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

@@ -203,11 +203,9 @@ 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)
-        }
+        self.node
+            .composed_parent_node_ref()
+            .map(Self::from_layout_js)
     }
 
     fn first_child(&self) -> Option<Self> {
@@ -745,12 +743,10 @@ 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)
-        }
+        self.element
+            .upcast()
+            .composed_parent_node_ref()
+            .and_then(as_element)
     }
 
     fn parent_node_is_shadow_root(&self) -> bool {

components/layout_thread_2020/dom_wrapper.rs

@@ -210,11 +210,9 @@ 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)
-        }
+        self.node
+            .composed_parent_node_ref()
+            .map(Self::from_layout_js)
     }
 
     fn first_child(&self) -> Option<Self> {
@@ -752,12 +750,10 @@ 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)
-        }
+        self.element
+            .upcast()
+            .composed_parent_node_ref()
+            .and_then(as_element)
     }
 
     fn parent_node_is_shadow_root(&self) -> bool {

components/script/dom/element.rs

@@ -985,32 +985,27 @@ 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();
-                match node.downcast::<Element>() {
-                    Some(elem) => {
-                        if let Some(attr) =
-                            elem.get_attr_val_for_layout(&ns!(xml), &local_name!("lang"))
-                        {
-                            return attr.to_owned();
-                        }
-                        if let Some(attr) =
-                            elem.get_attr_val_for_layout(&ns!(), &local_name!("lang"))
-                        {
-                            return attr.to_owned();
-                        }
-                    },
-                    None => continue,
-                }
+        let mut current_node = Some(self.upcast::<Node>());
+        while let Some(node) = current_node {
+            current_node = node.composed_parent_node_ref();
+            match node.downcast::<Element>() {
+                Some(elem) => {
+                    if let Some(attr) =
+                        elem.get_attr_val_for_layout(&ns!(xml), &local_name!("lang"))
+                    {
+                        return attr.to_owned();
+                    }
+                    if let Some(attr) = elem.get_attr_val_for_layout(&ns!(), &local_name!("lang")) {
+                        return attr.to_owned();
+                    }
+                },
+                None => continue,
             }
-            // TODO: Check meta tags for a pragma-set default language
-            // TODO: Check HTTP Content-Language header
-            String::new()
         }
+        // TODO: Check meta tags for a pragma-set default language
+        // TODO: Check HTTP Content-Language header
+        String::new()
     }
 
     #[inline]

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)]
-    unsafe fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>> {
-        let parent = (*self.unsafe_get()).parent_node.get_inner_as_layout();
-        if let Some(ref parent) = parent {
+    fn composed_parent_node_ref(self) -> Option<LayoutDom<'dom, Node>> {
+        let parent = self.parent_node_ref();
+        if let Some(parent) = parent {
             if let Some(shadow_root) = parent.downcast::<ShadowRoot>() {
                 return Some(shadow_root.get_host_for_layout().upcast());
             }