diff --git a/src/librustc/driver/driver.rs b/src/librustc/driver/driver.rs
index 7829fad69690..4fa475e99b56 100644
--- a/src/librustc/driver/driver.rs
+++ b/src/librustc/driver/driver.rs
@@ -26,6 +26,7 @@ use middle;
 use util::common::time;
 use util::ppaux;
 
+use std::cell::RefCell;
 use std::hashmap::{HashMap,HashSet};
 use std::io;
 use std::io::fs;
@@ -872,7 +873,7 @@ pub fn build_session_(sopts: @session::options,
         filesearch: filesearch,
         building_library: @mut false,
         working_dir: os::getcwd(),
-        lints: @mut HashMap::new(),
+        lints: RefCell::new(HashMap::new()),
         node_id: @mut 1,
         outputs: @mut ~[],
     }
diff --git a/src/librustc/driver/session.rs b/src/librustc/driver/session.rs
index 86a16f972004..69e0d2e3bf0e 100644
--- a/src/librustc/driver/session.rs
+++ b/src/librustc/driver/session.rs
@@ -28,6 +28,7 @@ use syntax::abi;
 use syntax::parse::token;
 use syntax;
 
+use std::cell::RefCell;
 use std::hashmap::{HashMap,HashSet};
 
 pub struct config {
@@ -211,7 +212,8 @@ pub struct Session_ {
     filesearch: @filesearch::FileSearch,
     building_library: @mut bool,
     working_dir: Path,
-    lints: @mut HashMap<ast::NodeId, ~[(lint::lint, codemap::Span, ~str)]>,
+    lints: RefCell<HashMap<ast::NodeId,
+                           ~[(lint::lint, codemap::Span, ~str)]>>,
     node_id: @mut ast::NodeId,
     outputs: @mut ~[OutputStyle],
 }
@@ -269,11 +271,12 @@ impl Session_ {
                     id: ast::NodeId,
                     sp: Span,
                     msg: ~str) {
-        match self.lints.find_mut(&id) {
+        let mut lints = self.lints.borrow_mut();
+        match lints.get().find_mut(&id) {
             Some(arr) => { arr.push((lint, sp, msg)); return; }
             None => {}
         }
-        self.lints.insert(id, ~[(lint, sp, msg)]);
+        lints.get().insert(id, ~[(lint, sp, msg)]);
     }
     pub fn next_node_id(&self) -> ast::NodeId {
         self.reserve_node_ids(1)
diff --git a/src/librustc/middle/lint.rs b/src/librustc/middle/lint.rs
index 6a5ad369c5ab..a5eec483367e 100644
--- a/src/librustc/middle/lint.rs
+++ b/src/librustc/middle/lint.rs
@@ -1425,7 +1425,8 @@ impl<'a> Visitor<()> for Context<'a> {
 
 impl<'a> IdVisitingOperation for Context<'a> {
     fn visit_id(&self, id: ast::NodeId) {
-        match self.tcx.sess.lints.pop(&id) {
+        let mut lints = self.tcx.sess.lints.borrow_mut();
+        match lints.get().pop(&id) {
             None => {}
             Some(l) => {
                 for (lint, span, msg) in l.move_iter() {
@@ -1477,7 +1478,8 @@ pub fn check_crate(tcx: ty::ctxt,
 
     // If we missed any lints added to the session, then there's a bug somewhere
     // in the iteration code.
-    for (id, v) in tcx.sess.lints.iter() {
+    let lints = tcx.sess.lints.borrow();
+    for (id, v) in lints.get().iter() {
         for &(lint, span, ref msg) in v.iter() {
             tcx.sess.span_bug(span, format!("unprocessed lint {:?} at {}: {}",
                                             lint,