auto merge of #4613 : saneyuki/servo/layoutjs, r=jdm

#4571

components/script/dom/bindings/codegen/CodegenRust.py

@@ -5220,7 +5220,7 @@ class GlobalGenRoots():
         descriptors = config.getDescriptors(register=True, isCallback=False)
         allprotos = [CGGeneric("#![allow(unused_imports)]\n"),
                      CGGeneric("use dom::types::*;\n"),
-                     CGGeneric("use dom::bindings::js::{JS, JSRef, Temporary};\n"),
+                     CGGeneric("use dom::bindings::js::{JS, JSRef, LayoutJS, Temporary};\n"),
                      CGGeneric("use dom::bindings::trace::JSTraceable;\n"),
                      CGGeneric("use dom::bindings::utils::Reflectable;\n"),
                      CGGeneric("use js::jsapi::JSTracer;\n\n"),
@@ -5279,6 +5279,17 @@ pub trait ${castTraitName} : Sized {
         }
     }
 
+    #[inline(always)]
+    #[allow(unrooted_must_root)]
+    fn to_layout_js<T: ${toBound}+Reflectable>(base: &LayoutJS<T>) -> Option<LayoutJS<Self>> {
+        unsafe {
+            match (*base.unsafe_get()).${checkFn}() {
+                true => Some(base.transmute_copy()),
+                false => None
+            }
+        }
+    }
+
     #[inline(always)]
     fn from_ref<'a, T: ${fromBound}+Reflectable>(derived: JSRef<'a, T>) -> JSRef<'a, Self> {
         unsafe { derived.transmute() }