Implement access tracking for containingUrl

lib/src/async_import_cache.dart

@@ -11,6 +11,7 @@ import 'package:path/path.dart' as p;
 import 'ast/sass.dart';
 import 'deprecation.dart';
 import 'importer.dart';
+import 'importer/canonicalize_context.dart';
 import 'importer/no_op.dart';
 import 'importer/utils.dart';
 import 'io.dart';
@@ -206,18 +207,17 @@ final class AsyncImportCache {
   /// that result is cacheable at all.
   Future<(AsyncCanonicalizeResult?, bool cacheable)> _canonicalize(
       AsyncImporter importer, Uri url, Uri? baseUrl, bool forImport) async {
-    var canonicalize = forImport
-        ? () => inImportRule(() => importer.canonicalize(url))
-        : () => importer.canonicalize(url);
-
     var passContainingUrl = baseUrl != null &&
         (url.scheme == '' || await importer.isNonCanonicalScheme(url.scheme));
-    var result = await withContainingUrl(
-        passContainingUrl ? baseUrl : null, canonicalize);
 
-    // TODO(sass/dart-sass#2208): Determine whether the containing URL was
-    // _actually_ accessed rather than assuming it was.
-    var cacheable = !passContainingUrl || importer is FilesystemImporter;
+    var canonicalizeContext =
+        CanonicalizeContext(passContainingUrl ? baseUrl : null, forImport);
+
+    var result = await withCanonicalizeContext(
+        canonicalizeContext, () => importer.canonicalize(url));
+
+    var cacheable =
+        !passContainingUrl || !canonicalizeContext.wasContainingUrlAccessed;
 
     if (result == null) return (null, cacheable);
 

lib/src/embedded/importer/file.dart

@@ -21,10 +21,12 @@ final class FileImporter extends ImporterBase {
       ..importerId = _importerId
       ..url = url.toString()
       ..fromImport = fromImport;
-    if (containingUrl case var containingUrl?) {
+    if (canonicalizeContext.containingUrlWithoutMarking
+        case var containingUrl?) {
       request.containingUrl = containingUrl.toString();
     }
     var response = dispatcher.sendFileImportRequest(request);
+    if (!response.containingUrlUnused) canonicalizeContext.containingUrl;
 
     switch (response.whichResult()) {
       case InboundMessage_FileImportResponse_Result.fileUrl:

lib/src/embedded/importer/host.dart

@@ -35,10 +35,12 @@ final class HostImporter extends ImporterBase {
       ..importerId = _importerId
       ..url = url.toString()
       ..fromImport = fromImport;
-    if (containingUrl case var containingUrl?) {
+    if (canonicalizeContext.containingUrlWithoutMarking
+        case var containingUrl?) {
       request.containingUrl = containingUrl.toString();
     }
     var response = dispatcher.sendCanonicalizeRequest(request);
+    if (!response.containingUrlUnused) canonicalizeContext.containingUrl;
 
     return switch (response.whichResult()) {
       InboundMessage_CanonicalizeResponse_Result.url =>

lib/src/import_cache.dart

@@ -5,7 +5,7 @@
 // DO NOT EDIT. This file was generated from async_import_cache.dart.
 // See tool/grind/synchronize.dart for details.
 //
-// Checksum: 37dd173d676ec6cf201a25b3cca9ac81d92b1433
+// Checksum: 36bc42050cf2eb3a43f36376c4f06c1708eee777
 //
 // ignore_for_file: unused_import
 
