Replace begin_styling with a centralized mechanism to set a node up for either styling or restyling.

We also make sure that an element never has an ElementData with ElementDataStyles::Uninitialized,
and eagerly call prepare_for_styling whenever an ElementData is instantiated.

MozReview-Commit-ID: 9YP6eSmdMt0

10 files changed, 127 insertions, 102 deletions

diff --git a/components/layout/traversal.rs b/components/layout/traversal.rs
index 49b8f1a32c6..d2ccad89d86 100644
--- a/components/layout/traversal.rs
+++ b/components/layout/traversal.rs
@@ -22,7 +22,7 @@ use style::traversal::{recalc_style_at, remove_from_bloom_filter};
 use style::traversal::RestyleResult;
 use style::traversal::take_thread_local_bloom_filter;
 use util::opts;
-use wrapper::{LayoutNodeHelpers, LayoutNodeLayoutData};
+use wrapper::{GetRawData, LayoutNodeHelpers, LayoutNodeLayoutData};
 
 pub struct RecalcStyleAndConstructFlows<'lc> {
     context: LayoutContext<'lc>,
@@ -131,9 +131,10 @@ impl<'lc, N> DomTraversalContext<N> for RecalcStyleAndConstructFlows<'lc>
         }
     }
 
-    fn ensure_element_data(element: &N::ConcreteElement) -> &AtomicRefCell<ElementData> {
+    #[allow(unsafe_code)]
+    unsafe fn ensure_element_data(element: &N::ConcreteElement) -> &AtomicRefCell<ElementData> {
         element.as_node().initialize_data();
-        element.get_style_data().unwrap()
+        element.get_data().unwrap()
     }
 
     fn local_context(&self) -> &LocalStyleContext {
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index 57bd44a5798..bc524395be2 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -33,10 +33,12 @@
 use core::nonzero::NonZero;
 use data::{LayoutDataFlags, PersistentLayoutData};
 use script_layout_interface::{OpaqueStyleAndLayoutData, PartialPersistentLayoutData};
-use script_layout_interface::wrapper_traits::{ThreadSafeLayoutElement, ThreadSafeLayoutNode};
+use script_layout_interface::wrapper_traits::{LayoutNode, ThreadSafeLayoutElement, ThreadSafeLayoutNode};
 use script_layout_interface::wrapper_traits::GetLayoutData;
 use style::atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use style::computed_values::content::{self, ContentItem};
+use style::dom::TElement;
+use style::traversal::prepare_for_styling;
 
 pub type NonOpaqueStyleAndLayoutData = AtomicRefCell<PersistentLayoutData>;
 
@@ -62,12 +64,24 @@ impl<T: GetLayoutData> LayoutNodeLayoutData for T {
     }
 }
 
+pub trait GetRawData {
+    fn get_raw_data(&self) -> Option<&NonOpaqueStyleAndLayoutData>;
+}
+
+impl<T: GetLayoutData> GetRawData for T {
+    fn get_raw_data(&self) -> Option<&NonOpaqueStyleAndLayoutData> {
+        self.get_style_and_layout_data().map(|opaque| {
+            let container = *opaque.ptr as *mut NonOpaqueStyleAndLayoutData;
+            unsafe { &*container }
+        })
+    }
+}
+
 pub trait LayoutNodeHelpers {
     fn initialize_data(&self);
-    fn get_raw_data(&self) -> Option<&NonOpaqueStyleAndLayoutData>;
 }
 
