Stop implementing Copy for JS<T>.

A copy of a JS<T> doesn't have the rooting properties of the original,
so it makes no sense for it to implement Copy.

components/script/dom/bindings/js.rs

@@ -32,9 +32,10 @@ use js::jsapi::{Heap, JSObject, JSTracer};
 use js::jsval::JSVal;
 use layout_interface::TrustedNodeAddress;
 use script_task::STACK_ROOTS;
-use std::cell::{Cell, UnsafeCell};
+use std::cell::UnsafeCell;
 use std::default::Default;
 use std::ops::Deref;
+use std::ptr;
 use util::mem::HeapSizeOf;
 
 /// A traced reference to a DOM object. Must only be used as a field in other
@@ -54,7 +55,7 @@ impl<T> HeapSizeOf for JS<T> {
 
 impl<T> JS<T> {
     /// Returns `LayoutJS<T>` containing the same pointer.
-    pub unsafe fn to_layout(self) -> LayoutJS<T> {
+    pub unsafe fn to_layout(&self) -> LayoutJS<T> {
         LayoutJS {
             ptr: self.ptr.clone()
         }
@@ -109,8 +110,6 @@ impl<T: Reflectable> LayoutJS<T> {
     }
 }
 
-impl<T> Copy for JS<T> {}
-
 impl<T> Copy for LayoutJS<T> {}
 
 impl<T> PartialEq for JS<T> {
@@ -208,56 +207,62 @@ impl MutHeapJSVal {
 /// `JS<T>`.
 #[must_root]
 #[derive(JSTraceable)]
-#[derive(HeapSizeOf)]
+pub struct MutHeap<T: HeapGCValue> {
-pub struct MutHeap<T: HeapGCValue + Copy> {
+    val: UnsafeCell<T>,
-    val: Cell<T>,
 }
 
-impl<T: HeapGCValue + Copy> MutHeap<T> {
+impl<T: HeapGCValue> MutHeap<T> {
     /// Create a new `MutHeap`.
     pub fn new(initial: T) -> MutHeap<T> {
         MutHeap {
-            val: Cell::new(initial),
+            val: UnsafeCell::new(initial),
         }
     }
 
     /// Set this `MutHeap` to the given value.
     pub fn set(&self, val: T) {
-        self.val.set(val)
+        unsafe { *self.val.get() = val; }
     }
 
     /// Set the value in this `MutHeap`.
     pub fn get(&self) -> T {
-        self.val.get()
+        unsafe { ptr::read(self.val.get()) }
+    }
+}
+
+impl<T: HeapGCValue> HeapSizeOf for MutHeap<T> {
+    fn heap_size_of_children(&self) -> usize {
+        // See comment on HeapSizeOf for JS<T>.
+        0
     }
 }
 
 /// A mutable holder for GC-managed values such as `JSval` and `JS<T>`, with
-/// nullability represented by an enclosing Option wrapper. Must be used in
+/// nullability represented by an enclosing Option wrapper. Roughly equivalent
-/// place of traditional internal mutability to ensure that the proper GC
+/// to a DOMRefCell<Option<JS<T>>>, but smaller; the cost is that values which
-/// barriers are enforced.
+/// are read must be immediately rooted.
 #[must_root]
-#[derive(JSTraceable, HeapSizeOf)]
+#[derive(JSTraceable)]
-pub struct MutNullableHeap<T: HeapGCValue + Copy> {
+pub struct MutNullableHeap<T: HeapGCValue> {
-    ptr: Cell<Option<T>>
+    ptr: UnsafeCell<Option<T>>
 }
 
-impl<T: HeapGCValue + Copy> MutNullableHeap<T> {
+impl<T: HeapGCValue> MutNullableHeap<T> {
     /// Create a new `MutNullableHeap`.
     pub fn new(initial: Option<T>) -> MutNullableHeap<T> {
         MutNullableHeap {
-            ptr: Cell::new(initial)
+            ptr: UnsafeCell::new(initial)
         }
     }
 
     /// Set this `MutNullableHeap` to the given value.
     pub fn set(&self, val: Option<T>) {
-        self.ptr.set(val);
+        unsafe { *self.ptr.get() = val; }
     }
 
     /// Retrieve a copy of the current optional inner value.
     pub fn get(&self) -> Option<T> {
-        self.ptr.get()
+        unsafe { ptr::read(self.ptr.get()) }
     }
 }
 
@@ -280,7 +285,7 @@ impl<T: Reflectable> MutNullableHeap<JS<T>> {
     /// Retrieve a copy of the inner optional `JS<T>` as `LayoutJS<T>`.
     /// For use by layout, which can't use safe types like Temporary.
     pub unsafe fn get_inner_as_layout(&self) -> Option<LayoutJS<T>> {
-        self.ptr.get().map(|js| js.to_layout())
+        ptr::read(self.ptr.get()).map(|js| js.to_layout())
     }
 
     /// Get a rooted value out of this object
@@ -290,15 +295,22 @@ impl<T: Reflectable> MutNullableHeap<JS<T>> {
     }
 }
 
-impl<T: HeapGCValue + Copy> Default for MutNullableHeap<T> {
+impl<T: HeapGCValue> Default for MutNullableHeap<T> {
     #[allow(unrooted_must_root)]
     fn default() -> MutNullableHeap<T> {
         MutNullableHeap {
-            ptr: Cell::new(None)
+            ptr: UnsafeCell::new(None)
         }
     }
 }
 
