Implement MutNullableJS for mutable, nullable member pointers to DOM objects.

11 files changed, 171 insertions, 109 deletions

diff --git a/components/script/dom/bindings/js.rs b/components/script/dom/bindings/js.rs
index a0521b17535..14f717f7455 100644
--- a/components/script/dom/bindings/js.rs
+++ b/components/script/dom/bindings/js.rs
@@ -54,6 +54,7 @@ use layout_interface::TrustedNodeAddress;
 use script_task::StackRoots;
 
 use std::cell::{Cell, RefCell};
+use std::default::Default;
 use std::kinds::marker::ContravariantLifetime;
 use std::mem;
 
@@ -192,6 +193,62 @@ impl<T: Reflectable> Reflectable for JS<T> {
     }
 }
 
+/// 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.
+#[must_root]
+#[jstraceable]
+pub struct MutNullableJS<T: Reflectable> {
+    ptr: Cell<Option<JS<T>>>
+}
+
+impl<T: Assignable<U>, U: Reflectable> MutNullableJS<U> {
+    pub fn new(initial: Option<T>) -> MutNullableJS<U> {
+        MutNullableJS {
+            ptr: Cell::new(initial.map(|initial| {
+                unsafe { initial.get_js() }
+            }))
+        }
+    }
+}
+
+impl<T: Reflectable> Default for MutNullableJS<T> {
+    fn default() -> MutNullableJS<T> {
+        MutNullableJS {
+            ptr: Cell::new(None)
+        }
+    }
+}
+
+impl<T: Reflectable> MutNullableJS<T> {
+    /// Store an unrooted value in this field. This is safe under the
+    /// assumption that `MutNullableJS<T>` values are only used as fields in
+    /// DOM types that are reachable in the GC graph, so this unrooted value
+    /// becomes transitively rooted for the lifetime of its new owner.
+    pub fn assign<U: Assignable<T>>(&self, val: Option<U>) {
+        self.ptr.set(val.map(|val| {
+            unsafe { val.get_js() }
+        }));
+    }
+
+    /// Set the inner value to null, without making API users jump through
+    /// useless type-ascription hoops.
+    pub fn clear(&self) {
+        self.assign(None::<JS<T>>);
+    }
+
+    /// Retrieve a copy of the current optional inner value.
+    pub fn get(&self) -> Option<Temporary<T>> {
+        self.ptr.get().map(Temporary::new)
+    }
+
+    /// Retrieve a copy of the inner optional `JS<T>`. For use by layout, which
+    /// can't use safe types like Temporary.
+    pub unsafe fn get_inner(&self) -> Option<JS<T>> {
+        self.ptr.get()
+    }
+}
+
 impl<T: Reflectable> JS<T> {
     /// Returns an unsafe pointer to the interior of this JS object without touching the borrow
     /// flags. This is the only method that be safely accessed from layout. (The fact that this
@@ -245,7 +302,7 @@ impl<'a, 'b, T: Reflectable> OptionalRootedReference<T> for Option<Option<Root<'
 /// Trait that allows extracting a `JS<T>` value from a variety of rooting-related containers,
 /// which in general is an unsafe operation since they can outlive the rooted lifetime of the
 /// original value.
-/*definitely not public*/ trait Assignable<T> {
+pub trait Assignable<T> {
     unsafe fn get_js(&self) -> JS<T>;
 }
 
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index b3261f6df39..cccb2658d40 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -21,7 +21,7 @@ use dom::bindings::error::{ErrorResult, Fallible, NotSupported, InvalidCharacter
 use dom::bindings::error::{HierarchyRequest, NamespaceError};
 use dom::bindings::global::GlobalRef;
 use dom::bindings::global;
-use dom::bindings::js::{JS, JSRef, Temporary, OptionalSettable, TemporaryPushable};
+use dom::bindings::js::{MutNullableJS, JS, JSRef, Temporary, OptionalSettable, TemporaryPushable};
 use dom::bindings::js::OptionalRootable;
 use dom::bindings::trace::{Traceable, Untraceable};
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
@@ -57,12 +57,14 @@ use hubbub::hubbub::{QuirksMode, NoQuirks, LimitedQuirks, FullQuirks};
 use layout_interface::{DocumentDamageLevel, ContentChangedDocumentDamage};
 use servo_util::namespace;
 use servo_util::str::{DOMString, split_html_space_chars};
+
 use string_cache::Atom;
+use url::Url;
 
 use std::collections::hashmap::HashMap;
 use std::ascii::StrAsciiExt;
 use std::cell::{Cell, RefCell};
-use url::Url;
+use std::default::Default;
 use time;
 
