diff --git a/src/librustc/middle/effect.rs b/src/librustc/middle/effect.rs
index aff37264f24e..d4c566d4a604 100644
--- a/src/librustc/middle/effect.rs
+++ b/src/librustc/middle/effect.rs
@@ -56,7 +56,8 @@ impl EffectCheckVisitor {
             UnsafeBlock(block_id) => {
                 // OK, but record this.
                 debug!("effect: recording unsafe block as used: {:?}", block_id);
-                let _ = self.tcx.used_unsafe.insert(block_id);
+                let mut used_unsafe = self.tcx.used_unsafe.borrow_mut();
+                let _ = used_unsafe.get().insert(block_id);
             }
             UnsafeFn => {}
         }
diff --git a/src/librustc/middle/lint.rs b/src/librustc/middle/lint.rs
index 72140d333730..6a5ad369c5ab 100644
--- a/src/librustc/middle/lint.rs
+++ b/src/librustc/middle/lint.rs
@@ -1001,8 +1001,9 @@ fn check_unused_unsafe(cx: &Context, e: &ast::Expr) {
     match e.node {
         // Don't warn about generated blocks, that'll just pollute the output.
         ast::ExprBlock(ref blk) => {
+            let used_unsafe = cx.tcx.used_unsafe.borrow();
             if blk.rules == ast::UnsafeBlock(ast::UserProvided) &&
-                !cx.tcx.used_unsafe.contains(&blk.id) {
+                !used_unsafe.get().contains(&blk.id) {
                 cx.span_lint(unused_unsafe, blk.span,
                              "unnecessary `unsafe` block");
             }
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 9166f25ea5da..b86fa8ade778 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -348,7 +348,7 @@ struct ctxt_ {
 
     // Set of used unsafe nodes (functions or blocks). Unsafe nodes not
     // present in this set can be warned about.
-    used_unsafe: @mut HashSet<ast::NodeId>,
+    used_unsafe: RefCell<HashSet<ast::NodeId>>,
 
     // Set of nodes which mark locals as mutable which end up getting used at
     // some point. Local variable definitions not in this set can be warned
@@ -1004,7 +1004,7 @@ pub fn mk_ctxt(s: session::Session,
         trait_impls: RefCell::new(HashMap::new()),
         inherent_impls: RefCell::new(HashMap::new()),
         impls: RefCell::new(HashMap::new()),
-        used_unsafe: @mut HashSet::new(),
+        used_unsafe: RefCell::new(HashSet::new()),
         used_mut_nodes: @mut HashSet::new(),
         impl_vtables: RefCell::new(HashMap::new()),
         populated_external_types: @mut HashSet::new(),