LibWeb: Make CSSStyleRule::qualified_layer_name() return a const-ref

And also make it inline. We were spending 8% of selector matching on
creating and destroying FlyString copies here. With this change, it's
now ~1%.

Userland/Libraries/LibWeb/CSS/CSSRule.cpp

@@ -48,38 +48,36 @@ void CSSRule::set_parent_style_sheet(CSSStyleSheet* parent_style_sheet)
     m_parent_style_sheet = parent_style_sheet;
 }
 
-FlyString const& CSSRule::parent_layer_internal_qualified_name() const
+FlyString const& CSSRule::parent_layer_internal_qualified_name_slow_case() const
 {
-    if (!m_cached_layer_name.has_value()) {
-        Vector<FlyString> layer_names;
-        for (auto* rule = parent_rule(); rule; rule = rule->parent_rule()) {
-            switch (rule->type()) {
-            case CSSRule::Type::Import:
-                // TODO: Handle `layer(foo)` in import rules once we implement that.
-                break;
+    Vector<FlyString> layer_names;
+    for (auto* rule = parent_rule(); rule; rule = rule->parent_rule()) {
+        switch (rule->type()) {
+        case CSSRule::Type::Import:
+            // TODO: Handle `layer(foo)` in import rules once we implement that.
+            break;
 
-            case CSSRule::Type::LayerBlock: {
-                auto& layer_block = static_cast<CSSLayerBlockRule const&>(*rule);
-                layer_names.append(layer_block.internal_name());
-                break;
-            }
-
-                // Ignore everything else
-                // Note that LayerStatement cannot have child rules so we still ignore it here.
-            case CSSRule::Type::LayerStatement:
-            case CSSRule::Type::Style:
-            case CSSRule::Type::Media:
-            case CSSRule::Type::FontFace:
-            case CSSRule::Type::Keyframes:
-            case CSSRule::Type::Keyframe:
-            case CSSRule::Type::Namespace:
-            case CSSRule::Type::Supports:
-                break;
-            }
+        case CSSRule::Type::LayerBlock: {
+            auto& layer_block = static_cast<CSSLayerBlockRule const&>(*rule);
+            layer_names.append(layer_block.internal_name());
+            break;
         }
 
-        m_cached_layer_name = MUST(String::join("."sv, layer_names.in_reverse()));
+            // Ignore everything else
+            // Note that LayerStatement cannot have child rules so we still ignore it here.
+        case CSSRule::Type::LayerStatement:
+        case CSSRule::Type::Style:
+        case CSSRule::Type::Media:
+        case CSSRule::Type::FontFace:
+        case CSSRule::Type::Keyframes:
+        case CSSRule::Type::Keyframe:
+        case CSSRule::Type::Namespace:
+        case CSSRule::Type::Supports:
+            break;
+        }
     }
+
+    m_cached_layer_name = MUST(String::join("."sv, layer_names.in_reverse()));
     return m_cached_layer_name.value();
 }
 

Userland/Libraries/LibWeb/CSS/CSSRule.h

@@ -58,7 +58,14 @@ protected:
 
     virtual void visit_edges(Cell::Visitor&) override;
 
-    FlyString const& parent_layer_internal_qualified_name() const;
+    [[nodiscard]] FlyString const& parent_layer_internal_qualified_name() const
+    {
+        if (!m_cached_layer_name.has_value())
+            return parent_layer_internal_qualified_name_slow_case();
+        return m_cached_layer_name.value();
+    }
+
+    [[nodiscard]] FlyString const& parent_layer_internal_qualified_name_slow_case() const;
 
     JS::GCPtr<CSSRule> m_parent_rule;
     JS::GCPtr<CSSStyleSheet> m_parent_style_sheet;

Userland/Libraries/LibWeb/CSS/CSSStyleRule.cpp

@@ -46,11 +46,6 @@ CSSStyleDeclaration* CSSStyleRule::style()
     return m_declaration;
 }
 
-FlyString CSSStyleRule::qualified_layer_name() const
-{
-    return parent_layer_internal_qualified_name();
-}
-
 // https://www.w3.org/TR/cssom/#serialize-a-css-rule
 String CSSStyleRule::serialized() const
 {

Userland/Libraries/LibWeb/CSS/CSSStyleRule.h

@@ -33,7 +33,7 @@ public:
 
     CSSStyleDeclaration* style();
 
-    FlyString qualified_layer_name() const;
+    [[nodiscard]] FlyString const& qualified_layer_name() const { return parent_layer_internal_qualified_name(); }
 
 private:
     CSSStyleRule(JS::Realm&, Vector<NonnullRefPtr<Selector>>&&, PropertyOwningCSSStyleDeclaration&);