 #[deriving(PartialEq)]
@@ -79,7 +81,7 @@ pub struct Document {
     reflector_: Reflector,
     pub window: JS<Window>,
     idmap: Traceable<RefCell<HashMap<Atom, Vec<JS<Element>>>>>,
-    implementation: Cell<Option<JS<DOMImplementation>>>,
+    implementation: MutNullableJS<DOMImplementation>,
     content_type: DOMString,
     last_modified: Traceable<RefCell<Option<DOMString>>>,
     pub encoding_name: Traceable<RefCell<DOMString>>,
@@ -288,7 +290,7 @@ impl Document {
             reflector_: Reflector::new(),
             window: JS::from_rooted(window),
             idmap: Traceable::new(RefCell::new(HashMap::new())),
-            implementation: Cell::new(None),
+            implementation: Default::default(),
             content_type: match content_type {
                 Some(string) => string.clone(),
                 None => match is_html_document {
@@ -382,7 +384,7 @@ impl<'a> DocumentMethods for JSRef<'a, Document> {
         if self.implementation.get().is_none() {
             self.implementation.assign(Some(DOMImplementation::new(self)));
         }
-        Temporary::new(self.implementation.get().as_ref().unwrap().clone())
+        self.implementation.get().unwrap()
     }
 
     // http://dom.spec.whatwg.org/#dom-document-url
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 2a3f53751f2..324cb02b2a7 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -12,7 +12,7 @@ use dom::bindings::codegen::Bindings::ElementBinding;
 use dom::bindings::codegen::Bindings::ElementBinding::ElementMethods;
 use dom::bindings::codegen::Bindings::NamedNodeMapBinding::NamedNodeMapMethods;
 use dom::bindings::codegen::InheritTypes::{ElementDerived, NodeCast};
-use dom::bindings::js::{JS, JSRef, Temporary, TemporaryPushable};
+use dom::bindings::js::{MutNullableJS, JS, JSRef, Temporary, TemporaryPushable};
 use dom::bindings::js::{OptionalSettable, OptionalRootable, Root};
 use dom::bindings::trace::Traceable;
 use dom::bindings::utils::{Reflectable, Reflector};
@@ -38,7 +38,8 @@ use servo_util::namespace;
 use servo_util::str::DOMString;
 
 use std::ascii::StrAsciiExt;
-use std::cell::{Cell, RefCell};
+use std::cell::RefCell;
+use std::default::Default;
 use std::mem;
 use string_cache::{Atom, Namespace};
 
@@ -51,8 +52,8 @@ pub struct Element {
     pub prefix: Option<DOMString>,
     pub attrs: RefCell<Vec<JS<Attr>>>,
     pub style_attribute: Traceable<RefCell<Option<style::PropertyDeclarationBlock>>>,
-    pub attr_list: Cell<Option<JS<NamedNodeMap>>>,
-    class_list: Cell<Option<JS<DOMTokenList>>>,
+    pub attr_list: MutNullableJS<NamedNodeMap>,
+    class_list: MutNullableJS<DOMTokenList>,
 }
 
 impl ElementDerived for EventTarget {
@@ -156,8 +157,8 @@ impl Element {
             namespace: namespace,
             prefix: prefix,
             attrs: RefCell::new(vec!()),
-            attr_list: Cell::new(None),
-            class_list: Cell::new(None),
+            attr_list: Default::default(),
+            class_list: Default::default(),
             style_attribute: Traceable::new(RefCell::new(None)),
         }
     }
@@ -557,31 +558,25 @@ impl<'a> ElementMethods for JSRef<'a, Element> {
 
     // http://dom.spec.whatwg.org/#dom-element-classlist
     fn ClassList(self) -> Temporary<DOMTokenList> {
-        match self.class_list.get() {
-            Some(class_list) => Temporary::new(class_list),
-            None => {
-                let class_list = DOMTokenList::new(self, "class").root();
-                self.class_list.assign(Some(class_list.deref().clone()));
-                Temporary::from_rooted(*class_list)
-            }
+        if self.class_list.get().is_none() {
+            let class_list = DOMTokenList::new(self, "class");
+            self.class_list.assign(Some(class_list));
         }
+        self.class_list.get().unwrap()
     }
 
