Introduce LayoutDom::to_layout_slice

It generalises <LayoutDom<Element>>::attrs.

components/script/dom/bindings/root.rs

@@ -736,6 +736,15 @@ where
         debug_assert!(thread_state::get().is_layout());
         self.value
     }
+
+    /// Transforms a slice of Dom<T> into a slice of LayoutDom<T>.
+    // FIXME(nox): This should probably be done through a ToLayout trait.
+    pub unsafe fn to_layout_slice(slice: &'dom [Dom<T>]) -> &'dom [LayoutDom<'dom, T>] {
+        // This doesn't compile if Dom and LayoutDom don't have the same
+        // representation.
+        let _ = mem::transmute::<Dom<T>, LayoutDom<T>>;
+        &*(slice as *const [Dom<T>] as *const [LayoutDom<T>])
+    }
 }
 
 /// Helper trait for safer manipulations of `Option<Heap<T>>` values.

components/script/dom/element.rs

@@ -598,14 +598,7 @@ impl<'dom> LayoutElementHelpers<'dom> for LayoutDom<'dom, Element> {
     #[allow(unsafe_code)]
     #[inline]
     fn attrs(self) -> &'dom [LayoutDom<'dom, Attr>] {
-        unsafe {
+        unsafe { LayoutDom::to_layout_slice(self.unsafe_get().attrs.borrow_for_layout()) }
-            // FIXME(nox): This should probably be done through a ToLayout trait.
-            let attrs: &[Dom<Attr>] = &self.unsafe_get().attrs.borrow_for_layout();
-            // This doesn't compile if Dom and LayoutDom don't have the same
-            // representation.
-            let _ = mem::transmute::<Dom<Attr>, LayoutDom<Attr>>;
-            &*(attrs as *const [Dom<Attr>] as *const [LayoutDom<Attr>])
-        }
     }
 
     #[inline]