diff --git a/components/script/dom/bindings/codegen/CodegenRust.py b/components/script/dom/bindings/codegen/CodegenRust.py
index 2bac83bc278..af6af8118cc 100644
--- a/components/script/dom/bindings/codegen/CodegenRust.py
+++ b/components/script/dom/bindings/codegen/CodegenRust.py
@@ -1920,10 +1920,10 @@ class CGIDLInterface(CGThing):
         }
         return string.Template("""\
 impl IDLInterface for ${type} {
-    fn get_prototype_id(_: Option<${type}>) -> PrototypeList::ID {
+    fn get_prototype_id() -> PrototypeList::ID {
         PrototypeList::ID::${type}
     }
-    fn get_prototype_depth(_: Option<${type}>) -> uint {
+    fn get_prototype_depth() -> uint {
         ${depth}
     }
 }
diff --git a/components/script/dom/bindings/conversions.rs b/components/script/dom/bindings/conversions.rs
index 62f71cea5d6..e60584a6416 100644
--- a/components/script/dom/bindings/conversions.rs
+++ b/components/script/dom/bindings/conversions.rs
@@ -57,14 +57,12 @@ use std::slice;
 
 /// A trait to retrieve the constants necessary to check if a `JSObject`
 /// implements a given interface.
-// FIXME (https://github.com/rust-lang/rfcs/pull/4)
-//       remove Option<Self> arguments.
 pub trait IDLInterface {
     /// Returns the prototype ID.
-    fn get_prototype_id(_: Option<Self>) -> PrototypeList::ID;
+    fn get_prototype_id() -> PrototypeList::ID;
     /// Returns the prototype depth, i.e., the number of interfaces this
     /// interface inherits from.
-    fn get_prototype_depth(_: Option<Self>) -> uint;
+    fn get_prototype_depth() -> uint;
 }
 
 /// A trait to convert Rust types to `JSVal`s.
@@ -489,8 +487,8 @@ pub fn unwrap_jsmanaged<T>(mut obj: *mut JSObject) -> Result<Unrooted<T>, ()>
             }
         }));
 
-        let proto_id = IDLInterface::get_prototype_id(None::<T>);
-        let proto_depth = IDLInterface::get_prototype_depth(None::<T>);
+        let proto_id = <T as IDLInterface>::get_prototype_id();
+        let proto_depth = <T as IDLInterface>::get_prototype_depth();
         if dom_class.interface_chain[proto_depth] == proto_id {
             debug!("good prototype");
             Ok(Unrooted::from_raw(unwrap(obj)))