     // http://dom.spec.whatwg.org/#dom-element-attributes
     fn Attributes(self) -> Temporary<NamedNodeMap> {
-        match self.attr_list.get() {
-            None => (),
-            Some(ref list) => return Temporary::new(list.clone()),
+        if self.attr_list.get().is_none() {
+            let doc = {
+                let node: JSRef<Node> = NodeCast::from_ref(self);
+                node.owner_doc().root()
+            };
+            let window = doc.deref().window.root();
+            let list = NamedNodeMap::new(*window, self);
+            self.attr_list.assign(Some(list));
         }
-
-        let doc = {
-            let node: JSRef<Node> = NodeCast::from_ref(self);
-            node.owner_doc().root()
-        };
-        let window = doc.deref().window.root();
-        let list = NamedNodeMap::new(*window, self);
-        self.attr_list.assign(Some(list));
-        Temporary::new(self.attr_list.get().as_ref().unwrap().clone())
+        self.attr_list.get().unwrap()
     }
 
     // http://dom.spec.whatwg.org/#dom-element-getattribute
diff --git a/components/script/dom/event.rs b/components/script/dom/event.rs
index 69caf3f137c..858ac31becd 100644
--- a/components/script/dom/event.rs
+++ b/components/script/dom/event.rs
@@ -6,12 +6,13 @@ use dom::bindings::codegen::Bindings::EventBinding;
 use dom::bindings::codegen::Bindings::EventBinding::{EventConstants, EventMethods};
 use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
-use dom::bindings::js::{JS, JSRef, Temporary};
+use dom::bindings::js::{MutNullableJS, JSRef, Temporary};
 use dom::bindings::trace::Traceable;
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
 use dom::eventtarget::EventTarget;
 use servo_util::str::DOMString;
 use std::cell::{Cell, RefCell};
+use std::default::Default;
 
 use time;
 
@@ -40,8 +41,8 @@ pub enum EventTypeId {
 pub struct Event {
     pub type_id: EventTypeId,
     reflector_: Reflector,
-    pub current_target: Cell<Option<JS<EventTarget>>>,
-    pub target: Cell<Option<JS<EventTarget>>>,
+    pub current_target: MutNullableJS<EventTarget>,
+    pub target: MutNullableJS<EventTarget>,
     type_: Traceable<RefCell<DOMString>>,
     pub phase: Traceable<Cell<EventPhase>>,
     pub canceled: Traceable<Cell<bool>>,
@@ -60,8 +61,8 @@ impl Event {
         Event {
             type_id: type_id,
             reflector_: Reflector::new(),
-            current_target: Cell::new(None),
-            target: Cell::new(None),
+            current_target: Default::default(),
+            target: Default::default(),
             phase: Traceable::new(Cell::new(PhaseNone)),
             type_: Traceable::new(RefCell::new("".to_string())),
             canceled: Traceable::new(Cell::new(false)),
@@ -108,11 +109,11 @@ impl<'a> EventMethods for JSRef<'a, Event> {
     }
 
     fn GetTarget(self) -> Option<Temporary<EventTarget>> {
-        self.target.get().as_ref().map(|target| Temporary::new(target.clone()))
+        self.target.get()
     }
 
     fn GetCurrentTarget(self) -> Option<Temporary<EventTarget>> {
-        self.current_target.get().as_ref().map(|target| Temporary::new(target.clone()))
+        self.current_target.get()
     }
 
     fn DefaultPrevented(self) -> bool {
@@ -158,7 +159,7 @@ impl<'a> EventMethods for JSRef<'a, Event> {
         self.stop_immediate.deref().set(false);
         self.canceled.deref().set(false);
         self.trusted.deref().set(false);
-        self.target.set(None);
+        self.target.clear();
         *self.type_.deref().borrow_mut() = type_;
         self.bubbles.deref().set(bubbles);
         self.cancelable.deref().set(cancelable);
diff --git a/components/script/dom/eventdispatcher.rs b/components/script/dom/eventdispatcher.rs
index 0fa26d60715..4c3bcc16fcb 100644
--- a/components/script/dom/eventdispatcher.rs
+++ b/components/script/dom/eventdispatcher.rs
@@ -133,7 +133,7 @@ pub fn dispatch_event<'a, 'b>(target: JSRef<'a, EventTarget>,
 
     event.dispatching.deref().set(false);
     event.phase.deref().set(PhaseNone);
-    event.current_target.set(None);
+    event.current_target.clear();
 
