diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index b34c6441b70..28e1b439775 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -102,17 +102,13 @@ class CastableObjectUnwrapper():
     """
     def __init__(self, descriptor, source, codeOnFailure):
         self.substitution = {
-            "type": descriptor.nativeType,
-            "depth": descriptor.interface.inheritanceDepth(),
-            "prototype": "PrototypeList::ID::" + descriptor.name,
-            "protoID": "PrototypeList::ID::" + descriptor.name + " as uint",
             "source": source,
             "codeOnFailure": CGIndenter(CGGeneric(codeOnFailure), 4).define(),
         }
 
     def __str__(self):
         return string.Template(
-"""match unwrap_jsmanaged(${source}, ${prototype}, ${depth}) {
+"""match unwrap_jsmanaged(${source}) {
   Ok(val) => val,
   Err(()) => {
 ${codeOnFailure}
diff --git a/components/script/dom/bindings/conversions.rs b/components/script/dom/bindings/conversions.rs
index 52150b26f9c..ceac869a19e 100644
--- a/components/script/dom/bindings/conversions.rs
+++ b/components/script/dom/bindings/conversions.rs
@@ -350,9 +350,7 @@ impl<T: Reflectable+IDLInterface> FromJSValConvertible<()> for JS<T> {
         if !value.is_object() {
             return Err(());
         }
-        unwrap_jsmanaged(value.to_object(),
-                         IDLInterface::get_prototype_id(None::<T>),
-                         IDLInterface::get_prototype_depth(None::<T>))
+        unwrap_jsmanaged(value.to_object())
     }
 }
 
diff --git a/components/script/dom/bindings/utils.rs b/components/script/dom/bindings/utils.rs
index 4f12d1dbc7d..048a54eee4a 100644
--- a/components/script/dom/bindings/utils.rs
+++ b/components/script/dom/bindings/utils.rs
@@ -123,9 +123,9 @@ pub unsafe fn get_dom_class(obj: *mut JSObject) -> Result<DOMClass, ()> {
 /// Returns Err(()) if `obj` is an opaque security wrapper or if the object is
 /// not a reflector for a DOM object of the given type (as defined by the
 /// proto_id and proto_depth).
-pub fn unwrap_jsmanaged<T: Reflectable>(mut obj: *mut JSObject,
-                                        proto_id: PrototypeList::ID,
-                                        proto_depth: uint) -> Result<JS<T>, ()> {
+pub fn unwrap_jsmanaged<T>(mut obj: *mut JSObject) -> Result<JS<T>, ()>
+    where T: Reflectable + IDLInterface
+{
     unsafe {
         let dom_class = try!(get_dom_class(obj).or_else(|_| {
             if IsWrapper(obj) == 1 {
@@ -145,6 +145,8 @@ pub fn unwrap_jsmanaged<T: Reflectable>(mut obj: *mut JSObject,
             }
         }));
 
+        let proto_id = IDLInterface::get_prototype_id(None::<T>);
+        let proto_depth = IDLInterface::get_prototype_depth(None::<T>);
         if dom_class.interface_chain[proto_depth] == proto_id {
             debug!("good prototype");
             Ok(JS::from_raw(unwrap(obj)))
@@ -648,12 +650,7 @@ pub extern fn outerize_global(_cx: *mut JSContext, obj: JSHandleObject) -> *mut
     unsafe {
         debug!("outerizing");
         let obj = *obj.unnamed_field1;
-        let win: Root<window::Window> =
-            unwrap_jsmanaged(obj,
-                             IDLInterface::get_prototype_id(None::<window::Window>),
-                             IDLInterface::get_prototype_depth(None::<window::Window>))
-            .unwrap()
-            .root();
+        let win: Root<window::Window> = unwrap_jsmanaged(obj).unwrap().root();
         win.browser_context().as_ref().unwrap().window_proxy()
     }
 }