4 files changed, 45 insertions, 13 deletions

diff --git a/components/script/dom/bindings/js.rs b/components/script/dom/bindings/js.rs
index c9f3e157b75..1f3cb7814e0 100644
--- a/components/script/dom/bindings/js.rs
+++ b/components/script/dom/bindings/js.rs
@@ -45,11 +45,13 @@
 //! - `OptionalSettable`: allows assigning `Option` values of `JSRef`/`Temporary` to fields of `Option<JS<T>>`
 //! - `RootedReference`: makes obtaining an `Option<JSRef<T>>` from an `Option<Root<T>>` easy
 
+use dom::bindings::trace::JSTraceable;
 use dom::bindings::utils::{Reflector, Reflectable};
 use dom::node::Node;
 use dom::xmlhttprequest::{XMLHttpRequest, TrustedXHRAddress};
 use dom::worker::{Worker, TrustedWorkerAddress};
 use js::jsapi::JSObject;
+use js::jsval::JSVal;
 use layout_interface::TrustedNodeAddress;
 use script_task::StackRoots;
 
@@ -194,6 +196,40 @@ impl<T: Reflectable> Reflectable for JS<T> {
     }
 }
 
+pub trait HeapGCValue: JSTraceable {
+}
+
+impl HeapGCValue for JSVal {
+}
+
+impl<T: Reflectable> HeapGCValue for JS<T> {
+}
+
+/// A mutable holder for a GC-owned SpiderMonkey value stored on the heap.
+/// Must be used in place of traditional interior mutability to ensure proper
+/// GC barriers are enforced.
+#[must_root]
+#[jstraceable]
+pub struct MutHeap<T: HeapGCValue+Copy> {
+    val: Cell<T>,
+}
+
+impl<T: HeapGCValue+Copy> MutHeap<T> {
+    pub fn new(initial: T) -> MutHeap<T> {
+        MutHeap {
+            val: Cell::new(initial),
+        }
+    }
+
+    pub fn set(&self, val: T) {
+        self.val.set(val)
+    }
+
+    pub fn get(&self) -> T {
+        self.val.get()
+    }
+}
+
 /// A mutable `JS<T>` value, with nullability represented by an enclosing
 /// Option wrapper. Must be used in place of traditional internal mutability
 /// to ensure that the proper GC barriers are enforced.
diff --git a/components/script/dom/customevent.rs b/components/script/dom/customevent.rs
index ee047e14c44..05390b6864f 100644
--- a/components/script/dom/customevent.rs
+++ b/components/script/dom/customevent.rs
@@ -8,19 +8,17 @@ use dom::bindings::codegen::Bindings::EventBinding::EventMethods;
 use dom::bindings::codegen::InheritTypes::{EventCast, CustomEventDerived};
 use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
-use dom::bindings::js::{JSRef, Temporary};
+use dom::bindings::js::{JSRef, Temporary, MutHeap};
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
 use dom::event::{Event, EventTypeId};
 use js::jsapi::JSContext;
 use js::jsval::{JSVal, NullValue};
 use servo_util::str::DOMString;
 
-use std::cell::Cell;
-
 #[dom_struct]
 pub struct CustomEvent {
     event: Event,
-    detail: Cell<JSVal>,
+    detail: MutHeap<JSVal>,
 }
 
 impl CustomEventDerived for Event {
@@ -33,7 +31,7 @@ impl CustomEvent {
     fn new_inherited(type_id: EventTypeId) -> CustomEvent {
         CustomEvent {
             event: Event::new_inherited(type_id),
-            detail: Cell::new(NullValue()),
+            detail: MutHeap::new(NullValue()),
         }
     }
 
diff --git a/components/script/dom/errorevent.rs b/components/script/dom/errorevent.rs
index 01e10003639..d4f048a8f47 100644
--- a/components/script/dom/errorevent.rs
+++ b/components/script/dom/errorevent.rs
@@ -8,7 +8,7 @@ use dom::bindings::codegen::Bindings::ErrorEventBinding::ErrorEventMethods;
 use dom::bindings::codegen::InheritTypes::{EventCast, ErrorEventDerived};
 use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
-use dom::bindings::js::{JSRef, Temporary};
+use dom::bindings::js::{JSRef, Temporary, MutHeap};
 use js::jsapi::JSContext;
 use dom::bindings::trace::JSTraceable;
 
@@ -27,7 +27,7 @@ pub struct ErrorEvent {
     filename: DOMRefCell<DOMString>,
     lineno: Cell<u32>,
     colno: Cell<u32>,
-    error: Cell<JSVal>
+    error: MutHeap<JSVal>,
 }
 
 impl ErrorEventDerived for Event {
@@ -44,7 +44,7 @@ impl ErrorEvent {
             filename: DOMRefCell::new("".to_string()),
             lineno: Cell::new(0),
             colno: Cell::new(0),
-            error: Cell::new(NullValue())
+            error: MutHeap::new(NullValue())
         }
     }
 
diff --git a/components/script/dom/treewalker.rs b/components/script/dom/treewalker.rs
index 1a25c7d9169..7f6d18fae4e 100644
--- a/components/script/dom/treewalker.rs
+++ b/components/script/dom/treewalker.rs
@@ -13,19 +13,17 @@ use dom::bindings::codegen::Bindings::NodeFilterBinding::NodeFilter;
 // use dom::bindings::codegen::Bindings::NodeFilterBinding::NodeFilterConstants;
 use dom::bindings::error::{ErrorResult, Fallible};
 use dom::bindings::global::GlobalRef;
-use dom::bindings::js::{JS, JSRef, OptionalRootable, Temporary};
+use dom::bindings::js::{JS, JSRef, OptionalRootable, Temporary, MutHeap};
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
 use dom::document::{Document, DocumentHelpers};
 use dom::node::{Node, NodeHelpers};
 
-use std::cell::Cell;
-
 // http://dom.spec.whatwg.org/#interface-treewalker
 #[dom_struct]
 pub struct TreeWalker {
     reflector_: Reflector,
     root_node: JS<Node>,
-    current_node: Cell<JS<Node>>,
+    current_node: MutHeap<JS<Node>>,
     what_to_show: u32,
     filter: Filter
 }
@@ -37,7 +35,7 @@ impl TreeWalker {
         TreeWalker {
             reflector_: Reflector::new(),
             root_node: JS::from_rooted(root_node),
-            current_node: Cell::new(JS::from_rooted(root_node)),
+            current_node: MutHeap::new(JS::from_rooted(root_node)),
             what_to_show: what_to_show,
             filter: filter
         }