     !event.DefaultPrevented()
 }
diff --git a/components/script/dom/mouseevent.rs b/components/script/dom/mouseevent.rs
index 9efc5a97d3e..2332417da18 100644
--- a/components/script/dom/mouseevent.rs
+++ b/components/script/dom/mouseevent.rs
@@ -9,7 +9,7 @@ use dom::bindings::codegen::InheritTypes::{UIEventCast, MouseEventDerived};
 use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
 use dom::bindings::global;
-use dom::bindings::js::{JS, JSRef, RootedReference, Temporary, OptionalSettable};
+use dom::bindings::js::{MutNullableJS, JSRef, RootedReference, Temporary, OptionalSettable};
 use dom::bindings::trace::Traceable;
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
 use dom::event::{Event, MouseEventTypeId};
@@ -18,6 +18,7 @@ use dom::uievent::UIEvent;
 use dom::window::Window;
 use servo_util::str::DOMString;
 use std::cell::Cell;
+use std::default::Default;
 
 #[jstraceable]
 #[must_root]
@@ -32,7 +33,7 @@ pub struct MouseEvent {
     pub alt_key: Traceable<Cell<bool>>,
     pub meta_key: Traceable<Cell<bool>>,
     pub button: Traceable<Cell<i16>>,
-    pub related_target: Cell<Option<JS<EventTarget>>>
+    pub related_target: MutNullableJS<EventTarget>
 }
 
 impl MouseEventDerived for Event {
@@ -54,7 +55,7 @@ impl MouseEvent {
             alt_key: Traceable::new(Cell::new(false)),
             meta_key: Traceable::new(Cell::new(false)),
             button: Traceable::new(Cell::new(0)),
-            related_target: Cell::new(None)
+            related_target: Default::default(),
         }
     }
 
@@ -142,7 +143,7 @@ impl<'a> MouseEventMethods for JSRef<'a, MouseEvent> {
     }
 
     fn GetRelatedTarget(self) -> Option<Temporary<EventTarget>> {
-        self.related_target.get().clone().map(|target| Temporary::new(target))
+        self.related_target.get()
     }
 
     fn InitMouseEvent(self,
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 7e9ce5ec907..632335d14cc 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -22,9 +22,9 @@ use dom::bindings::codegen::InheritTypes::HTMLOptGroupElementDerived;
 use dom::bindings::error::{Fallible, NotFound, HierarchyRequest, Syntax};
 use dom::bindings::global::GlobalRef;
 use dom::bindings::global;
-use dom::bindings::js::{JS, JSRef, RootedReference, Temporary, Root, OptionalUnrootable};
+use dom::bindings::js::{JS, JSRef, RootedReference, Temporary, Root};
 use dom::bindings::js::{OptionalSettable, TemporaryPushable, OptionalRootedRootable};
-use dom::bindings::js::{ResultRootable, OptionalRootable};
+use dom::bindings::js::{ResultRootable, OptionalRootable, MutNullableJS};
 use dom::bindings::trace::{Traceable, Untraceable};
 use dom::bindings::utils;
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
@@ -57,7 +57,8 @@ use js::jsapi::{JSContext, JSObject, JSRuntime};
 use js::jsfriendapi;
 use libc;
 use libc::uintptr_t;
-use std::cell::{Cell, RefCell, Ref, RefMut};
+use std::cell::{RefCell, Ref, RefMut};
+use std::default::Default;
 use std::iter::{Map, Filter};
 use std::mem;
 use style;
@@ -80,25 +81,25 @@ pub struct Node {
     type_id: NodeTypeId,
 
     /// The parent of this node.
-    parent_node: Cell<Option<JS<Node>>>,
+    parent_node: MutNullableJS<Node>,
 
     /// The first child of this node.
-    first_child: Cell<Option<JS<Node>>>,
+    first_child: MutNullableJS<Node>,
 
     /// The last child of this node.
-    last_child: Cell<Option<JS<Node>>>,
+    last_child: MutNullableJS<Node>,
 
     /// The next sibling of this node.
-    next_sibling: Cell<Option<JS<Node>>>,
+    next_sibling: MutNullableJS<Node>,
 
     /// The previous sibling of this node.
-    prev_sibling: Cell<Option<JS<Node>>>,
+    prev_sibling: MutNullableJS<Node>,
 
     /// The document that this node belongs to.
-    owner_doc: Cell<Option<JS<Document>>>,
+    owner_doc: MutNullableJS<Document>,
 
     /// The live list of children return by .childNodes.
-    child_list: Cell<Option<JS<NodeList>>>,
+    child_list: MutNullableJS<NodeList>,
 
     /// A bitfield of flags for node items.
     flags: Traceable<RefCell<NodeFlags>>,
@@ -358,9 +359,9 @@ impl<'a> PrivateNodeHelpers for JSRef<'a, Node> {
             }
         }
 
-        child.prev_sibling.set(None);
-        child.next_sibling.set(None);
-        child.parent_node.set(None);
+        child.prev_sibling.clear();
+        child.next_sibling.clear();
+        child.parent_node.clear();
     }
 }
 
