Auto merge of #16967 - emilio:after, r=heycam,emilio

Bug 1366144: Correctly diff ::before and ::after pseudo-element styles if there's no generated content. r=heycam

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components/style/properties/gecko.mako.rs

@@ -141,6 +141,13 @@ impl ComputedValues {
         self.get_box().clone_display() == longhands::display::computed_value::T::contents
     }
 
+    /// Returns true if the value of the `content` property would make a
+    /// pseudo-element not rendered.
+    #[inline]
+    pub fn ineffective_content_property(&self) -> bool {
+        self.get_counters().ineffective_content_property()
+    }
+
     % for style_struct in data.style_structs:
     #[inline]
     pub fn clone_${style_struct.name_lower}(&self) -> Arc<style_structs::${style_struct.name}> {
@@ -4204,6 +4211,10 @@ clip-path
 
 <%self:impl_trait style_struct_name="Counters"
                   skip_longhands="content counter-increment counter-reset">
+    pub fn ineffective_content_property(&self) -> bool {
+        self.gecko.mContents.is_empty()
+    }
+
     pub fn set_content(&mut self, v: longhands::content::computed_value::T) {
         use properties::longhands::content::computed_value::T;
         use properties::longhands::content::computed_value::ContentItem;

components/style/properties/properties.mako.rs

@@ -1813,6 +1813,19 @@ impl ComputedValues {
     /// Since this isn't supported in Servo, this is always false for Servo.
     pub fn is_display_contents(&self) -> bool { false }
 
+    #[inline]
+    /// Returns whether the "content" property for the given style is completely
+    /// ineffective, and would yield an empty `::before` or `::after`
+    /// pseudo-element.
+    pub fn ineffective_content_property(&self) -> bool {
+        use properties::longhands::content::computed_value::T;
+        match self.get_counters().content {
+            T::normal |
+            T::none => true,
+            T::Content(ref items) => items.is_empty(),
+        }
+    }
+
     /// Whether the current style is multicolumn.
     #[inline]
     pub fn is_multicol(&self) -> bool {