Don't add rule feature data for rules which may never match.

:host and :host-context must always be in the rightmost compound with
no other simple selectors except a succeding pseudo element in order to
match as the host element is feature-less in that context. It's however
not an invalid selector according to the CSS Scoping spec, so we
shouldn't drop it at parse time.

We could potentially skip adding other selectors to rulesets as well
including selectors like:

:hover:not(:hover), div:not(div), ::content and ::slotted not in
shadow trees, etc.

R=kochi@chromium.org,ericwilligers@chromium.org
BUG=489481

Review URL: https://codereview.chromium.org/1703893002

Cr-Commit-Position: refs/heads/master@{#376116}

third_party/WebKit/Source/core/css/CSSSelector.h

@@ -253,7 +253,6 @@ public:
 
     bool isAdjacentSelector() const { return m_relation == DirectAdjacent || m_relation == IndirectAdjacent; }
     bool isShadowSelector() const { return m_relation == ShadowPseudo || m_relation == ShadowDeep; }
-    bool isSiblingPseudo() const;
     bool isAttributeSelector() const { return m_match >= FirstAttributeSelectorMatch; }
     bool isHostPseudoClass() const { return m_pseudoType == PseudoHost || m_pseudoType == PseudoHostContext; }
     bool isInsertionPointCrossing() const { return m_pseudoType == PseudoHostContext || m_pseudoType == PseudoContent || m_pseudoType == PseudoSlotted; }
@@ -365,22 +364,6 @@ inline CSSSelector::AttributeMatchType CSSSelector::attributeMatchType() const
     return m_data.m_rareData->m_bits.m_attributeMatchType;
 }
 
-inline bool CSSSelector::isSiblingPseudo() const
-{
-    PseudoType type = pseudoType();
-    return type == PseudoEmpty
-        || type == PseudoFirstChild
-        || type == PseudoFirstOfType
-        || type == PseudoLastChild
-        || type == PseudoLastOfType
-        || type == PseudoOnlyChild
-        || type == PseudoOnlyOfType
-        || type == PseudoNthChild
-        || type == PseudoNthOfType
-        || type == PseudoNthLastChild
-        || type == PseudoNthLastOfType;
-}
-
 inline bool CSSSelector::isASCIILower(const AtomicString& value)
 {
     for (size_t i = 0; i < value.length(); ++i) {

third_party/WebKit/Source/core/css/RuleFeature.cpp

@@ -556,55 +556,90 @@ void RuleFeatureSet::addFeaturesToInvalidationSets(const CSSSelector* selector,
     }
 }
 
-void RuleFeatureSet::collectFeaturesFromRuleData(const RuleData& ruleData)
+RuleFeatureSet::SelectorPreMatch RuleFeatureSet::collectFeaturesFromRuleData(const RuleData& ruleData)
 {
-    updateInvalidationSets(ruleData);
-
     FeatureMetadata metadata;
-    collectFeaturesFromSelector(ruleData.selector(), metadata);
+    if (collectFeaturesFromSelector(ruleData.selector(), metadata) == SelectorNeverMatches)
+        return SelectorNeverMatches;
+
     m_metadata.add(metadata);
 
     if (metadata.foundSiblingSelector)
         siblingRules.append(RuleFeature(ruleData.rule(), ruleData.selectorIndex(), ruleData.hasDocumentSecurityOrigin()));
     if (ruleData.containsUncommonAttributeSelector())
         uncommonAttributeRules.append(RuleFeature(ruleData.rule(), ruleData.selectorIndex(), ruleData.hasDocumentSecurityOrigin()));
+
+    updateInvalidationSets(ruleData);
+    return SelectorMayMatch;
 }
 
-void RuleFeatureSet::collectFeaturesFromSelector(const CSSSelector& selector, RuleFeatureSet::FeatureMetadata& metadata)
+RuleFeatureSet::SelectorPreMatch RuleFeatureSet::collectFeaturesFromSelector(const CSSSelector& selector, RuleFeatureSet::FeatureMetadata& metadata)
 {
     unsigned maxDirectAdjacentSelectors = 0;
+    CSSSelector::Relation relation = CSSSelector::Descendant;
+    bool foundHostPseudo = false;
 
