Create a replacement for Cell<SM primitive>. Fixes #4337.

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.