diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 22974ebd711d..c7be0b7ca4ca 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -1006,7 +1006,7 @@ pub fn mk_ctxt(s: session::Session,
         impls: RefCell::new(HashMap::new()),
         used_unsafe: @mut HashSet::new(),
         used_mut_nodes: @mut HashSet::new(),
-        impl_vtables: @mut HashMap::new(),
+        impl_vtables: RefCell::new(HashMap::new()),
         populated_external_types: @mut HashSet::new(),
         populated_external_traits: @mut HashSet::new(),
 
@@ -3980,8 +3980,9 @@ pub fn lookup_item_type(cx: ctxt,
 pub fn lookup_impl_vtables(cx: ctxt,
                            did: ast::DefId)
                      -> typeck::impl_res {
+    let mut impl_vtables = cx.impl_vtables.borrow_mut();
     lookup_locally_or_in_crate_store(
-        "impl_vtables", did, cx.impl_vtables,
+        "impl_vtables", did, impl_vtables.get(),
         || csearch::get_impl_vtables(cx, did) )
 }
 
diff --git a/src/librustc/middle/typeck/check/vtable.rs b/src/librustc/middle/typeck/check/vtable.rs
index 5ddb2c78d192..bdcbf4e128a7 100644
--- a/src/librustc/middle/typeck/check/vtable.rs
+++ b/src/librustc/middle/typeck/check/vtable.rs
@@ -778,7 +778,9 @@ pub fn resolve_impl(ccx: @mut CrateCtxt,
         self_vtables: self_vtable_res
     };
     let impl_def_id = ast_util::local_def(impl_item.id);
-    ccx.tcx.impl_vtables.insert(impl_def_id, res);
+
+    let mut impl_vtables = ccx.tcx.impl_vtables.borrow_mut();
+    impl_vtables.get().insert(impl_def_id, res);
 }
 
 impl visit::Visitor<()> for @FnCtxt {
diff --git a/src/librustc/middle/typeck/mod.rs b/src/librustc/middle/typeck/mod.rs
index c606e264865b..10f9cc289ea5 100644
--- a/src/librustc/middle/typeck/mod.rs
+++ b/src/librustc/middle/typeck/mod.rs
@@ -68,6 +68,7 @@ use util::common::time;
 use util::ppaux::Repr;
 use util::ppaux;
 
+use std::cell::RefCell;
 use std::hashmap::HashMap;
 use std::result;
 use extra::list::List;
@@ -225,7 +226,7 @@ impl Repr for impl_res {
     }
 }
 
-pub type impl_vtable_map = @mut HashMap<ast::DefId, impl_res>;
+pub type impl_vtable_map = RefCell<HashMap<ast::DefId, impl_res>>;
 
 pub struct CrateCtxt {
     // A mapping from method call sites to traits that have that method.