Auto merge of #26083 - servo:layout-2020-more-cleanups, r=jdm

More layout cleanups from the introduction of a lifetime in LayoutDom<T>

What can I say, the follow-up fixes just kept coming to my door one by one, I couldn't just tell them to go away.

1 files changed, 18 insertions, 0 deletions

diff --git a/components/script/dom/bindings/root.rs b/components/script/dom/bindings/root.rs
index 9afb421abbb..4f4d376e951 100644
--- a/components/script/dom/bindings/root.rs
+++ b/components/script/dom/bindings/root.rs
@@ -441,6 +441,15 @@ where
         debug_assert!(thread_state::get().is_layout());
         self.value.downcast::<U>().map(|value| LayoutDom { value })
     }
+
+    /// Returns whether this inner object is a U.
+    pub fn is<U>(&self) -> bool
+    where
+        U: DerivedFrom<T>,
+    {
+        debug_assert!(thread_state::get().is_layout());
+        self.value.is::<U>()
+    }
 }
 
 impl<T> LayoutDom<'_, T>
@@ -736,6 +745,15 @@ 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.