-impl<T: GetLayoutData> LayoutNodeHelpers for T {
+impl<T: LayoutNode> LayoutNodeHelpers for T {
     fn initialize_data(&self) {
         if self.get_raw_data().is_none() {
             let ptr: *mut NonOpaqueStyleAndLayoutData =
@@ -75,16 +89,12 @@ impl<T: GetLayoutData> LayoutNodeHelpers for T {
             let opaque = OpaqueStyleAndLayoutData {
                 ptr: unsafe { NonZero::new(ptr as *mut AtomicRefCell<PartialPersistentLayoutData>) }
             };
-            self.init_style_and_layout_data(opaque);
+            unsafe { self.init_style_and_layout_data(opaque) };
+            if let Some(el) = self.as_element() {
+                let _ = prepare_for_styling(el, el.get_data().unwrap());
+            }
         };
     }
-
-    fn get_raw_data(&self) -> Option<&NonOpaqueStyleAndLayoutData> {
-        self.get_style_and_layout_data().map(|opaque| {
-            let container = *opaque.ptr as *mut NonOpaqueStyleAndLayoutData;
-            unsafe { &*container }
-        })
-    }
 }
 
 pub trait ThreadSafeLayoutNodeHelpers {
diff --git a/components/script/layout_wrapper.rs b/components/script/layout_wrapper.rs
index 8e3bbbb737b..2120f4ae5cc 100644
--- a/components/script/layout_wrapper.rs
+++ b/components/script/layout_wrapper.rs
@@ -56,7 +56,7 @@ use std::mem::transmute;
 use std::sync::Arc;
 use std::sync::atomic::Ordering;
 use string_cache::{Atom, Namespace};
-use style::atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
+use style::atomic_refcell::{AtomicRef, AtomicRefCell};
 use style::attr::AttrValue;
 use style::computed_values::display;
 use style::context::SharedStyleContext;
@@ -252,6 +252,10 @@ impl<'ln> LayoutNode for ServoLayoutNode<'ln> {
         self.script_type_id().into()
     }
 
+    unsafe fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
+        self.get_jsmanaged().init_style_and_layout_data(data);
+    }
+
     fn has_changed(&self) -> bool {
         unsafe { self.node.get_flag(HAS_CHANGED) }
     }
@@ -269,42 +273,24 @@ impl<'ln> GetLayoutData for ServoLayoutNode<'ln> {
             self.get_jsmanaged().get_style_and_layout_data()
         }
     }
-
-    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
-        unsafe {
-            self.get_jsmanaged().init_style_and_layout_data(data);
-        }
-    }
 }
 
 impl<'le> GetLayoutData for ServoLayoutElement<'le> {
     fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
         self.as_node().get_style_and_layout_data()
     }
-
-    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
-        self.as_node().init_style_and_layout_data(data)
-    }
 }
 
 impl<'ln> GetLayoutData for ServoThreadSafeLayoutNode<'ln> {
     fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
         self.node.get_style_and_layout_data()
     }
-
-    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
-        self.node.init_style_and_layout_data(data)
-    }
 }
 
 impl<'le> GetLayoutData for ServoThreadSafeLayoutElement<'le> {
     fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData> {
         self.element.as_node().get_style_and_layout_data()
     }
-
-    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData) {
-        self.element.as_node().init_style_and_layout_data(data)
-    }
 }
 
 impl<'ln> ServoLayoutNode<'ln> {
@@ -506,20 +492,11 @@ impl<'le> TElement for ServoLayoutElement<'le> {
         old_value - 1
     }
 
-    fn begin_styling(&self) -> AtomicRefMut<ElementData> {
-        let mut data = self.mutate_data().unwrap();
-        data.gather_previous_styles(|| None);
-        data
-    }
-
     fn borrow_data(&self) -> Option<AtomicRef<ElementData>> {
-        self.get_style_data().map(|d| d.borrow())
+        self.get_data().map(|d| d.borrow())
     }
 
-}
-
-impl<'le> LayoutElement for ServoLayoutElement<'le> {
-    fn get_style_data(&self) -> Option<&AtomicRefCell<ElementData>> {
+    fn get_data(&self) -> Option<&AtomicRefCell<ElementData>> {
         unsafe {
             self.get_style_and_layout_data().map(|d| {
                 let ppld: &AtomicRefCell<PartialPersistentLayoutData> = &**d.ptr;
@@ -551,10 +528,6 @@ impl<'le> ServoLayoutElement<'le> {
         }
     }
 