@@ -18,6 +18,7 @@ import 'package:path/path.dart' as p;
 import 'ast/sass.dart';
 import 'deprecation.dart';
 import 'importer.dart';
+import 'importer/canonicalize_context.dart';
 import 'importer/no_op.dart';
 import 'importer/utils.dart';
 import 'io.dart';
@@ -206,18 +207,17 @@ final class ImportCache {
   /// that result is cacheable at all.
   (CanonicalizeResult?, bool cacheable) _canonicalize(
       Importer importer, Uri url, Uri? baseUrl, bool forImport) {
-    var canonicalize = forImport
-        ? () => inImportRule(() => importer.canonicalize(url))
-        : () => importer.canonicalize(url);
-
     var passContainingUrl = baseUrl != null &&
         (url.scheme == '' || importer.isNonCanonicalScheme(url.scheme));
-    var result =
-        withContainingUrl(passContainingUrl ? baseUrl : null, canonicalize);
 
-    // TODO(sass/dart-sass#2208): Determine whether the containing URL was
-    // _actually_ accessed rather than assuming it was.
-    var cacheable = !passContainingUrl || importer is FilesystemImporter;
+    var canonicalizeContext =
+        CanonicalizeContext(passContainingUrl ? baseUrl : null, forImport);
+
+    var result = withCanonicalizeContext(
+        canonicalizeContext, () => importer.canonicalize(url));
+
+    var cacheable =
+        !passContainingUrl || !canonicalizeContext.wasContainingUrlAccessed;
 
     if (result == null) return (null, cacheable);
 

lib/src/importer/async.dart

@@ -6,6 +6,7 @@ import 'dart:async';
 
 import 'package:meta/meta.dart';
 
+import 'canonicalize_context.dart';
 import 'result.dart';
 import 'utils.dart' as utils;
 
@@ -54,7 +55,19 @@ abstract class AsyncImporter {
   /// Outside of that context, its value is undefined and subject to change.
   @protected
   @nonVirtual
-  Uri? get containingUrl => utils.containingUrl;
+  Uri? get containingUrl => utils.canonicalizeContext.containingUrl;
+
+  /// The canonicalize context of the stylesheet that caused the current
+  /// [canonicalize] invocation.
+  ///
+  /// Subclasses should only access this from within calls to [canonicalize].
+  /// Outside of that context, its value is undefined and subject to change.
+  ///
+  /// @nodoc
+  @internal
+  @protected
+  @nonVirtual
+  CanonicalizeContext get canonicalizeContext => utils.canonicalizeContext;
 
   /// If [url] is recognized by this importer, returns its canonical format.
   ///

lib/src/importer/canonicalize_context.dart

@@ -0,0 +1,47 @@
+// Copyright 2024 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import 'dart:async';
+
+import 'package:meta/meta.dart';
+
+/// Contextual information used by importers' `canonicalize` method.
+@internal
+final class CanonicalizeContext {
+  /// Whether the Sass compiler is currently evaluating an `@import` rule.
+  bool get fromImport => _fromImport;
+  bool _fromImport;
+
+  /// The URL of the stylesheet that contains the current load.
+  Uri? get containingUrl {
+    _wasContainingUrlAccessed = true;
+    return _containingUrl;
+  }
+
+  final Uri? _containingUrl;
+
+  /// Returns the same value as [containingUrl], but doesn't mark it accessed.
+  Uri? get containingUrlWithoutMarking => _containingUrl;
+
+  /// Whether [containingUrl] has been accessed.
+  ///
+  /// This is used to determine whether canonicalize result is cacheable.
+  bool get wasContainingUrlAccessed => _wasContainingUrlAccessed;
+  var _wasContainingUrlAccessed = false;
+
+  /// Runs [callback] in a context with specificed [fromImport].
+  T withFromImport<T>(bool fromImport, T callback()) {
+    assert(Zone.current[#_canonicalizeContext] == this);
+
+    var oldFromImport = _fromImport;
+    _fromImport = fromImport;
+    try {
+      return callback();
+    } finally {
+      _fromImport = oldFromImport;
+    }
+  }
+
+  CanonicalizeContext(this._containingUrl, this._fromImport);
+}

lib/src/importer/js_to_dart/async.dart

@@ -13,6 +13,7 @@ import '../../js/url.dart';
 import '../../js/utils.dart';
 import '../../util/nullable.dart';
 import '../async.dart';
+import '../canonicalize_context.dart';
 import '../result.dart';
 import 'utils.dart';
 
