style: Make the allocation of AuthorStyles for ShadowRoot lazy.

So that we don't waste a bunch of memory with stuff like <svg:use>. I
plan to shrink AuthorStyles further, but this should help regardless, and isn't
very complex.

Differential Revision: https://phabricator.services.mozilla.com/D4618

components/style/dom.rs

@@ -342,7 +342,7 @@ pub trait TShadowRoot: Sized + Copy + Clone + PartialEq {
     fn host(&self) -> <Self::ConcreteNode as TNode>::ConcreteElement;
 
     /// Get the style data for this ShadowRoot.
-    fn style_data<'a>(&self) -> &'a CascadeData
+    fn style_data<'a>(&self) -> Option<&'a CascadeData>
     where
         Self: 'a;
 
@@ -824,30 +824,36 @@ pub trait TElement:
 
         if let Some(shadow) = self.containing_shadow() {
             doc_rules_apply = false;
+            if let Some(data) = shadow.style_data() {
                 f(
-                shadow.style_data(),
+                    data,
                     self.as_node().owner_doc().quirks_mode(),
                     Some(shadow.host()),
                 );
             }
+        }
 
         if let Some(shadow) = self.shadow_root() {
+            if let Some(data) = shadow.style_data() {
                 f(
-                shadow.style_data(),
+                    data,
                     self.as_node().owner_doc().quirks_mode(),
                     Some(shadow.host()),
                 );
             }
+        }
 
         let mut current = self.assigned_slot();
         while let Some(slot) = current {
             // Slots can only have assigned nodes when in a shadow tree.
             let shadow = slot.containing_shadow().unwrap();
+            if let Some(data) = shadow.style_data() {
                 f(
-                shadow.style_data(),
+                    data,
                     self.as_node().owner_doc().quirks_mode(),
                     Some(shadow.host()),
                 );
+            }
             current = slot.assigned_slot();
         }
 

components/style/gecko/wrapper.rs

@@ -167,15 +167,14 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
     }
 
     #[inline]
-    fn style_data<'a>(&self) -> &'a CascadeData
+    fn style_data<'a>(&self) -> Option<&'a CascadeData>
     where
         Self: 'a,
     {
-        debug_assert!(!self.0.mServoStyles.mPtr.is_null());
-
         let author_styles = unsafe {
-            &*(self.0.mServoStyles.mPtr as *const structs::RawServoAuthorStyles
-                as *const bindings::RawServoAuthorStyles)
+            (self.0.mServoStyles.mPtr
+                as *const structs::RawServoAuthorStyles
+                as *const bindings::RawServoAuthorStyles).as_ref()?
         };
 
 
@@ -187,7 +186,7 @@ impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
                 author_styles.stylesheets.dirty()
         );
 
-        &author_styles.data
+        Some(&author_styles.data)
     }
 
     #[inline]

components/style/stylist.rs

@@ -1206,7 +1206,7 @@ impl Stylist {
         // to add some sort of AuthorScoped cascade level or something.
         if matches_author_rules {
             if let Some(shadow) = rule_hash_target.shadow_root() {
-                if let Some(map) = shadow.style_data().host_rules(pseudo_element) {
+                if let Some(map) = shadow.style_data().and_then(|data| data.host_rules(pseudo_element)) {
                     context.with_shadow_host(Some(rule_hash_target), |context| {
                         map.get_all_matching_rules(
                             element,
@@ -1233,8 +1233,7 @@ impl Stylist {
 
             for slot in slots.iter().rev() {
                 let shadow = slot.containing_shadow().unwrap();
-                let styles = shadow.style_data();
-                if let Some(map) = styles.slotted_rules(pseudo_element) {
+                if let Some(map) = shadow.style_data().and_then(|data| data.slotted_rules(pseudo_element)) {
                     context.with_shadow_host(Some(shadow.host()), |context| {
                         map.get_all_matching_rules(
                             element,
@@ -1253,7 +1252,7 @@ impl Stylist {
             if let Some(containing_shadow) = rule_hash_target.containing_shadow() {
                 let cascade_data = containing_shadow.style_data();
                 let host = containing_shadow.host();
-                if let Some(map) = cascade_data.normal_rules(pseudo_element) {
+                if let Some(map) = cascade_data.and_then(|data| data.normal_rules(pseudo_element)) {
                     context.with_shadow_host(Some(host), |context| {
                         map.get_all_matching_rules(
                             element,
@@ -1283,6 +1282,11 @@ impl Stylist {
                 //
                 // See: https://github.com/w3c/svgwg/issues/505
                 //
+                // FIXME(emilio, bug 1487259): We now do after bug 1483882, we
+                // should jump out of the <svg:use> shadow tree chain now.
+                //
+                // Unless the used node is cross-doc, I guess, in which case doc
+                // rules are probably ok...
                 let host_is_svg_use =
                     host.is_svg_element() &&
                     host.local_name() == &*local_name!("use");
@@ -1431,11 +1435,15 @@ impl Stylist {
         // [2]: https://github.com/w3c/csswg-drafts/issues/1995
         // [3]: https://bugzil.la/1458189
         if let Some(shadow) = element.shadow_root() {
-            try_find_in!(shadow.style_data());
+            if let Some(data) = shadow.style_data() {
+                try_find_in!(data);
+            }
         }
 
         if let Some(shadow) = element.containing_shadow() {
-            try_find_in!(shadow.style_data());
+            if let Some(data) = shadow.style_data() {
+                try_find_in!(data);
+            }
         } else {
             try_find_in!(self.cascade_data.author);
         }