-    fn mutate_data(&self) -> Option<AtomicRefMut<ElementData>> {
-        self.get_style_data().map(|d| d.borrow_mut())
-    }
-
     fn get_partial_layout_data(&self) -> Option<&AtomicRefCell<PartialPersistentLayoutData>> {
         unsafe {
             self.get_style_and_layout_data().map(|d| &**d.ptr)
@@ -836,7 +809,7 @@ impl<'ln> ThreadSafeLayoutNode for ServoThreadSafeLayoutNode<'ln> {
         // also not visible to script.)
         debug_assert!(self.is_text_node());
         let parent = self.node.parent_node().unwrap().as_element().unwrap();
-        let parent_data = parent.get_style_data().unwrap().borrow();
+        let parent_data = parent.get_data().unwrap().borrow();
         parent_data.current_styles().primary.clone()
     }
 
@@ -1089,10 +1062,12 @@ impl<'le> ThreadSafeLayoutElement for ServoThreadSafeLayoutElement<'le> {
     }
 
     fn get_style_data(&self) -> Option<&AtomicRefCell<ElementData>> {
-        self.element.get_style_data()
+        self.element.get_data()
     }
 }
 
+impl<'le> LayoutElement for ServoLayoutElement<'le> {}
+
 /// This implementation of `::selectors::Element` is used for implementing lazy
 /// pseudo-elements.
 ///
diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index 470685979b7..5f712046eea 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -73,7 +73,6 @@ impl<T> PseudoElementType<T> {
 /// Trait to abstract access to layout data across various data structures.
 pub trait GetLayoutData {
     fn get_style_and_layout_data(&self) -> Option<OpaqueStyleAndLayoutData>;
-    fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData);
 }
 
 /// A wrapper so that layout can access only the methods that it should have access to. Layout must
@@ -85,6 +84,8 @@ pub trait LayoutNode: GetLayoutData + TNode {
     /// Returns the type ID of this node.
     fn type_id(&self) -> LayoutNodeType;
 
+    unsafe fn init_style_and_layout_data(&self, data: OpaqueStyleAndLayoutData);
+
     fn has_changed(&self) -> bool;
 
     unsafe fn clear_dirty_bits(&self);
@@ -274,7 +275,6 @@ pub trait DangerousThreadSafeLayoutNode: ThreadSafeLayoutNode {
 }
 
 pub trait LayoutElement: Clone + Copy + Sized + Debug + GetLayoutData + TElement {
-    fn get_style_data(&self) -> Option<&AtomicRefCell<ElementData>>;
 }
 
 pub trait ThreadSafeLayoutElement: Clone + Copy + Sized + Debug +
diff --git a/components/style/atomic_refcell.rs b/components/style/atomic_refcell.rs
index a45a682b638..409d669b595 100644
--- a/components/style/atomic_refcell.rs
+++ b/components/style/atomic_refcell.rs
@@ -27,10 +27,10 @@ impl<T> AtomicRefCell<T> {
         AtomicRefCell(RwLock::new(value))
     }
     pub fn borrow(&self) -> AtomicRef<T> {
-        self.0.try_read().unwrap()
+        self.0.try_read().expect("already mutably borrowed")
     }
     pub fn borrow_mut(&self) -> AtomicRefMut<T> {
-        self.0.try_write().unwrap()
+        self.0.try_write().expect("already borrowed")
     }
 }
 
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 7ab817ac2c6..46949946607 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -6,7 +6,7 @@
 
 #![allow(unsafe_code)]
 
-use atomic_refcell::{AtomicRef, AtomicRefMut};
+use atomic_refcell::{AtomicRef, AtomicRefCell};
 use data::{ElementStyles, ElementData};
 use element_state::ElementState;
 use parking_lot::RwLock;
@@ -243,30 +243,43 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
             Stop
         };
 
