),
         interner: RefCell::new(HashMap::new()),
         next_id: @mut primitives::LAST_PRIMITIVE_ID,
@@ -4483,8 +4483,9 @@ pub fn visitor_object_ty(tcx: ctxt,
 }
 
 pub fn item_variances(tcx: ctxt, item_id: ast::DefId) -> @ItemVariances {
+    let mut item_variance_map = tcx.item_variance_map.borrow_mut();
     lookup_locally_or_in_crate_store(
-        "item_variance_map", item_id, tcx.item_variance_map,
+        "item_variance_map", item_id, item_variance_map.get(),
         || @csearch::get_item_variances(tcx.cstore, item_id))
 }
 
diff --git a/src/librustc/middle/typeck/variance.rs b/src/librustc/middle/typeck/variance.rs
index 219f9689a257..3ae43d606617 100644
--- a/src/librustc/middle/typeck/variance.rs
+++ b/src/librustc/middle/typeck/variance.rs
@@ -363,7 +363,10 @@ impl<'a> Visitor<()> for TermsContext<'a> {
                 // "invalid item id" from "item id with no
                 // parameters".
                 if self.num_inferred() == inferreds_on_entry {
-                    let newly_added = self.tcx.item_variance_map.insert(
+                    let mut item_variance_map = self.tcx
+                                                    .item_variance_map
+                                                    .borrow_mut();
+                    let newly_added = item_variance_map.get().insert(
                         ast_util::local_def(item.id),
                         self.empty_variances);
                     assert!(newly_added);
@@ -876,7 +879,6 @@ impl<'a> SolveContext<'a> {
         // item id).
 
         let tcx = self.terms_cx.tcx;
-        let item_variance_map = tcx.item_variance_map;
         let solutions = &self.solutions;
         let inferred_infos = &self.terms_cx.inferred_infos;
         let mut index = 0;
@@ -919,8 +921,9 @@ impl<'a> SolveContext<'a> {
                 tcx.sess.span_err(ast_map::node_span(tcx.items, item_id), found);
             }
 
-            let newly_added = item_variance_map.insert(item_def_id,
-                                                       @item_variances);
+            let mut item_variance_map = tcx.item_variance_map.borrow_mut();
+            let newly_added = item_variance_map.get().insert(item_def_id,
+                                                             @item_variances);
             assert!(newly_added);
         }
     }