diff --git a/src/librustc/middle/borrowck/check_loans.rs b/src/librustc/middle/borrowck/check_loans.rs
index 5c50b8c55a9..7ed7735db45 100644
--- a/src/librustc/middle/borrowck/check_loans.rs
+++ b/src/librustc/middle/borrowck/check_loans.rs
@@ -419,7 +419,10 @@ impl<'a> CheckLoanCtxt<'a> {
                         derefs: deref_count
                     };
                     debug!("Inserting write guard at {:?}", key);
-                    this.bccx.write_guard_map.insert(key);
+                    let mut write_guard_map = this.bccx
+                                                  .write_guard_map
+                                                  .borrow_mut();
+                    write_guard_map.get().insert(key);
                 }
 
                 _ => {}
diff --git a/src/librustc/middle/borrowck/mod.rs b/src/librustc/middle/borrowck/mod.rs
index dc3dff0046e..431b7b0e2c8 100644
--- a/src/librustc/middle/borrowck/mod.rs
+++ b/src/librustc/middle/borrowck/mod.rs
@@ -82,7 +82,7 @@ pub fn check_crate(
         moved_variables_set: moved_variables_set,
         capture_map: capture_map,
         root_map: root_map(),
-        write_guard_map: @mut HashSet::new(),
+        write_guard_map: @RefCell::new(HashSet::new()),
         stats: @mut BorrowStats {
             loaned_paths_same: 0,
             loaned_paths_imm: 0,
@@ -217,7 +217,7 @@ pub struct root_map_key {
 
 // A set containing IDs of expressions of gc'd type that need to have a write
 // guard.
-pub type write_guard_map = @mut HashSet<root_map_key>;
+pub type write_guard_map = @RefCell<HashSet<root_map_key>>;
 
 pub type BckResult<T> = Result<T, BckError>;
 
diff --git a/src/librustc/middle/const_eval.rs b/src/librustc/middle/const_eval.rs
index 43bbd3881f9..1b3a3d91838 100644
--- a/src/librustc/middle/const_eval.rs
+++ b/src/librustc/middle/const_eval.rs
@@ -123,7 +123,7 @@ pub fn lookup_variant_by_id(tcx: ty::ctxt,
             root_map: @RefCell::new(HashMap::new()),
             method_map: @mut HashMap::new(),
             vtable_map: @RefCell::new(HashMap::new()),
-            write_guard_map: @mut HashSet::new(),
+            write_guard_map: @RefCell::new(HashSet::new()),
             capture_map: @RefCell::new(HashMap::new())
         };
         let e = match csearch::maybe_get_item_ast(tcx, enum_def,
@@ -173,7 +173,7 @@ pub fn lookup_const_by_id(tcx: ty::ctxt,
             root_map: @RefCell::new(HashMap::new()),
             method_map: @mut HashMap::new(),
             vtable_map: @RefCell::new(HashMap::new()),
-            write_guard_map: @mut HashSet::new(),
+            write_guard_map: @RefCell::new(HashSet::new()),
             capture_map: @RefCell::new(HashMap::new())
         };
         let e = match csearch::maybe_get_item_ast(tcx, def_id,
diff --git a/src/librustc/middle/trans/write_guard.rs b/src/librustc/middle/trans/write_guard.rs
index 922abffac9e..3b60dc3dd37 100644
--- a/src/librustc/middle/trans/write_guard.rs
+++ b/src/librustc/middle/trans/write_guard.rs
@@ -56,7 +56,8 @@ pub fn root_and_write_guard(datum: &Datum,
     // Perform the write guard, if necessary.
     //
     // (Note: write-guarded values are always boxes)
-    if ccx.maps.write_guard_map.contains(&key) {
+    let write_guard_map = ccx.maps.write_guard_map.borrow();
+    if write_guard_map.get().contains(&key) {
         perform_write_guard(datum, bcx, span)
     } else {
         bcx