-        match self.borrow_data() {
-            // No node data, no style on the frame.
+        let mut mode = match self.borrow_data() {
+            // No element data, no style on the frame.
             None if !self.frame_has_style() => Initial,
-            // No node data, style on the frame.
+            // No element data, style on the frame.
             None => mode_for_descendants,
+            // We have element data. Decide below.
             Some(d) => {
-                if d.restyle_data.is_some() || self.deprecated_dirty_bit_is_set() {
-                    Restyle
-                } else {
-                    debug_assert!(!self.frame_has_style()); // display:none etc
+                if d.has_current_styles() {
+                    // The element has up-to-date style.
+                    debug_assert!(!self.frame_has_style());
+                    debug_assert!(d.restyle_data.is_none());
                     mode_for_descendants
+                } else {
+                    // The element needs processing.
+                    if d.previous_styles().is_some() {
+                        Restyle
+                    } else {
+                        Initial
+                    }
                 }
             },
+        };
+
+        // Handle the deprecated dirty bit. This should go away soon.
+        if mode != Initial && self.deprecated_dirty_bit_is_set() {
+            mode = Restyle;
         }
-    }
+        mode
 
-    /// Sets up the appropriate data structures to style a node, returing a
-    /// mutable handle to the node data upon which further style calculations
-    /// can be performed.
-    fn begin_styling(&self) -> AtomicRefMut<ElementData>;
+    }
 
     /// Immutable borrows the ElementData.
     fn borrow_data(&self) -> Option<AtomicRef<ElementData>>;
 
+    /// Gets a reference to the ElementData container.
+    fn get_data(&self) -> Option<&AtomicRefCell<ElementData>>;
+
     /// Properly marks nodes as dirty in response to restyle hints.
     fn note_restyle_hint<C: DomTraversalContext<Self::ConcreteNode>>(&self, hint: RestyleHint) {
         // Bail early if there's no restyling to do.
@@ -285,13 +298,13 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
 
         // Process hints.
         if hint.contains(RESTYLE_SELF) {
-            unsafe { C::ensure_element_data(self).borrow_mut().ensure_restyle_data(); }
+            unsafe { let _ = C::prepare_for_styling(self); }
         // XXX(emilio): For now, dirty implies dirty descendants if found.
         } else if hint.contains(RESTYLE_DESCENDANTS) {
             unsafe { self.set_dirty_descendants(); }
             let mut current = self.first_child_element();
             while let Some(el) = current {
-                unsafe { C::ensure_element_data(&el).borrow_mut().ensure_restyle_data(); }
+                unsafe { let _ = C::prepare_for_styling(&el); }
                 current = el.next_sibling_element();
             }
         }
@@ -299,7 +312,7 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
         if hint.contains(RESTYLE_LATER_SIBLINGS) {
             let mut next = ::selectors::Element::next_sibling_element(self);
             while let Some(sib) = next {
-                unsafe { C::ensure_element_data(&sib).borrow_mut().ensure_restyle_data() };
+                unsafe { let _ = C::prepare_for_styling(&sib); }
                 next = ::selectors::Element::next_sibling_element(&sib);
             }
         }
diff --git a/components/style/gecko/traversal.rs b/components/style/gecko/traversal.rs
index 0825d83665f..4334cbeb6e0 100644
--- a/components/style/gecko/traversal.rs
+++ b/components/style/gecko/traversal.rs
@@ -55,7 +55,7 @@ impl<'lc, 'ln> DomTraversalContext<GeckoNode<'ln>> for RecalcStyleOnly<'lc> {
         }
     }
 
-    fn ensure_element_data<'a>(element: &'a GeckoElement<'ln>) -> &'a AtomicRefCell<ElementData> {
+    unsafe fn ensure_element_data<'a>(element: &'a GeckoElement<'ln>) -> &'a AtomicRefCell<ElementData> {
         element.ensure_data()
     }
 
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index 045240565fb..d57d4ecd4a0 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -5,7 +5,7 @@
 #![allow(unsafe_code)]
 
