Don't reconstruct the layout object when going from no pseudo to pseudo with no content for ::before and ::after.

Fixes Gecko bug 1376073.  r=emilio

components/style/context.rs

@@ -232,7 +232,7 @@ impl Clone for EagerPseudoCascadeInputs {
 impl EagerPseudoCascadeInputs {
     /// Construct inputs from previous cascade results, if any.
     fn new_from_style(styles: &EagerPseudoStyles) -> Self {
-        EagerPseudoCascadeInputs(styles.as_array().map(|styles| {
+        EagerPseudoCascadeInputs(styles.as_optional_array().map(|styles| {
             let mut inputs: [Option<CascadeInputs>; EAGER_PSEUDO_COUNT] = Default::default();
             for i in 0..EAGER_PSEUDO_COUNT {
                 inputs[i] = styles[i].as_ref().map(|s| CascadeInputs::new_from_style(s));

components/style/data.rs

@@ -162,6 +162,13 @@ impl fmt::Debug for EagerPseudoArray {
     }
 }
 
+// Can't use [None; EAGER_PSEUDO_COUNT] here because it complains
+// about Copy not being implemented for our Arc type.
+#[cfg(feature = "gecko")]
+const EMPTY_PSEUDO_ARRAY: &'static EagerPseudoArrayInner = &[None, None, None, None];
+#[cfg(feature = "servo")]
+const EMPTY_PSEUDO_ARRAY: &'static EagerPseudoArrayInner = &[None, None, None];
+
 impl EagerPseudoStyles {
     /// Returns whether there are any pseudo styles.
     pub fn is_empty(&self) -> bool {
@@ -169,13 +176,19 @@ impl EagerPseudoStyles {
     }
 
     /// Grabs a reference to the list of styles, if they exist.
-    pub fn as_array(&self) -> Option<&EagerPseudoArrayInner> {
+    pub fn as_optional_array(&self) -> Option<&EagerPseudoArrayInner> {
         match self.0 {
             None => None,
             Some(ref x) => Some(&x.0),
         }
     }
 
+    /// Grabs a reference to the list of styles or a list of None if
+    /// there are no styles to be had.
+    pub fn as_array(&self) -> &EagerPseudoArrayInner {
+        self.as_optional_array().unwrap_or(EMPTY_PSEUDO_ARRAY)
+    }
+
     /// Returns a reference to the style for a given eager pseudo, if it exists.
     pub fn get(&self, pseudo: &PseudoElement) -> Option<&Arc<ComputedValues>> {
         debug_assert!(pseudo.is_eager());

components/style/gecko/pseudo_element.rs

@@ -12,6 +12,8 @@ use cssparser::{ToCss, serialize_identifier};
 use gecko_bindings::structs::{self, CSSPseudoElementType};
 use properties::{PropertyFlags, APPLIES_TO_FIRST_LETTER, APPLIES_TO_FIRST_LINE};
 use properties::APPLIES_TO_PLACEHOLDER;
+use properties::ComputedValues;
+use properties::longhands::display::computed_value as display;
 use selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
 use string_cache::Atom;
@@ -132,4 +134,19 @@ impl PseudoElement {
             _ => None,
         }
     }
+
+    /// Whether this pseudo-element should actually exist if it has
+    /// the given styles.
+    pub fn should_exist(&self, style: &ComputedValues) -> bool
+    {
+        let display = style.get_box().clone_display();
+        if display == display::T::none {
+            return false;
+        }
+        if self.is_before_or_after() && style.ineffective_content_property() {
+            return false;
+        }
+
+        true
+    }
 }

components/style/matching.rs

@@ -522,18 +522,13 @@ pub trait MatchMethods : TElement {
         );
 
         if data.styles.pseudos.is_empty() && old_styles.pseudos.is_empty() {
-            return cascade_requirement;
+            // This is the common case; no need to examine pseudos here.
-        }
-
-        // If it matched a different number of pseudos, reconstruct.
-        if data.styles.pseudos.is_empty() != old_styles.pseudos.is_empty() {
-            data.restyle.damage |= RestyleDamage::reconstruct();
             return cascade_requirement;
         }
 
         let pseudo_styles =
-            old_styles.pseudos.as_array().unwrap().iter().zip(
+            old_styles.pseudos.as_array().iter().zip(
-            data.styles.pseudos.as_array().unwrap().iter());
+            data.styles.pseudos.as_array().iter());
 
         for (i, (old, new)) in pseudo_styles.enumerate() {
             match (old, new) {
@@ -548,11 +543,24 @@ pub trait MatchMethods : TElement {
                 }
                 (&None, &None) => {},
                 _ => {
+                    // It's possible that we're switching from not having
+                    // ::before/::after at all to having styles for them but not
+                    // actually having a useful pseudo-element.  Check for that
+                    // case.
+                    let pseudo = PseudoElement::from_eager_index(i);
+                    let new_pseudo_should_exist =
+                        new.as_ref().map_or(false,
+                                            |s| pseudo.should_exist(s));
+                    let old_pseudo_should_exist =
+                        old.as_ref().map_or(false,
+                                            |s| pseudo.should_exist(s));
+                    if new_pseudo_should_exist != old_pseudo_should_exist {
                         data.restyle.damage |= RestyleDamage::reconstruct();
                         return cascade_requirement;
                     }
                 }
             }
+        }
 
         cascade_requirement
     }
@@ -790,6 +798,9 @@ pub trait MatchMethods : TElement {
         }
 
         if pseudo.map_or(false, |p| p.is_before_or_after()) {
+            // FIXME(bz) This duplicates some of the logic in
+            // PseudoElement::should_exist, but it's not clear how best to share
+            // that logic without redoing the "get the display" work.
             let old_style_generates_no_pseudo =
                 old_style_is_display_none ||
                 old_values.ineffective_content_property();

components/style/servo/selector_parser.rs

@@ -13,7 +13,9 @@ use dom::{OpaqueNode, TElement, TNode};
 use element_state::ElementState;
 use fnv::FnvHashMap;
 use invalidation::element::element_wrapper::ElementSnapshot;
+use properties::ComputedValues;
 use properties::PropertyFlags;
+use properties::longhands::display::computed_value as display;
 use selector_parser::{AttrValue as SelectorAttrValue, ElementExt, PseudoElementCascadeType, SelectorParser};
 use selectors::Element;
 use selectors::attr::{AttrSelectorOperation, NamespaceConstraint, CaseSensitivity};
@@ -181,6 +183,21 @@ impl PseudoElement {
     pub fn property_restriction(&self) -> Option<PropertyFlags> {
         None
     }
+
+    /// Whether this pseudo-element should actually exist if it has
+    /// the given styles.
+    pub fn should_exist(&self, style: &ComputedValues) -> bool
+    {
+        let display = style.get_box().clone_display();
+        if display == display::T::none {
+            return false;
+        }
+        if self.is_before_or_after() && style.ineffective_content_property() {
+            return false;
+        }
+
+        true
+    }
 }
 
 /// The type used for storing pseudo-class string arguments.