Introduce RootedTraceableBox.

components/script/dom/bindings/conversions.rs

@@ -37,6 +37,7 @@ use dom::bindings::js::Root;
 use dom::bindings::num::Finite;
 use dom::bindings::reflector::{DomObject, Reflector};
 use dom::bindings::str::{ByteString, DOMString, USVString};
+use dom::bindings::trace::{JSTraceable, RootedTraceableBox};
 use dom::bindings::utils::DOMClass;
 use js;
 pub use js::conversions::{FromJSValConvertible, ToJSValConvertible, ConversionResult};
@@ -117,6 +118,22 @@ impl <T: DomObject + IDLInterface> FromJSValConvertible for Root<T> {
     }
 }
 
+impl <T: FromJSValConvertible + JSTraceable> FromJSValConvertible for RootedTraceableBox<T> {
+    type Config = T::Config;
+
+    unsafe fn from_jsval(cx: *mut JSContext,
+                         value: HandleValue,
+                         config: Self::Config)
+                         -> Result<ConversionResult<Self>, ()> {
+        T::from_jsval(cx, value, config).map(|result| {
+            match result {
+                ConversionResult::Success(v) => ConversionResult::Success(RootedTraceableBox::new(v)),
+                ConversionResult::Failure(e) => ConversionResult::Failure(e),
+            }
+        })
+    }
+}
+
 /// Convert `id` to a `DOMString`, assuming it is string-valued.
 ///
 /// Handling of invalid UTF-16 in strings depends on the relevant option.