 
-use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
+use atomic_refcell::{AtomicRef, AtomicRefCell};
 use data::ElementData;
 use dom::{LayoutIterator, NodeInfo, TDocument, TElement, TNode, TRestyleDamage, UnsafeNode};
 use dom::{OpaqueNode, PresentationalHintsSynthetizer};
@@ -337,12 +337,8 @@ impl<'le> GeckoElement<'le> {
                                                .get(pseudo).map(|c| c.clone()))
     }
 
-    fn get_node_data(&self) -> Option<&AtomicRefCell<ElementData>> {
-        unsafe { self.raw_node().mServoData.get().as_ref() }
-    }
-
     pub fn ensure_data(&self) -> &AtomicRefCell<ElementData> {
-        match self.get_node_data() {
+        match self.get_data() {
             Some(x) => x,
             None => {
                 let ptr = Box::into_raw(Box::new(AtomicRefCell::new(ElementData::new())));
@@ -431,7 +427,7 @@ impl<'le> TElement for GeckoElement<'le> {
     fn has_dirty_descendants(&self) -> bool {
         // Return true unconditionally if we're not yet styled. This is a hack
         // and should go away soon.
-        if self.get_node_data().is_none() {
+        if self.get_data().is_none() {
             return true;
         }
         self.flags() & (NODE_HAS_DIRTY_DESCENDANTS_FOR_SERVO as u32) != 0
@@ -449,15 +445,14 @@ impl<'le> TElement for GeckoElement<'le> {
         panic!("Atomic child count not implemented in Gecko");
     }
 
-    fn begin_styling(&self) -> AtomicRefMut<ElementData> {
-        let mut data = self.ensure_data().borrow_mut();
-        data.gather_previous_styles(|| self.get_styles_from_frame());
-        data
+    fn borrow_data(&self) -> Option<AtomicRef<ElementData>> {
+        self.get_data().map(|x| x.borrow())
     }
 
-    fn borrow_data(&self) -> Option<AtomicRef<ElementData>> {
-        self.get_node_data().map(|x| x.borrow())
+    fn get_data(&self) -> Option<&AtomicRefCell<ElementData>> {
+        unsafe { self.raw_node().mServoData.get().as_ref() }
     }
+
 }
 
 impl<'le> PartialEq for GeckoElement<'le> {
diff --git a/components/style/matching.rs b/components/style/matching.rs
index a5216d14416..e90ff962a65 100644
--- a/components/style/matching.rs
+++ b/components/style/matching.rs
@@ -8,10 +8,11 @@
 
 use animation;
 use arc_ptr_eq;
+use atomic_refcell::AtomicRefMut;
 use cache::{LRUCache, SimpleHashCache};
 use cascade_info::CascadeInfo;
 use context::{SharedStyleContext, StyleContext};
-use data::{ElementStyles, PseudoStyles};
+use data::{ElementData, ElementStyles, PseudoStyles};
 use dom::{TElement, TNode, TRestyleDamage, UnsafeNode};
 use properties::{CascadeFlags, ComputedValues, SHAREABLE, cascade};
 use properties::longhands::display::computed_value as display;
@@ -423,6 +424,7 @@ impl StyleSharingCandidateCache {
 
     pub fn insert_if_possible<E: TElement>(&mut self,
                                            element: &E,
+                                           style: &Arc<ComputedValues>,
                                            relations: StyleRelations) {
         use traversal::relations_are_shareable;
 
@@ -441,9 +443,6 @@ impl StyleSharingCandidateCache {
             return;
         }
 