@@ -461,25 +462,25 @@ impl<'m, 'n> NodeHelpers<'m, 'n> for JSRef<'n, Node> {
     }
 
     fn parent_node(self) -> Option<Temporary<Node>> {
-        self.deref().parent_node.get().map(|node| Temporary::new(node))
+        self.deref().parent_node.get()
     }
 
     fn first_child(self) -> Option<Temporary<Node>> {
-        self.deref().first_child.get().map(|node| Temporary::new(node))
+        self.deref().first_child.get()
     }
 
     fn last_child(self) -> Option<Temporary<Node>> {
-        self.deref().last_child.get().map(|node| Temporary::new(node))
+        self.deref().last_child.get()
     }
 
     /// Returns the previous sibling of this node. Fails if this node is borrowed mutably.
     fn prev_sibling(self) -> Option<Temporary<Node>> {
-        self.deref().prev_sibling.get().map(|node| Temporary::new(node))
+        self.deref().prev_sibling.get()
     }
 
     /// Returns the next sibling of this node. Fails if this node is borrowed mutably.
     fn next_sibling(self) -> Option<Temporary<Node>> {
-        self.deref().next_sibling.get().map(|node| Temporary::new(node))
+        self.deref().next_sibling.get()
     }
 
     #[inline]
@@ -578,7 +579,7 @@ impl<'m, 'n> NodeHelpers<'m, 'n> for JSRef<'n, Node> {
 
     fn is_parent_of(self, child: JSRef<Node>) -> bool {
         match child.parent_node() {
-            Some(parent) if parent == Temporary::from_rooted(self) => true,
+            Some(ref parent) if parent == &Temporary::from_rooted(self) => true,
             _ => false
         }
     }
@@ -651,7 +652,7 @@ impl<'m, 'n> NodeHelpers<'m, 'n> for JSRef<'n, Node> {
     }
 
     fn owner_doc(self) -> Temporary<Document> {
-        Temporary::new(self.owner_doc.get().as_ref().unwrap().clone())
+        self.owner_doc.get().unwrap()
     }
 
     fn set_owner_doc(self, document: JSRef<Document>) {
@@ -777,32 +778,32 @@ impl LayoutNodeHelpers for JS<Node> {
 
     #[inline]
     unsafe fn parent_node_ref(&self) -> Option<JS<Node>> {
-        (*self.unsafe_get()).parent_node.get()
+        (*self.unsafe_get()).parent_node.get_inner()
     }
 
     #[inline]
     unsafe fn first_child_ref(&self) -> Option<JS<Node>> {
-        (*self.unsafe_get()).first_child.get()
+        (*self.unsafe_get()).first_child.get_inner()
     }
 
     #[inline]
     unsafe fn last_child_ref(&self) -> Option<JS<Node>> {
-        (*self.unsafe_get()).last_child.get()
+        (*self.unsafe_get()).last_child.get_inner()
     }
 
     #[inline]
     unsafe fn prev_sibling_ref(&self) -> Option<JS<Node>> {
-        (*self.unsafe_get()).prev_sibling.get()
+        (*self.unsafe_get()).prev_sibling.get_inner()
     }
 
     #[inline]
     unsafe fn next_sibling_ref(&self) -> Option<JS<Node>> {
-        (*self.unsafe_get()).next_sibling.get()
+        (*self.unsafe_get()).next_sibling.get_inner()
     }
 
     #[inline]
     unsafe fn owner_doc_for_layout(&self) -> JS<Document> {
-        (*self.unsafe_get()).owner_doc.get().unwrap()
+        (*self.unsafe_get()).owner_doc.get_inner().unwrap()
     }
 }
 
@@ -1025,13 +1026,13 @@ impl Node {
             eventtarget: EventTarget::new_inherited(NodeTargetTypeId(type_id)),
             type_id: type_id,
 
-            parent_node: Cell::new(None),
-            first_child: Cell::new(None),
-            last_child: Cell::new(None),
-            next_sibling: Cell::new(None),
-            prev_sibling: Cell::new(None),
-            owner_doc: Cell::new(doc.unrooted()),
-            child_list: Cell::new(None),
+            parent_node: Default::default(),
+            first_child: Default::default(),
+            last_child: Default::default(),
+            next_sibling: Default::default(),
+            prev_sibling: Default::default(),
+            owner_doc: MutNullableJS::new(doc),
+            child_list: Default::default(),
 
             flags: Traceable::new(RefCell::new(NodeFlags::new(type_id))),
 
@@ -1304,7 +1305,7 @@ impl Node {
     fn pre_remove(child: JSRef<Node>, parent: JSRef<Node>) -> Fallible<Temporary<Node>> {
         // Step 1.
         match child.parent_node() {
-            Some(node) if node != Temporary::from_rooted(parent) => return Err(NotFound),
+            Some(ref node) if node != &Temporary::from_rooted(parent) => return Err(NotFound),
             _ => ()
         }
 