     for (const CSSSelector* current = &selector; current; current = current->tagHistory()) {
-        if (current->pseudoType() == CSSSelector::PseudoFirstLine)
+        switch (current->pseudoType()) {
+        case CSSSelector::PseudoFirstLine:
             metadata.usesFirstLineRules = true;
-        if (current->pseudoType() == CSSSelector::PseudoWindowInactive)
+            break;
+        case CSSSelector::PseudoWindowInactive:
             metadata.usesWindowInactiveSelector = true;
-        if (current->relation() == CSSSelector::DirectAdjacent) {
+            break;
+        case CSSSelector::PseudoEmpty:
+        case CSSSelector::PseudoFirstChild:
+        case CSSSelector::PseudoFirstOfType:
+        case CSSSelector::PseudoLastChild:
+        case CSSSelector::PseudoLastOfType:
+        case CSSSelector::PseudoOnlyChild:
+        case CSSSelector::PseudoOnlyOfType:
+        case CSSSelector::PseudoNthChild:
+        case CSSSelector::PseudoNthOfType:
+        case CSSSelector::PseudoNthLastChild:
+        case CSSSelector::PseudoNthLastOfType:
+            if (!metadata.foundInsertionPointCrossing)
+                metadata.foundSiblingSelector = true;
+            break;
+        case CSSSelector::PseudoHost:
+        case CSSSelector::PseudoHostContext:
+            if (relation == CSSSelector::SubSelector)
+                return SelectorNeverMatches;
+            if (!current->isLastInTagHistory() && current->tagHistory()->match() != CSSSelector::PseudoElement)
+                return SelectorNeverMatches;
+            foundHostPseudo = true;
+            // fall through.
+        default:
+            if (const CSSSelectorList* selectorList = current->selectorList()) {
+                for (const CSSSelector* subSelector = selectorList->first(); subSelector; subSelector = CSSSelectorList::next(*subSelector))
+                    collectFeaturesFromSelector(*subSelector, metadata);
+            }
+            break;
+        }
+
+        if (current->relationIsAffectedByPseudoContent() || current->pseudoType() == CSSSelector::PseudoSlotted)
+            metadata.foundInsertionPointCrossing = true;
+
+        relation = current->relation();
+
+        if (foundHostPseudo && relation != CSSSelector::SubSelector)
+            return SelectorNeverMatches;
+
+        if (relation == CSSSelector::DirectAdjacent) {
             maxDirectAdjacentSelectors++;
         } else if (maxDirectAdjacentSelectors
-            && ((current->relation() != CSSSelector::SubSelector) || current->isLastInTagHistory())) {
+            && ((relation != CSSSelector::SubSelector) || current->isLastInTagHistory())) {
             if (maxDirectAdjacentSelectors > metadata.maxDirectAdjacentSelectors)
                 metadata.maxDirectAdjacentSelectors = maxDirectAdjacentSelectors;
             maxDirectAdjacentSelectors = 0;
         }
-        if (!metadata.foundInsertionPointCrossing && current->isSiblingPseudo())
-            metadata.foundSiblingSelector = true;
 
-        if (const CSSSelectorList* selectorList = current->selectorList()) {
-            for (const CSSSelector* subSelector = selectorList->first(); subSelector; subSelector = CSSSelectorList::next(*subSelector))
-                collectFeaturesFromSelector(*subSelector, metadata);
-        }
-        if (current->relationIsAffectedByPseudoContent()
-            || current->pseudoType() == CSSSelector::PseudoHost
-            || current->pseudoType() == CSSSelector::PseudoHostContext
-            || current->pseudoType() == CSSSelector::PseudoSlotted) {
-            metadata.foundInsertionPointCrossing = true;
-        }
         if (!metadata.foundInsertionPointCrossing && current->isAdjacentSelector())
             metadata.foundSiblingSelector = true;
     }
 
     ASSERT(!maxDirectAdjacentSelectors);
+    return SelectorMayMatch;
 }
 
 void RuleFeatureSet::FeatureMetadata::add(const FeatureMetadata& other)

third_party/WebKit/Source/core/css/RuleFeature.h

@@ -59,7 +59,9 @@ public:
     void add(const RuleFeatureSet&);
     void clear();
 
-    void collectFeaturesFromRuleData(const RuleData&);
+    enum SelectorPreMatch { SelectorNeverMatches, SelectorMayMatch };
+
+    SelectorPreMatch collectFeaturesFromRuleData(const RuleData&);
 
     bool usesSiblingRules() const { return !siblingRules.isEmpty(); }
     bool usesFirstLineRules() const { return m_metadata.usesFirstLineRules; }
@@ -118,7 +120,7 @@ private:
         unsigned maxDirectAdjacentSelectors = 0;
     };
 
-    void collectFeaturesFromSelector(const CSSSelector&, FeatureMetadata&);
+    SelectorPreMatch collectFeaturesFromSelector(const CSSSelector&, FeatureMetadata&);
 
     InvalidationSet& ensureClassInvalidationSet(const AtomicString& className, InvalidationType);
     InvalidationSet& ensureAttributeInvalidationSet(const AtomicString& attributeName, InvalidationType);

third_party/WebKit/Source/core/css/RuleSet.cpp

@@ -78,12 +78,8 @@ static bool containsUncommonAttributeSelector(const CSSSelector& selector)
             return true;
         if (selectorListContainsUncommonAttributeSelector(current))
             return true;
-        if (current->relationIsAffectedByPseudoContent()
-            || current->pseudoType() == CSSSelector::PseudoHost
-            || current->pseudoType() == CSSSelector::PseudoHostContext
-            || current->pseudoType() == CSSSelector::PseudoSlotted) {
+        if (current->relationIsAffectedByPseudoContent() || current->pseudoType() == CSSSelector::PseudoSlotted)
             return false;
-        }
         if (current->relation() != CSSSelector::SubSelector) {
             current = current->tagHistory();
             break;
@@ -222,7 +218,8 @@ bool RuleSet::findBestRuleSetAndAdd(const CSSSelector& component, RuleData& rule
 void RuleSet::addRule(StyleRule* rule, unsigned selectorIndex, AddRuleFlags addRuleFlags)
 {
     RuleData ruleData(rule, selectorIndex, m_ruleCount++, addRuleFlags);
-    m_features.collectFeaturesFromRuleData(ruleData);
+    if (m_features.collectFeaturesFromRuleData(ruleData) == RuleFeatureSet::SelectorNeverMatches)
+        return;
 
     if (!findBestRuleSetAndAdd(ruleData.selector(), ruleData)) {
         // If we didn't find a specialized map to stick it in, file under universal rules.