-        let data = element.borrow_data().unwrap();
-        let style = &data.current_styles().primary;
-
         let box_style = style.get_box();
         if box_style.transition_property_count() > 0 {
             debug!("Failing to insert to the cache: transitions");
@@ -695,7 +694,8 @@ pub trait MatchMethods : TElement {
     unsafe fn share_style_if_possible(&self,
                                       style_sharing_candidate_cache:
                                         &mut StyleSharingCandidateCache,
-                                      shared_context: &SharedStyleContext)
+                                      shared_context: &SharedStyleContext,
+                                      data: &mut AtomicRefMut<ElementData>)
                                       -> StyleSharingResult<Self::ConcreteRestyleDamage> {
         if opts::get().disable_share_style_cache {
             return StyleSharingResult::CannotShare
@@ -715,7 +715,6 @@ pub trait MatchMethods : TElement {
             match sharing_result {
                 Ok(shared_style) => {
                     // Yay, cache hit. Share the style.
-                    let mut data = self.begin_styling();
 
                     // TODO: add the display: none optimisation here too! Even
                     // better, factor it out/make it a bit more generic so Gecko
@@ -855,6 +854,7 @@ pub trait MatchMethods : TElement {
 
     unsafe fn cascade_node<'a, Ctx>(&self,
                                     context: &Ctx,
+                                    mut data: AtomicRefMut<ElementData>,
                                     parent: Option<Self>,
                                     applicable_declarations: &ApplicableDeclarations)
                                     -> RestyleResult
@@ -864,7 +864,6 @@ pub trait MatchMethods : TElement {
         let parent_data = parent.as_ref().map(|x| x.borrow_data().unwrap());
         let parent_style = parent_data.as_ref().map(|x| &x.current_styles().primary);
 
-        let mut data = self.begin_styling();
         let mut new_styles;
 
         let mut applicable_declarations_cache =
diff --git a/components/style/traversal.rs b/components/style/traversal.rs
index 5d5e5e6f418..38ed543d017 100644
--- a/components/style/traversal.rs
+++ b/components/style/traversal.rs
@@ -4,7 +4,7 @@
 
 //! Traversing the DOM tree; the bloom filter.
 
-use atomic_refcell::AtomicRefCell;
+use atomic_refcell::{AtomicRefCell, AtomicRefMut};
 use context::{LocalStyleContext, SharedStyleContext, StyleContext};
 use data::ElementData;
 use dom::{OpaqueNode, StylingMode, TElement, TNode, UnsafeNode};
@@ -12,6 +12,7 @@ use matching::{ApplicableDeclarations, MatchMethods, StyleSharingResult};
 use selectors::bloom::BloomFilter;
 use selectors::matching::StyleRelations;
 use std::cell::RefCell;
+use std::mem;
 use std::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT, Ordering};
 use tid::tid;
 use util::opts;
@@ -156,6 +157,18 @@ pub fn remove_from_bloom_filter<'a, N, C>(context: &C, root: OpaqueNode, node: N
     };
 }
 
+pub fn prepare_for_styling<E: TElement>(element: E,
+                                        data: &AtomicRefCell<ElementData>)
+                                        -> AtomicRefMut<ElementData> {
+    let mut d = data.borrow_mut();
+    d.gather_previous_styles(|| element.get_styles_from_frame());
+    if d.previous_styles().is_some() {
+        d.ensure_restyle_data();
+    }
+
+    d
+}
+
 pub trait DomTraversalContext<N: TNode> {
     type SharedContext: Sync + 'static;
 
@@ -200,7 +213,17 @@ pub trait DomTraversalContext<N: TNode> {
     /// Ensures the existence of the ElementData, and returns it. This can't live
     /// on TNode because of the trait-based separation between Servo's script
     /// and layout crates.
-    fn ensure_element_data(element: &N::ConcreteElement) -> &AtomicRefCell<ElementData>;
+    ///
+    /// This is only safe to call in top-down traversal before processing the
+    /// children of |element|.
+    unsafe fn ensure_element_data(element: &N::ConcreteElement) -> &AtomicRefCell<ElementData>;
+
+    /// Sets up the appropriate data structures to style or restyle a node,
+    /// returing a mutable handle to the node data upon which further style
+    /// calculations can be performed.
+    unsafe fn prepare_for_styling(element: &N::ConcreteElement) -> AtomicRefMut<ElementData> {
+        prepare_for_styling(*element, Self::ensure_element_data(element))
+    }
 