@@ -1535,7 +1536,7 @@ impl<'a> NodeMethods for JSRef<'a, Node> {
 
     // http://dom.spec.whatwg.org/#dom-node-parentnode
     fn GetParentNode(self) -> Option<Temporary<Node>> {
-        self.parent_node.get().map(|node| Temporary::new(node))
+        self.parent_node.get()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-parentelement
@@ -1558,34 +1559,34 @@ impl<'a> NodeMethods for JSRef<'a, Node> {
     fn ChildNodes(self) -> Temporary<NodeList> {
         match self.child_list.get() {
             None => (),
-            Some(ref list) => return Temporary::new(list.clone()),
+            Some(list) => return list,
         }
 
         let doc = self.owner_doc().root();
         let window = doc.deref().window.root();
         let child_list = NodeList::new_child_list(*window, self);
         self.child_list.assign(Some(child_list));
-        Temporary::new(self.child_list.get().as_ref().unwrap().clone())
+        self.child_list.get().unwrap()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-firstchild
     fn GetFirstChild(self) -> Option<Temporary<Node>> {
-        self.first_child.get().map(|node| Temporary::new(node))
+        self.first_child.get()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-lastchild
     fn GetLastChild(self) -> Option<Temporary<Node>> {
-        self.last_child.get().map(|node| Temporary::new(node))
+        self.last_child.get()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-previoussibling
     fn GetPreviousSibling(self) -> Option<Temporary<Node>> {
-        self.prev_sibling.get().map(|node| Temporary::new(node))
+        self.prev_sibling.get()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-nextsibling
     fn GetNextSibling(self) -> Option<Temporary<Node>> {
-        self.next_sibling.get().map(|node| Temporary::new(node))
+        self.next_sibling.get()
     }
 
     // http://dom.spec.whatwg.org/#dom-node-nodevalue
diff --git a/components/script/dom/uievent.rs b/components/script/dom/uievent.rs
index eac29a4ab24..86501d47324 100644
--- a/components/script/dom/uievent.rs
+++ b/components/script/dom/uievent.rs
@@ -9,7 +9,7 @@ use dom::bindings::codegen::InheritTypes::{EventCast, UIEventDerived};
 use dom::bindings::error::Fallible;
 use dom::bindings::global::GlobalRef;
 use dom::bindings::global;
-use dom::bindings::js::{JS, JSRef, RootedReference, Temporary, OptionalSettable};
+use dom::bindings::js::{MutNullableJS, JSRef, RootedReference, Temporary, OptionalSettable};
 use dom::bindings::trace::Traceable;
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
 use dom::event::{Event, EventTypeId, UIEventTypeId};
@@ -17,12 +17,13 @@ use dom::window::Window;
 use servo_util::str::DOMString;
 
 use std::cell::Cell;
+use std::default::Default;
 
 #[jstraceable]
 #[must_root]
 pub struct UIEvent {
     pub event: Event,
-    view: Cell<Option<JS<Window>>>,
+    view: MutNullableJS<Window>,
     detail: Traceable<Cell<i32>>
 }
 
@@ -36,7 +37,7 @@ impl UIEvent {
     pub fn new_inherited(type_id: EventTypeId) -> UIEvent {
         UIEvent {
             event: Event::new_inherited(type_id),
-            view: Cell::new(None),
+            view: Default::default(),
             detail: Traceable::new(Cell::new(0)),
         }
     }
@@ -70,7 +71,7 @@ impl UIEvent {
 
 impl<'a> UIEventMethods for JSRef<'a, UIEvent> {
     fn GetView(self) -> Option<Temporary<Window>> {
-        self.view.get().map(|view| Temporary::new(view))
+        self.view.get()
     }
 
