diff --git a/src/librustc/middle/trans/adt.rs b/src/librustc/middle/trans/adt.rs
index 9b18b7b1a31a..e06dae6654c8 100644
--- a/src/librustc/middle/trans/adt.rs
+++ b/src/librustc/middle/trans/adt.rs
@@ -118,13 +118,18 @@ pub fn represent_node(bcx: @Block, node: ast::NodeId) -> @Repr {
 /// Decides how to represent a given type.
 pub fn represent_type(cx: &mut CrateContext, t: ty::t) -> @Repr {
     debug!("Representing: {}", ty_to_str(cx.tcx, t));
-    match cx.adt_reprs.find(&t) {
-        Some(repr) => return *repr,
-        None => { }
+    {
+        let adt_reprs = cx.adt_reprs.borrow();
+        match adt_reprs.get().find(&t) {
+            Some(repr) => return *repr,
+            None => {}
+        }
     }
+
     let repr = @represent_type_uncached(cx, t);
     debug!("Represented as: {:?}", repr)
-    cx.adt_reprs.insert(t, repr);
+    let mut adt_reprs = cx.adt_reprs.borrow_mut();
+    adt_reprs.get().insert(t, repr);
     return repr;
 }
 
diff --git a/src/librustc/middle/trans/context.rs b/src/librustc/middle/trans/context.rs
index 449862c898d1..23e6e897f594 100644
--- a/src/librustc/middle/trans/context.rs
+++ b/src/librustc/middle/trans/context.rs
@@ -94,7 +94,7 @@ pub struct CrateContext {
      module_data: RefCell<HashMap<~str, ValueRef>>,
      lltypes: RefCell<HashMap<ty::t, Type>>,
      llsizingtypes: RefCell<HashMap<ty::t, Type>>,
-     adt_reprs: HashMap<ty::t, @adt::Repr>,
+     adt_reprs: RefCell<HashMap<ty::t, @adt::Repr>>,
      symbol_hasher: Sha256,
      type_hashcodes: HashMap<ty::t, @str>,
      type_short_names: HashMap<ty::t, ~str>,
@@ -205,7 +205,7 @@ impl CrateContext {
                   module_data: RefCell::new(HashMap::new()),
                   lltypes: RefCell::new(HashMap::new()),
                   llsizingtypes: RefCell::new(HashMap::new()),
-                  adt_reprs: HashMap::new(),
+                  adt_reprs: RefCell::new(HashMap::new()),
                   symbol_hasher: symbol_hasher,
                   type_hashcodes: HashMap::new(),
                   type_short_names: HashMap::new(),