     fn local_context(&self) -> &LocalStyleContext;
 }
@@ -267,6 +290,7 @@ fn ensure_element_styled_internal<'a, E, C>(element: E,
     // probably not necessary since we're likely to be matching only a few
     // nodes, at best.
     let mut applicable_declarations = ApplicableDeclarations::new();
+    let data = prepare_for_styling(element, element.get_data().unwrap());
     let stylist = &context.shared_context().stylist;
 
     element.match_element(&**stylist,
@@ -274,7 +298,7 @@ fn ensure_element_styled_internal<'a, E, C>(element: E,
                           &mut applicable_declarations);
 
     unsafe {
-        element.cascade_node(context, parent, &applicable_declarations);
+        element.cascade_node(context, data, parent, &applicable_declarations);
     }
 }
 
@@ -295,6 +319,8 @@ pub fn recalc_style_at<'a, E, C, D>(context: &'a C,
     let mode = element.styling_mode();
     debug_assert!(mode != StylingMode::Stop, "Parent should not have enqueued us");
     if mode != StylingMode::Traverse {
+        let mut data = unsafe { D::prepare_for_styling(&element) };
+
         // Check to see whether we can share a style with someone.
         let style_sharing_candidate_cache =
             &mut context.local_context().style_sharing_candidate_cache.borrow_mut();
@@ -302,7 +328,8 @@ pub fn recalc_style_at<'a, E, C, D>(context: &'a C,
         let sharing_result = if element.parent_element().is_none() {
             StyleSharingResult::CannotShare
         } else {
-            unsafe { element.share_style_if_possible(style_sharing_candidate_cache, context.shared_context()) }
+            unsafe { element.share_style_if_possible(style_sharing_candidate_cache,
+                                                     context.shared_context(), &mut data) }
         };
 
         // Otherwise, match and cascade selectors.
@@ -334,14 +361,19 @@ pub fn recalc_style_at<'a, E, C, D>(context: &'a C,
 
                 // Perform the CSS cascade.
                 unsafe {
-                    restyle_result = element.cascade_node(context,
+                    restyle_result = element.cascade_node(context, data,
                                                           element.parent_element(),
                                                           &applicable_declarations);
                 }
 
                 // Add ourselves to the LRU cache.
                 if let Some(element) = shareable_element {
-                    style_sharing_candidate_cache.insert_if_possible(&element, relations);
+                    style_sharing_candidate_cache.insert_if_possible(&element,
+                                                                     &element.borrow_data()
+                                                                             .unwrap()
+                                                                             .current_styles()
+                                                                             .primary,
+                                                                     relations);
                 }
             }
             StyleSharingResult::StyleWasShared(index, damage, cached_restyle_result) => {
@@ -350,6 +382,10 @@ pub fn recalc_style_at<'a, E, C, D>(context: &'a C,
                     STYLE_SHARING_CACHE_HITS.fetch_add(1, Ordering::Relaxed);
                 }
                 style_sharing_candidate_cache.touch(index);
+
+                // Drop the mutable borrow early, since Servo's set_restyle_damage also borrows.
+                mem::drop(data);
+
                 element.set_restyle_damage(damage);
             }
         }
@@ -361,11 +397,7 @@ pub fn recalc_style_at<'a, E, C, D>(context: &'a C,
     if mode == StylingMode::Restyle && restyle_result == RestyleResult::Continue {
         for kid in element.as_node().children() {
             if let Some(kid) = kid.as_element() {
-                let mut data = D::ensure_element_data(&kid).borrow_mut();
-                data.gather_previous_styles(|| kid.get_styles_from_frame());
-                if data.previous_styles().is_some() {
-                    data.ensure_restyle_data();
-                }
+                unsafe { let _ = D::prepare_for_styling(&kid); }
             }
         }
     }