     fn Detail(self) -> i32 {
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index d0c33ebc1ca..f48fb5a2e43 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -8,7 +8,7 @@ use dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
 use dom::bindings::codegen::InheritTypes::EventTargetCast;
 use dom::bindings::error::{Fallible, InvalidCharacter};
 use dom::bindings::global;
-use dom::bindings::js::{JS, JSRef, Temporary, OptionalSettable};
+use dom::bindings::js::{MutNullableJS, JS, JSRef, Temporary, OptionalSettable};
 use dom::bindings::trace::{Traceable, Untraceable};
 use dom::bindings::utils::{Reflectable, Reflector};
 use dom::browsercontext::BrowserContext;
@@ -44,6 +44,7 @@ use std::cell::{Cell, RefCell};
 use std::cmp;
 use std::comm::{channel, Sender};
 use std::comm::Select;
+use std::default::Default;
 use std::hash::{Hash, sip};
 use std::io::timer::Timer;
 use std::ptr;
@@ -81,16 +82,16 @@ pub struct Window {
     eventtarget: EventTarget,
     pub script_chan: ScriptChan,
     pub control_chan: ScriptControlChan,
-    console: Cell<Option<JS<Console>>>,
-    location: Cell<Option<JS<Location>>>,
-    navigator: Cell<Option<JS<Navigator>>>,
+    console: MutNullableJS<Console>,
+    location: MutNullableJS<Location>,
+    navigator: MutNullableJS<Navigator>,
     pub image_cache_task: ImageCacheTask,
     pub active_timers: Traceable<RefCell<HashMap<TimerId, TimerHandle>>>,
     next_timer_handle: Traceable<Cell<i32>>,
     pub compositor: Untraceable<Box<ScriptListener+'static>>,
     pub browser_context: Traceable<RefCell<Option<BrowserContext>>>,
     pub page: Rc<Page>,
-    performance: Cell<Option<JS<Performance>>>,
+    performance: MutNullableJS<Performance>,
     pub navigationStart: u64,
     pub navigationStartPrecise: f64,
     screen: Cell<Option<JS<Screen>>>,
@@ -225,7 +226,7 @@ impl<'a> WindowMethods for JSRef<'a, Window> {
             let location = Location::new(self, page);
             self.location.assign(Some(location));
         }
-        Temporary::new(self.location.get().as_ref().unwrap().clone())
+        self.location.get().unwrap()
     }
 
     fn Console(self) -> Temporary<Console> {
@@ -233,7 +234,7 @@ impl<'a> WindowMethods for JSRef<'a, Window> {
             let console = Console::new(&global::Window(self));
             self.console.assign(Some(console));
         }
-        Temporary::new(self.console.get().as_ref().unwrap().clone())
+        self.console.get().unwrap()
     }
 
     fn Navigator(self) -> Temporary<Navigator> {
@@ -241,7 +242,7 @@ impl<'a> WindowMethods for JSRef<'a, Window> {
             let navigator = Navigator::new(self);
             self.navigator.assign(Some(navigator));
         }
-        Temporary::new(self.navigator.get().as_ref().unwrap().clone())
+        self.navigator.get().unwrap()
     }
 
     fn SetTimeout(self, _cx: *mut JSContext, callback: JSVal, timeout: i32) -> i32 {
@@ -289,7 +290,7 @@ impl<'a> WindowMethods for JSRef<'a, Window> {
             let performance = Performance::new(self);
             self.performance.assign(Some(performance));
         }
-        Temporary::new(self.performance.get().as_ref().unwrap().clone())
+        self.performance.get().unwrap()
     }
 
     fn GetOnclick(self) -> Option<EventHandlerNonNull> {
@@ -529,16 +530,16 @@ impl Window {
             eventtarget: EventTarget::new_inherited(WindowTypeId),
             script_chan: script_chan,
             control_chan: control_chan,
-            console: Cell::new(None),
+            console: Default::default(),
             compositor: Untraceable::new(compositor),
             page: page,
-            location: Cell::new(None),
-            navigator: Cell::new(None),
+            location: Default::default(),
+            navigator: Default::default(),
             image_cache_task: image_cache_task,
             active_timers: Traceable::new(RefCell::new(HashMap::new())),
             next_timer_handle: Traceable::new(Cell::new(0)),
             browser_context: Traceable::new(RefCell::new(None)),
-            performance: Cell::new(None),
+            performance: Default::default(),
             navigationStart: time::get_time().sec as u64,
             navigationStartPrecise: time::precise_time_s(),
             screen: Cell::new(None),
diff --git a/components/script/dom/xmlhttprequest.rs b/components/script/dom/xmlhttprequest.rs
index 7bb38cf49df..dd3ababc8cb 100644
--- a/components/script/dom/xmlhttprequest.rs
+++ b/components/script/dom/xmlhttprequest.rs
@@ -13,7 +13,7 @@ use dom::bindings::conversions::ToJSValConvertible;
 use dom::bindings::error::{Error, ErrorResult, Fallible, InvalidState, InvalidAccess};
 use dom::bindings::error::{Network, Syntax, Security, Abort, Timeout};
 use dom::bindings::global::{GlobalField, GlobalRef, WorkerField};
-use dom::bindings::js::{JS, JSRef, Temporary, OptionalRootedRootable};
+use dom::bindings::js::{MutNullableJS, JS, JSRef, Temporary, OptionalRootedRootable};
 use dom::bindings::str::ByteString;
 use dom::bindings::trace::{Traceable, Untraceable};
 use dom::bindings::utils::{Reflectable, Reflector, reflect_dom_object};
@@ -53,6 +53,7 @@ use servo_util::task::spawn_named;
 use std::ascii::StrAsciiExt;
 use std::cell::{Cell, RefCell};
 use std::comm::{Sender, Receiver, channel};
+use std::default::Default;
 use std::io::{BufReader, MemWriter, Timer};
 use std::from_str::FromStr;
 use std::path::BytesContainer;
@@ -115,7 +116,7 @@ pub struct XMLHttpRequest {
     status_text: Traceable<RefCell<ByteString>>,
     response: Traceable<RefCell<ByteString>>,
     response_type: Traceable<Cell<XMLHttpRequestResponseType>>,
-    response_xml: Cell<Option<JS<Document>>>,
+    response_xml: MutNullableJS<Document>,
     response_headers: Untraceable<RefCell<ResponseHeaderCollection>>,
 