@@ -38,11 +39,8 @@ final class JSToDartAsyncImporter extends AsyncImporter {
   }
 
   FutureOr<Uri?> canonicalize(Uri url) async {
-    var result = wrapJSExceptions(() => _canonicalize(
-        url.toString(),
-        CanonicalizeContext(
-            fromImport: fromImport,
-            containingUrl: containingUrl.andThen(dartToJSUrl))));
+    var result = wrapJSExceptions(
+        () => _canonicalize(url.toString(), canonicalizeContext));
     if (isPromise(result)) result = await promiseToFuture(result as Promise);
     if (result == null) return null;
 

lib/src/importer/js_to_dart/async_file.dart

@@ -8,11 +8,10 @@ import 'package:cli_pkg/js.dart';
 import 'package:node_interop/js.dart';
 import 'package:node_interop/util.dart';
 
-import '../../js/importer.dart';
 import '../../js/url.dart';
 import '../../js/utils.dart';
-import '../../util/nullable.dart';
 import '../async.dart';
+import '../canonicalize_context.dart';
 import '../filesystem.dart';
 import '../result.dart';
 import '../utils.dart';
@@ -28,11 +27,8 @@ final class JSToDartAsyncFileImporter extends AsyncImporter {
   FutureOr<Uri?> canonicalize(Uri url) async {
     if (url.scheme == 'file') return FilesystemImporter.cwd.canonicalize(url);
 
-    var result = wrapJSExceptions(() => _findFileUrl(
-        url.toString(),
-        CanonicalizeContext(
-            fromImport: fromImport,
-            containingUrl: containingUrl.andThen(dartToJSUrl))));
+    var result = wrapJSExceptions(
+        () => _findFileUrl(url.toString(), canonicalizeContext));
     if (isPromise(result)) result = await promiseToFuture(result as Promise);
     if (result == null) return null;
     if (!isJSUrl(result)) {

lib/src/importer/js_to_dart/file.dart

@@ -6,10 +6,9 @@ import 'package:cli_pkg/js.dart';
 import 'package:node_interop/js.dart';
 
 import '../../importer.dart';
-import '../../js/importer.dart';
 import '../../js/url.dart';
 import '../../js/utils.dart';
-import '../../util/nullable.dart';
+import '../canonicalize_context.dart';
 import '../utils.dart';
 
 /// A wrapper for a potentially-asynchronous JS API file importer that exposes
@@ -23,11 +22,8 @@ final class JSToDartFileImporter extends Importer {
   Uri? canonicalize(Uri url) {
     if (url.scheme == 'file') return FilesystemImporter.cwd.canonicalize(url);
 
-    var result = wrapJSExceptions(() => _findFileUrl(
-        url.toString(),
-        CanonicalizeContext(
-            fromImport: fromImport,
-            containingUrl: containingUrl.andThen(dartToJSUrl))));
+    var result = wrapJSExceptions(
+        () => _findFileUrl(url.toString(), canonicalizeContext));
     if (result == null) return null;
 
     if (isPromise(result)) {

lib/src/importer/js_to_dart/sync.dart

@@ -10,6 +10,7 @@ import '../../js/importer.dart';
 import '../../js/url.dart';
 import '../../js/utils.dart';
 import '../../util/nullable.dart';
+import '../canonicalize_context.dart';
 import 'utils.dart';
 
 /// A wrapper for a synchronous JS API importer that exposes it as a Dart
@@ -34,11 +35,8 @@ final class JSToDartImporter extends Importer {
   }
 