+impl<T: HeapGCValue> HeapSizeOf for MutNullableHeap<T> {
+    fn heap_size_of_children(&self) -> usize {
+        // See comment on HeapSizeOf for JS<T>.
+        0
+    }
+}
+
 impl<T: Reflectable> LayoutJS<T> {
     /// Returns an unsafe pointer to the interior of this JS object. This is
     /// the only method that be safely accessed from layout. (The fact that

components/script/dom/browsercontext.rs

@@ -54,7 +54,7 @@ impl BrowsingContext {
     }
 
     pub fn frame_element(&self) -> Option<Root<Element>> {
-        self.frame_element.map(Root::from_rooted)
+        self.frame_element.as_ref().map(JS::root)
     }
 
     pub fn window_proxy(&self) -> *mut JSObject {

components/script/dom/canvasrenderingcontext2d.rs

@@ -763,7 +763,7 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
                 serialize(rgba, &mut result).unwrap();
                 StringOrCanvasGradientOrCanvasPattern::eString(result)
             },
-            CanvasFillOrStrokeStyle::Gradient(gradient) => {
+            CanvasFillOrStrokeStyle::Gradient(ref gradient) => {
                 StringOrCanvasGradientOrCanvasPattern::eCanvasGradient(gradient.root())
             },
         }
@@ -803,7 +803,7 @@ impl CanvasRenderingContext2DMethods for CanvasRenderingContext2D {
                 serialize(rgba, &mut result).unwrap();
                 StringOrCanvasGradientOrCanvasPattern::eString(result)
             },
-            CanvasFillOrStrokeStyle::Gradient(gradient) => {
+            CanvasFillOrStrokeStyle::Gradient(ref gradient) => {
                 StringOrCanvasGradientOrCanvasPattern::eCanvasGradient(gradient.root())
             },
         }

components/script/dom/htmlcanvaselement.rs

@@ -32,7 +32,7 @@ const DEFAULT_WIDTH: u32 = 300;
 const DEFAULT_HEIGHT: u32 = 150;
 
 #[must_root]
-#[derive(JSTraceable, Clone, Copy, HeapSizeOf)]
+#[derive(JSTraceable, Clone, HeapSizeOf)]
 pub enum CanvasContext {
     Context2d(JS<CanvasRenderingContext2D>),
     WebGL(JS<WebGLRenderingContext>),

components/script/dom/htmlinputelement.rs

@@ -758,7 +758,7 @@ impl Activatable for HTMLInputElement {
             InputType::InputRadio => {
                 // We want to restore state only if the element had been changed in the first place
                 if cache.was_mutable {
-                    let old_checked: Option<Root<HTMLInputElement>> = cache.checked_radio.map(|t| t.root());
+                    let old_checked = cache.checked_radio.as_ref().map(|t| t.root());
                     let name = self.get_radio_group_name();
                     match old_checked {
                         Some(ref o) => {

components/script/dom/nodelist.rs

@@ -64,7 +64,7 @@ impl NodeListMethods for NodeList {
     fn Item(&self, index: u32) -> Option<Root<Node>> {
         match self.list_type {
             NodeListType::Simple(ref elems) => {
-                elems.get(index as usize).map(|node| Root::from_rooted(*node))
+                elems.get(index as usize).map(|node| node.root())
             },
             NodeListType::Children(ref list) => list.item(index),
         }

components/script/dom/webglrenderingcontext.rs

@@ -12,7 +12,7 @@ use dom::bindings::codegen::InheritTypes::{EventCast, EventTargetCast, NodeCast}
 use dom::bindings::codegen::UnionTypes::ImageDataOrHTMLImageElementOrHTMLCanvasElementOrHTMLVideoElement;
 use dom::bindings::conversions::ToJSValConvertible;
 use dom::bindings::global::{GlobalField, GlobalRef};
-use dom::bindings::js::{JS, LayoutJS, Root};
+use dom::bindings::js::{JS, LayoutJS, MutNullableHeap, Root};
 use dom::bindings::utils::{Reflector, reflect_dom_object};
 use dom::event::{EventBubbles, EventCancelable};
 use dom::htmlcanvaselement::HTMLCanvasElement;
@@ -78,8 +78,8 @@ pub struct WebGLRenderingContext {
     canvas: JS<HTMLCanvasElement>,
     last_error: Cell<Option<WebGLError>>,
     texture_unpacking_settings: Cell<TextureUnpacking>,
-    bound_texture_2d: Cell<Option<JS<WebGLTexture>>>,
+    bound_texture_2d: MutNullableHeap<JS<WebGLTexture>>,
-    bound_texture_cube_map: Cell<Option<JS<WebGLTexture>>>,
+    bound_texture_cube_map: MutNullableHeap<JS<WebGLTexture>>,
 }
 
 impl WebGLRenderingContext {
@@ -104,8 +104,8 @@ impl WebGLRenderingContext {
                 canvas: JS::from_ref(canvas),
                 last_error: Cell::new(None),
                 texture_unpacking_settings: Cell::new(CONVERT_COLORSPACE),
-                bound_texture_2d: Cell::new(None),
+                bound_texture_2d: MutNullableHeap::new(None),
-                bound_texture_cube_map: Cell::new(None),
+                bound_texture_cube_map: MutNullableHeap::new(None),
             }
         })
     }