Make LayoutNodeHelpers::composed_parent_node_ref be safe

For clarity, I introduce <LayoutDom<Element>>::parent_node_ref to contain
the remaining unsafety bits out of composed_parent_node_ref which is more
complex than just a field access.

components/layout_thread/dom_wrapper.rs

@@ -203,12 +203,10 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
     type ConcreteShadowRoot = ServoShadowRoot<'ln>;
 
     fn parent_node(&self) -> Option<Self> {
-        unsafe {
         self.node
             .composed_parent_node_ref()
             .map(Self::from_layout_js)
     }
-    }
 
     fn first_child(&self) -> Option<Self> {
         self.node.first_child_ref().map(Self::from_layout_js)
@@ -745,13 +743,11 @@ impl<'le> ::selectors::Element for ServoLayoutElement<'le> {
     }
 
     fn parent_element(&self) -> Option<ServoLayoutElement<'le>> {
-        unsafe {
         self.element
             .upcast()
             .composed_parent_node_ref()
             .and_then(as_element)
     }
-    }
 
     fn parent_node_is_shadow_root(&self) -> bool {
         match self.as_node().parent_node() {

components/layout_thread_2020/dom_wrapper.rs

@@ -210,12 +210,10 @@ impl<'ln> TNode for ServoLayoutNode<'ln> {
     type ConcreteShadowRoot = ServoShadowRoot<'ln>;
 
     fn parent_node(&self) -> Option<Self> {
-        unsafe {
         self.node
             .composed_parent_node_ref()
             .map(Self::from_layout_js)
     }
-    }
 
     fn first_child(&self) -> Option<Self> {
         self.node.first_child_ref().map(Self::from_layout_js)
@@ -752,13 +750,11 @@ impl<'le> ::selectors::Element for ServoLayoutElement<'le> {
     }
 
     fn parent_element(&self) -> Option<ServoLayoutElement<'le>> {
-        unsafe {
         self.element
             .upcast()
             .composed_parent_node_ref()
             .and_then(as_element)
     }
-    }
 
     fn parent_node_is_shadow_root(&self) -> bool {
         match self.as_node().parent_node() {

components/script/dom/element.rs

@@ -985,9 +985,7 @@ impl<'dom> LayoutElementHelpers<'dom> for LayoutDom<'dom, Element> {
         unsafe { &(*self.unsafe_get()).namespace }
     }
 
-    #[allow(unsafe_code)]
     fn get_lang_for_layout(self) -> String {
-        unsafe {
         let mut current_node = Some(self.upcast::<Node>());
         while let Some(node) = current_node {
             current_node = node.composed_parent_node_ref();
@@ -998,9 +996,7 @@ impl<'dom> LayoutElementHelpers<'dom> for LayoutDom<'dom, Element> {
                     {
                         return attr.to_owned();
                     }
-                        if let Some(attr) =
+                    if let Some(attr) = elem.get_attr_val_for_layout(&ns!(), &local_name!("lang")) {
-                            elem.get_attr_val_for_layout(&ns!(), &local_name!("lang"))
-                        {
                         return attr.to_owned();
                     }
                 },
@@ -1011,7 +1007,6 @@ impl<'dom> LayoutElementHelpers<'dom> for LayoutDom<'dom, Element> {
         // TODO: Check HTTP Content-Language header
         String::new()
     }
-    }
 
     #[inline]
     #[allow(unsafe_code)]

components/script/dom/node.rs

@@ -1307,7 +1307,7 @@ pub unsafe fn from_untrusted_node_address(
 pub trait LayoutNodeHelpers<'dom> {
     fn type_id_for_layout(self) -> NodeTypeId;
 
-    unsafe fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>>;
+    fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>>;
     fn first_child_ref(self) -> Option<LayoutDom<'dom, Node>>;
     fn last_child_ref(self) -> Option<LayoutDom<'dom, Node>>;
     fn prev_sibling_ref(self) -> Option<LayoutDom<'dom, Node>>;
@@ -1339,6 +1339,14 @@ pub trait LayoutNodeHelpers<'dom> {
     fn opaque(self) -> OpaqueNode;
 }
 
+impl<'dom> LayoutDom<'dom, Node> {
+    #[inline]
+    #[allow(unsafe_code)]
+    fn parent_node_ref(self) -> Option<LayoutDom<'dom, Node>> {
+        unsafe { self.unsafe_get().parent_node.get_inner_as_layout() }
+    }
+}
+
 impl<'dom> LayoutNodeHelpers<'dom> for LayoutDom<'dom, Node> {
     #[inline]
     #[allow(unsafe_code)]
@@ -1352,10 +1360,9 @@ impl<'dom> LayoutNodeHelpers<'dom> for LayoutDom<'dom, Node> {
     }
 
     #[inline]
-    #[allow(unsafe_code)]
+    fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>> {
-    unsafe fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>> {
+        let parent = self.parent_node_ref();
-        let parent = (*self.unsafe_get()).parent_node.get_inner_as_layout();
+        if let Some(parent) = parent {
-        if let Some(ref parent) = parent {
             if let Some(shadow_root) = parent.downcast::<ShadowRoot>() {
                 return Some(shadow_root.get_host_for_layout().upcast());
             }