     // Associated concepts
@@ -149,7 +150,7 @@ impl XMLHttpRequest {
             status_text: Traceable::new(RefCell::new(ByteString::new(vec!()))),
             response: Traceable::new(RefCell::new(ByteString::new(vec!()))),
             response_type: Traceable::new(Cell::new(_empty)),
-            response_xml: Cell::new(None),
+            response_xml: Default::default(),
             response_headers: Untraceable::new(RefCell::new(ResponseHeaderCollection::new())),
 
             request_method: Untraceable::new(RefCell::new(Get)),
@@ -667,7 +668,7 @@ impl<'a> XMLHttpRequestMethods for JSRef<'a, XMLHttpRequest> {
         }
     }
     fn GetResponseXML(self) -> Option<Temporary<Document>> {
-        self.response_xml.get().map(|response| Temporary::new(response))
+        self.response_xml.get()
     }
 }
 
diff --git a/components/script/page.rs b/components/script/page.rs
index 41083ca2250..b25be6122bb 100644
--- a/components/script/page.rs
+++ b/components/script/page.rs
@@ -5,7 +5,7 @@
 use dom::attr::AttrHelpers;
 use dom::bindings::codegen::Bindings::DocumentBinding::DocumentMethods;
 use dom::bindings::codegen::InheritTypes::{NodeCast, ElementCast};
-use dom::bindings::js::{JS, JSRef, Temporary};
+use dom::bindings::js::{MutNullableJS, JS, JSRef, Temporary};
 use dom::bindings::js::OptionalRootable;
 use dom::bindings::trace::{Traceable, Untraceable};
 use dom::bindings::utils::GlobalStaticData;
@@ -30,12 +30,14 @@ use servo_net::resource_task::ResourceTask;
 use servo_util::str::DOMString;
 use std::cell::{Cell, RefCell, Ref, RefMut};
 use std::comm::{channel, Receiver, Empty, Disconnected};
+use std::default::Default;
 use std::mem::replace;
 use std::rc::Rc;
 use url::Url;
 
 /// Encapsulates a handle to a frame and its associated layout information.
 #[jstraceable]
+#[allow(unrooted_must_root)] // FIXME(#3543) should be must_root.
 pub struct Page {
     /// Pipeline id associated with this page.
     pub id: PipelineId,
@@ -78,7 +80,7 @@ pub struct Page {
     pub resize_event: Untraceable<Cell<Option<WindowSizeData>>>,
 
     /// Pending scroll to fragment event, if any
-    pub fragment_node: Cell<Option<JS<Element>>>,
+    pub fragment_node: MutNullableJS<Element>,
 
     /// Associated resource task for use by DOM objects like XMLHttpRequest
     pub resource_task: Untraceable<ResourceTask>,
@@ -152,7 +154,7 @@ impl Page {
             url: Untraceable::new(RefCell::new(None)),
             next_subpage_id: Traceable::new(Cell::new(SubpageId(0))),
             resize_event: Untraceable::new(Cell::new(None)),
-            fragment_node: Cell::new(None),
+            fragment_node: Default::default(),
             last_reflow_id: Traceable::new(Cell::new(0)),
             resource_task: Untraceable::new(resource_task),
             constellation_chan: Untraceable::new(constellation_chan),