diff --git a/src/librustc/middle/typeck/check/method.rs b/src/librustc/middle/typeck/check/method.rs
index 0d0c26baf9c8..efee8c235354 100644
--- a/src/librustc/middle/typeck/check/method.rs
+++ b/src/librustc/middle/typeck/check/method.rs
@@ -142,8 +142,8 @@ pub fn lookup(
         m_name: m_name,
         supplied_tps: supplied_tps,
         impl_dups: impl_dups,
-        inherent_candidates: @mut ~[],
-        extension_candidates: @mut ~[],
+        inherent_candidates: @RefCell::new(~[]),
+        extension_candidates: @RefCell::new(~[]),
         deref_args: deref_args,
         check_traits: check_traits,
         autoderef_receiver: autoderef_receiver,
@@ -176,8 +176,8 @@ pub struct LookupContext<'a> {
     m_name: ast::Name,
     supplied_tps: &'a [ty::t],
     impl_dups: @RefCell<HashSet<DefId>>,
-    inherent_candidates: @mut ~[Candidate],
-    extension_candidates: @mut ~[Candidate],
+    inherent_candidates: @RefCell<~[Candidate]>,
+    extension_candidates: @RefCell<~[Candidate]>,
     deref_args: check::DerefArgs,
     check_traits: CheckTraitsFlag,
     autoderef_receiver: AutoderefReceiverFlag,
@@ -279,8 +279,8 @@ impl<'a> LookupContext<'a> {
     // Candidate collection (see comment at start of file)
 
     fn reset_candidates(&self) {
-        *self.inherent_candidates = ~[];
-        *self.extension_candidates = ~[];
+        self.inherent_candidates.set(~[]);
+        self.extension_candidates.set(~[]);
     }
 
     fn push_inherent_candidates(&self, self_ty: ty::t) {
@@ -358,8 +358,10 @@ impl<'a> LookupContext<'a> {
                 let opt_impl_infos = trait_impls.get().find(trait_did);
                 for impl_infos in opt_impl_infos.iter() {
                     for impl_info in impl_infos.iter() {
+                        let mut extension_candidates =
+                            self.extension_candidates.borrow_mut();
                         self.push_candidates_from_impl(
-                            self.extension_candidates, *impl_info);
+                            extension_candidates.get(), *impl_info);
 
                     }
                 }
@@ -511,7 +513,9 @@ impl<'a> LookupContext<'a> {
                                        pos, this_bound_idx);
 
                     debug!("pushing inherent candidate for param: {:?}", cand);
-                    self.inherent_candidates.push(cand);
+                    let mut inherent_candidates = self.inherent_candidates
+                                                      .borrow_mut();
+                    inherent_candidates.get().push(cand);
                 }
                 None => {
                     debug!("trait doesn't contain method: {:?}",
@@ -533,8 +537,10 @@ impl<'a> LookupContext<'a> {
         let opt_impl_infos = inherent_impls.get().find(&did);
         for impl_infos in opt_impl_infos.iter() {
             for impl_info in impl_infos.iter() {
-                self.push_candidates_from_impl(
-                    self.inherent_candidates, *impl_info);
+                let mut inherent_candidates = self.inherent_candidates
+                                                  .borrow_mut();
+                self.push_candidates_from_impl(inherent_candidates.get(),
+                                               *impl_info);
             }
         }
     }
@@ -828,7 +834,8 @@ impl<'a> LookupContext<'a> {
         // existing code.
 
         debug!("searching inherent candidates");
-        match self.consider_candidates(rcvr_ty, self.inherent_candidates) {
+        let mut inherent_candidates = self.inherent_candidates.borrow_mut();
+        match self.consider_candidates(rcvr_ty, inherent_candidates.get()) {
             None => {}
             Some(mme) => {
                 return Some(mme);
@@ -836,7 +843,8 @@ impl<'a> LookupContext<'a> {
         }
 
         debug!("searching extension candidates");
-        match self.consider_candidates(rcvr_ty, self.extension_candidates) {
+        let mut extension_candidates = self.extension_candidates.borrow_mut();
+        match self.consider_candidates(rcvr_ty, extension_candidates.get()) {
             None => {
                 return None;
             }
@@ -847,9 +855,9 @@ impl<'a> LookupContext<'a> {
     }
 
     fn consider_candidates(&self,
-                               rcvr_ty: ty::t,
-                               candidates: &mut ~[Candidate])
-                               -> Option<method_map_entry> {
+                           rcvr_ty: ty::t,
+                           candidates: &mut ~[Candidate])
+                           -> Option<method_map_entry> {
         // XXX(pcwalton): Do we need to clone here?
         let relevant_candidates: ~[Candidate] =
             candidates.iter().map(|c| (*c).clone()).