Filter out non-applying properties when cascading style for ::first-letter/::first-line/::placeholder. r=emilio

Part 3 of the fix for Gecko bug 1382786 <https://bugzilla.mozilla.org/show_bug.cgi?id=1382786>.

components/style/gecko/pseudo_element.rs

@@ -10,6 +10,8 @@
 
 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 selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
 use string_cache::Atom;
@@ -119,4 +121,15 @@ impl PseudoElement {
             _ => self.clone(),
         }
     }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        match *self {
+            PseudoElement::FirstLetter => Some(APPLIES_TO_FIRST_LETTER),
+            PseudoElement::FirstLine => Some(APPLIES_TO_FIRST_LINE),
+            PseudoElement::Placeholder => Some(APPLIES_TO_PLACEHOLDER),
+            _ => None,
+        }
+    }
 }

components/style/properties/properties.mako.rs

@@ -2872,11 +2872,26 @@ pub fn cascade(
                 &[]
             };
             let node_importance = node.importance();
+
+            let property_restriction = pseudo.and_then(|p| p.property_restriction());
+
             declarations
                 .iter()
                 // Yield declarations later in source order (with more precedence) first.
                 .rev()
                 .filter_map(move |&(ref declaration, declaration_importance)| {
+                    if let Some(property_restriction) = property_restriction {
+                        // declaration.id() is either a longhand or a custom
+                        // property.  Custom properties are always allowed, but
+                        // longhands are only allowed if they have our
+                        // property_restriction flag set.
+                        if let PropertyDeclarationId::Longhand(id) = declaration.id() {
+                            if !id.flags().contains(property_restriction) {
+                                return None
+                            }
+                        }
+                    }
+
                     if declaration_importance == node_importance {
                         Some((declaration, cascade_level))
                     } else {

components/style/servo/selector_parser.rs

@@ -13,6 +13,7 @@ use dom::{OpaqueNode, TElement, TNode};
 use element_state::ElementState;
 use fnv::FnvHashMap;
 use invalidation::element::element_wrapper::ElementSnapshot;
+use properties::PropertyFlags;
 use selector_parser::{AttrValue as SelectorAttrValue, ElementExt, PseudoElementCascadeType, SelectorParser};
 use selectors::Element;
 use selectors::attr::{AttrSelectorOperation, NamespaceConstraint, CaseSensitivity};
@@ -39,6 +40,12 @@ pub enum PseudoElement {
     Before,
     Selection,
     // If/when :first-letter is added, update is_first_letter accordingly.
+
+    // If/when ::first-letter, ::first-line, or ::placeholder are added, adjust
+    // our property_restriction implementation to do property filtering for
+    // them.  Also, make sure the UA sheet has the !important rules some of the
+    // APPLIES_TO_PLACEHOLDER properties expect!
+
     // Non-eager pseudos.
     DetailsSummary,
     DetailsContent,
@@ -169,6 +176,12 @@ impl PseudoElement {
 
     /// Stub, only Gecko needs this
     pub fn pseudo_info(&self) { () }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        None
+    }
 }
 
 /// The type used for storing pseudo-class string arguments.