Introduce TraversalFlags to represents target elements of the traversal we are about to do.

components/layout_thread/lib.rs

@@ -120,7 +120,7 @@ use style::stylesheets::{Origin, Stylesheet, UserAgentStylesheets};
 use style::stylist::Stylist;
 use style::thread_state;
 use style::timer::Timer;
-use style::traversal::{DomTraversal, TraversalDriver};
+use style::traversal::{DomTraversal, TraversalDriver, TraversalFlags};
 
 /// Information needed by the layout thread.
 pub struct LayoutThread {
@@ -1143,7 +1143,7 @@ impl LayoutThread {
             let stylist = &<RecalcStyleAndConstructFlows as
                             DomTraversal<ServoLayoutElement>>::shared_context(&traversal).stylist;
             <RecalcStyleAndConstructFlows as
-             DomTraversal<ServoLayoutElement>>::pre_traverse(element, stylist, /* skip_root = */ false)
+             DomTraversal<ServoLayoutElement>>::pre_traverse(element, stylist, TraversalFlags::empty())
         };
 
         if token.should_traverse() {

components/style/traversal.rs

@@ -31,6 +31,28 @@ pub struct PerLevelTraversalData {
     pub current_dom_depth: Option<usize>,
 }
 
+bitflags! {
+    /// Represents that target elements of the traversal.
+    pub flags TraversalFlags: u8 {
+        /// Traverse only unstyled children.
+        const UNSTYLED_CHILDREN_ONLY = 0x01,
+        /// Traverse only elements for animation restyles
+        const ANIMATION_ONLY = 0x02,
+    }
+}
+
+impl TraversalFlags {
+    /// Returns true if the traversal is for animation-only restyles.
+    pub fn for_animation_only(&self) -> bool {
+        self.contains(ANIMATION_ONLY)
+    }
+
+    /// Returns true if the traversal is for unstyled children.
+    pub fn for_unstyled_children_only(&self) -> bool {
+        self.contains(UNSTYLED_CHILDREN_ONLY)
+    }
+}
+
 /// This structure exists to enforce that callers invoke pre_traverse, and also
 /// to pass information from the pre-traversal into the primary traversal.
 pub struct PreTraverseToken {
@@ -109,12 +131,13 @@ pub trait DomTraversal<E: TElement> : Sync {
     /// a traversal is needed. Returns a token that allows the caller to prove
     /// that the call happened.
     ///
-    /// The unstyled_children_only parameter is used in Gecko to style newly-
+    /// The traversal_flag is used in Gecko.
+    /// If traversal_flag::UNSTYLED_CHILDREN_ONLY is specified, style newly-
     /// appended children without restyling the parent.
-    fn pre_traverse(root: E, stylist: &Stylist, unstyled_children_only: bool)
+    fn pre_traverse(root: E, stylist: &Stylist, traversal_flags: TraversalFlags)
                     -> PreTraverseToken
     {
-        if unstyled_children_only {
+        if traversal_flags.for_unstyled_children_only() {
             return PreTraverseToken {
                 traverse: true,
                 unstyled_children_only: true,

ports/geckolib/glue.rs

@@ -83,7 +83,8 @@ use style::stylesheets::StylesheetLoader as StyleStylesheetLoader;
 use style::supports::parse_condition_or_declaration;
 use style::thread_state;
 use style::timer::Timer;
-use style::traversal::{resolve_style, DomTraversal, TraversalDriver};
+use style::traversal::{resolve_style, DomTraversal, TraversalDriver, TraversalFlags};
+use style::traversal::UNSTYLED_CHILDREN_ONLY;
 use style_traits::ToCss;
 use super::stylesheet_loader::StylesheetLoader;
 
@@ -143,7 +144,7 @@ fn create_shared_context<'a>(guard: &'a SharedRwLockReadGuard,
 }
 
 fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
-                    unstyled_children_only: bool) {
+                    traversal_flags: TraversalFlags) {
     // When new content is inserted in a display:none subtree, we will call into
     // servo to try to style it. Detect that here and bail out.
     if let Some(parent) = element.parent_element() {
@@ -155,7 +156,7 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
 
     let per_doc_data = PerDocumentStyleData::from_ffi(raw_data).borrow();
 
-    let token = RecalcStyleOnly::pre_traverse(element, &per_doc_data.stylist, unstyled_children_only);
+    let token = RecalcStyleOnly::pre_traverse(element, &per_doc_data.stylist, traversal_flags);
     if !token.should_traverse() {
         return;
     }
@@ -192,8 +193,13 @@ pub extern "C" fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
                                         behavior: structs::TraversalRootBehavior) -> bool {
     let element = GeckoElement(root);
     debug!("Servo_TraverseSubtree: {:?}", element);
-    traverse_subtree(element, raw_data,
+
-                     behavior == structs::TraversalRootBehavior::UnstyledChildrenOnly);
+    let traversal_flags = match behavior {
+        structs::TraversalRootBehavior::UnstyledChildrenOnly => UNSTYLED_CHILDREN_ONLY,
+        _ => TraversalFlags::empty(),
+    };
+
+    traverse_subtree(element, raw_data, traversal_flags);
 
     element.has_dirty_descendants() || element.mutate_data().unwrap().has_restyle()
 }