Remove the heuristic where long selector lists wouldn't be trimmed (#2255)

Testing this against the `@extend`-heavy stylesheets in
vinceliuice/Colloid-gtk-theme, trimming everywhere actually *improves*
performance rather than reducing it.

Author: nex3

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 1.77.5
+
+* Fully trim redundant selectors generated by `@extend`.
+
 ## 1.77.4
 
 ### Embedded Sass

lib/src/ast/selector/compound.dart

@@ -42,6 +42,18 @@ final class CompoundSelector extends Selector {
   SimpleSelector? get singleSimple =>
       components.length == 1 ? components.first : null;
 
+  /// Whether any simple selector in this contains a selector that requires
+  /// complex non-local reasoning to determine whether it's a super- or
+  /// sub-selector.
+  ///
+  /// This includes both pseudo-elements and pseudo-selectors that take
+  /// selectors as arguments.
+  ///
+  /// #nodoc
+  @internal
+  late final bool hasComplicatedSuperselectorSemantics = components
+      .any((component) => component.hasComplicatedSuperselectorSemantics);
+
   CompoundSelector(Iterable<SimpleSelector> components, super.span)
       : components = List.unmodifiable(components) {
     if (this.components.isEmpty) {

lib/src/ast/selector/pseudo.dart

@@ -67,6 +67,10 @@ final class PseudoSelector extends SimpleSelector {
   bool get isHostContext =>
       isClass && name == 'host-context' && selector != null;
 
+  @internal
+  bool get hasComplicatedSuperselectorSemantics =>
+      isElement || selector != null;
+
   /// The non-selector argument passed to this selector.
   ///
   /// This is `null` if there's no argument. If [argument] and [selector] are

lib/src/ast/selector/simple.dart

@@ -34,6 +34,16 @@ abstract base class SimpleSelector extends Selector {
   /// sequence will contain 1000 simple selectors.
   int get specificity => 1000;
 
+  /// Whether this requires complex non-local reasoning to determine whether
+  /// it's a super- or sub-selector.
+  ///
+  /// This includes both pseudo-elements and pseudo-selectors that take
+  /// selectors as arguments.
+  ///
+  /// #nodoc
+  @internal
+  bool get hasComplicatedSuperselectorSemantics => false;
+
   SimpleSelector(super.span);
 
   /// Parses a simple selector from [contents].

lib/src/extend/extension_store.dart

@@ -901,13 +901,6 @@ class ExtensionStore {
   // document, and thus should never be trimmed.
   List<ComplexSelector> _trim(List<ComplexSelector> selectors,
       bool isOriginal(ComplexSelector complex)) {
-    // Avoid truly horrific quadratic behavior.
-    //
-    // TODO(nweiz): I think there may be a way to get perfect trimming without
-    // going quadratic by building some sort of trie-like data structure that
-    // can be used to look up superselectors.
-    if (selectors.length > 100) return selectors;
-
     // This is n² on the sequences, but only comparing between separate
     // sequences should limit the quadratic behavior. We iterate from last to
     // first and reverse the result so that, if two selectors are identical, we

lib/src/extend/functions.dart

@@ -646,24 +646,28 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
     var component1 = complex1[i1];
     if (component1.combinators.length > 1) return false;
     if (remaining1 == 1) {
-      var parents = complex2.sublist(i2, complex2.length - 1);
-      if (parents.any((parent) => parent.combinators.length > 1)) return false;
-
-      return compoundIsSuperselector(
-          component1.selector, complex2.last.selector,
-          parents: parents);
+      if (complex2.any((parent) => parent.combinators.length > 1)) {
+        return false;
+      } else {
+        return compoundIsSuperselector(
+            component1.selector, complex2.last.selector,
+            parents: component1.selector.hasComplicatedSuperselectorSemantics
+                ? complex2.sublist(i2, complex2.length - 1)
+                : null);
+      }
     }
 
     // Find the first index [endOfSubselector] in [complex2] such that
     // `complex2.sublist(i2, endOfSubselector + 1)` is a subselector of
     // [component1.selector].
     var endOfSubselector = i2;
-    List<ComplexSelectorComponent>? parents;
     while (true) {
       var component2 = complex2[endOfSubselector];
       if (component2.combinators.length > 1) return false;
       if (compoundIsSuperselector(component1.selector, component2.selector,
-          parents: parents)) {
+          parents: component1.selector.hasComplicatedSuperselectorSemantics
+              ? complex2.sublist(i2, endOfSubselector)
+              : null)) {
         break;
       }
 
@@ -675,13 +679,10 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
         // to match.
         return false;
       }
-
-      parents ??= [];
-      parents.add(component2);
     }
 
     if (!_compatibleWithPreviousCombinator(
-        previousCombinator, parents ?? const [])) {
+        previousCombinator, complex2.take(endOfSubselector).skip(i2))) {
       return false;
     }
 
@@ -717,8 +718,8 @@ bool complexIsSuperselector(List<ComplexSelectorComponent> complex1,
 /// Returns whether [parents] are valid intersitial components between one
 /// complex superselector and another, given that the earlier complex
 /// superselector had the combinator [previous].
-bool _compatibleWithPreviousCombinator(
-    CssValue<Combinator>? previous, List<ComplexSelectorComponent> parents) {
+bool _compatibleWithPreviousCombinator(CssValue<Combinator>? previous,
+    Iterable<ComplexSelectorComponent> parents) {
   if (parents.isEmpty) return true;
   if (previous == null) return true;
 
@@ -754,6 +755,13 @@ bool _isSupercombinator(
 bool compoundIsSuperselector(
     CompoundSelector compound1, CompoundSelector compound2,
     {Iterable<ComplexSelectorComponent>? parents}) {
+  if (!compound1.hasComplicatedSuperselectorSemantics &&
+      !compound2.hasComplicatedSuperselectorSemantics) {
+    if (compound1.components.length > compound2.components.length) return false;
+    return compound1.components
+        .every((simple1) => compound2.components.any(simple1.isSuperselector));
+  }
+
   // Pseudo elements effectively change the target of a compound selector rather
   // than narrowing the set of elements to which it applies like other
   // selectors. As such, if either selector has a pseudo element, they both must

pkg/sass_api/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 10.4.5
+
+* No user-visible changes.
+
 ## 10.4.4
 
 * No user-visible changes.

pkg/sass_api/pubspec.yaml

@@ -2,15 +2,15 @@ name: sass_api
 # Note: Every time we add a new Sass AST node, we need to bump the *major*
 # version because it's a breaking change for anyone who's implementing the
 # visitor interface(s).
-version: 10.4.4
+version: 10.4.5
 description: Additional APIs for Dart Sass.
 homepage: https://github.com/sass/dart-sass
 
 environment:
   sdk: ">=3.0.0 <4.0.0"
 
 dependencies:
-  sass: 1.77.4
+  sass: 1.77.5
 
 dev_dependencies:
   dartdoc: ">=6.0.0 <9.0.0"

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass
-version: 1.77.4
+version: 1.77.5
 description: A Sass implementation in Dart.
 homepage: https://github.com/sass/dart-sass