diff --git a/src/librustc/metadata/encoder.rs b/src/librustc/metadata/encoder.rs
index c04d7b1418fd..00ec1fd9fe6e 100644
--- a/src/librustc/metadata/encoder.rs
+++ b/src/librustc/metadata/encoder.rs
@@ -1084,7 +1084,8 @@ fn encode_info_for_item(ecx: &EncodeContext,
       item_impl(_, ref opt_trait, ty, ref ast_methods) => {
         // We need to encode information about the default methods we
         // have inherited, so we drive this based on the impl structure.
-        let imp = tcx.impls.get(&def_id);
+        let impls = tcx.impls.borrow();
+        let imp = impls.get().get(&def_id);
 
         add_to_index();
         ebml_w.start_tag(tag_items_data_item);
diff --git a/src/librustc/middle/trans/meth.rs b/src/librustc/middle/trans/meth.rs
index 2fdc444920a7..6b7963a226cf 100644
--- a/src/librustc/middle/trans/meth.rs
+++ b/src/librustc/middle/trans/meth.rs
@@ -305,7 +305,8 @@ pub fn method_with_name(ccx: &CrateContext,
         }
     }
 
-    let imp = ccx.tcx.impls.find(&impl_id)
+    let impls = ccx.tcx.impls.borrow();
+    let imp = impls.get().find(&impl_id)
         .expect("could not find impl while translating");
     let meth = imp.methods.iter().find(|m| m.ident.name == name)
         .expect("could not find method while translating");
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 976a4f3af68c..8722aba15997 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -344,7 +344,7 @@ struct ctxt_ {
     // Note that this contains all of the impls that we know about,
     // including ones in other crates. It's not clear that this is the best
     // way to do it.
-    impls: @mut HashMap<ast::DefId, @Impl>,
+    impls: RefCell<HashMap<ast::DefId, @Impl>>,
 
     // Set of used unsafe nodes (functions or blocks). Unsafe nodes not
     // present in this set can be warned about.
@@ -1007,7 +1007,7 @@ pub fn mk_ctxt(s: session::Session,
         destructors: RefCell::new(HashSet::new()),
         trait_impls: RefCell::new(HashMap::new()),
         inherent_impls: RefCell::new(HashMap::new()),
-        impls:  @mut HashMap::new(),
+        impls: RefCell::new(HashMap::new()),
         used_unsafe: @mut HashSet::new(),
         used_mut_nodes: @mut HashSet::new(),
         impl_vtables: @mut HashMap::new(),
@@ -4563,7 +4563,8 @@ pub fn populate_implementations_for_type_if_necessary(tcx: ctxt,
         }
 
         // Store the implementation info.
-        tcx.impls.insert(implementation_def_id, implementation);
+        let mut impls = tcx.impls.borrow_mut();
+        impls.get().insert(implementation_def_id, implementation);
     });
 
     tcx.populated_external_types.insert(type_id);
@@ -4599,7 +4600,8 @@ pub fn populate_implementations_for_trait_if_necessary(
         }
 
         // Store the implementation info.
-        tcx.impls.insert(implementation_def_id, implementation);
+        let mut impls = tcx.impls.borrow_mut();
+        impls.get().insert(implementation_def_id, implementation);
     });
 
     tcx.populated_external_traits.insert(trait_id);
diff --git a/src/librustc/middle/typeck/coherence.rs b/src/librustc/middle/typeck/coherence.rs
index f4ffd03cf641..1fcb8d80c7bd 100644
--- a/src/librustc/middle/typeck/coherence.rs
+++ b/src/librustc/middle/typeck/coherence.rs
@@ -313,7 +313,8 @@ impl CoherenceChecker {
             }
         }
 
-        tcx.impls.insert(implementation.did, implementation);
+        let mut impls = tcx.impls.borrow_mut();
+        impls.get().insert(implementation.did, implementation);
     }
 
     // Creates default method IDs and performs type substitutions for an impl
@@ -665,7 +666,8 @@ impl CoherenceChecker {
             }
         }
 
-        tcx.impls.insert(implementation.did, implementation);
+        let mut impls = tcx.impls.borrow_mut();
+        impls.get().insert(implementation.did, implementation);
     }
 
     // Adds implementations and traits from external crates to the coherence