diff --git a/src/librustc/middle/const_eval.rs b/src/librustc/middle/const_eval.rs
index 248ff1fdd16..a81964aad82 100644
--- a/src/librustc/middle/const_eval.rs
+++ b/src/librustc/middle/const_eval.rs
@@ -154,9 +154,12 @@ pub fn lookup_const_by_id(tcx: ty::ctxt,
             Some(_) => None
         }
     } else {
-        match tcx.extern_const_statics.find(&def_id) {
-            Some(&e) => return e,
-            None => {}
+        {
+            let extern_const_statics = tcx.extern_const_statics.borrow();
+            match extern_const_statics.get().find(&def_id) {
+                Some(&e) => return e,
+                None => {}
+            }
         }
         let maps = astencode::Maps {
             root_map: @mut HashMap::new(),
@@ -173,8 +176,12 @@ pub fn lookup_const_by_id(tcx: ty::ctxt,
             },
             _ => None
         };
-        tcx.extern_const_statics.insert(def_id, e);
-        return e;
+        {
+            let mut extern_const_statics = tcx.extern_const_statics
+                                              .borrow_mut();
+            extern_const_statics.get().insert(def_id, e);
+            return e;
+        }
     }
 }
 
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 8722aba1599..2acf37697d1 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -368,7 +368,7 @@ struct ctxt_ {
 
     // These two caches are used by const_eval when decoding external statics
     // and variants that are found.
-    extern_const_statics: @mut HashMap<ast::DefId, Option<@ast::Expr>>,
+    extern_const_statics: RefCell<HashMap<ast::DefId, Option<@ast::Expr>>>,
     extern_const_variants: @mut HashMap<ast::DefId, Option<@ast::Expr>>,
 }
 
@@ -1014,7 +1014,7 @@ pub fn mk_ctxt(s: session::Session,
         populated_external_types: @mut HashSet::new(),
         populated_external_traits: @mut HashSet::new(),
 
-        extern_const_statics: @mut HashMap::new(),
+        extern_const_statics: RefCell::new(HashMap::new()),
         extern_const_variants: @mut HashMap::new(),
      }
 }