   Uri? canonicalize(Uri url) {
-    var result = wrapJSExceptions(() => _canonicalize(
-        url.toString(),
-        CanonicalizeContext(
-            fromImport: fromImport,
-            containingUrl: containingUrl.andThen(dartToJSUrl))));
+    var result = wrapJSExceptions(
+        () => _canonicalize(url.toString(), canonicalizeContext));
     if (result == null) return null;
     if (isJSUrl(result)) return jsToDartUrl(result as JSUrl);
 

lib/src/importer/utils.dart

@@ -7,6 +7,7 @@ import 'dart:async';
 import 'package:path/path.dart' as p;
 
 import '../io.dart';
+import './canonicalize_context.dart';
 
 /// Whether the Sass compiler is currently evaluating an `@import` rule.
 ///
@@ -15,30 +16,35 @@ import '../io.dart';
 /// canonicalization should be identical for `@import` and `@use` rules. It's
 /// admittedly hacky to set this globally, but `@import` will eventually be
 /// removed, at which point we can delete this and have one consistent behavior.
-bool get fromImport => Zone.current[#_inImportRule] as bool? ?? false;
-
-/// The URL of the stylesheet that contains the current load.
-Uri? get containingUrl => switch (Zone.current[#_containingUrl]) {
+bool get fromImport =>
+    ((Zone.current[#_canonicalizeContext] as CanonicalizeContext?)
+            ?.fromImport ??
+        false);
+
+/// The CanonicalizeContext of the current load.
+CanonicalizeContext get canonicalizeContext =>
+    switch (Zone.current[#_canonicalizeContext]) {
       null => throw StateError(
-          "containingUrl may only be accessed within a call to canonicalize()."),
-      #_none => null,
-      Uri url => url,
+          "canonicalizeContext may only be accessed within a call to canonicalize()."),
+      CanonicalizeContext context => context,
       var value => throw StateError(
-          "Unexpected Zone.current[#_containingUrl] value $value.")
+          "Unexpected Zone.current[#_canonicalizeContext] value $value.")
     };
 
 /// Runs [callback] in a context where [fromImport] returns `true` and
 /// [resolveImportPath] uses `@import` semantics rather than `@use` semantics.
 T inImportRule<T>(T callback()) =>
-    runZoned(callback, zoneValues: {#_inImportRule: true});
+    switch (Zone.current[#_canonicalizeContext]) {
+      null => runZoned(callback,
+          zoneValues: {#_canonicalizeContext: CanonicalizeContext(null, true)}),
+      CanonicalizeContext context => context.withFromImport(true, callback),
+      var value => throw StateError(
+          "Unexpected Zone.current[#_canonicalizeContext] value $value.")
+    };
 
-/// Runs [callback] in a context where [containingUrl] returns [url].
-///
-/// If [when] is `false`, runs [callback] without setting [containingUrl].
-T withContainingUrl<T>(Uri? url, T callback()) =>
-    // Use #_none as a sentinel value so we can distinguish a containing URL
-    // that's set to null from one that's unset at all.
-    runZoned(callback, zoneValues: {#_containingUrl: url ?? #_none});
+/// Runs [callback] in the given context.
+T withCanonicalizeContext<T>(CanonicalizeContext? context, T callback()) =>
+    runZoned(callback, zoneValues: {#_canonicalizeContext: context});
 
 /// Resolves an imported path using the same logic as the filesystem importer.
 ///

lib/src/js.dart

@@ -7,6 +7,7 @@ import 'package:js/js_util.dart';
 import 'js/exception.dart';
 import 'js/deprecations.dart';
 import 'js/exports.dart';
+import 'js/importer/canonicalize_context.dart';
 import 'js/compile.dart';
 import 'js/compiler.dart';
 import 'js/legacy.dart';
@@ -64,6 +65,7 @@ void main() {
       "dart2js\t${const String.fromEnvironment('dart-version')}\t"
       "(Dart Compiler)\t[Dart]";
 
+  updateCanonicalizeContextPrototype();
   updateSourceSpanPrototype();
 
   // Legacy API

lib/src/js/importer.dart

@@ -4,6 +4,7 @@
 
 import 'package:js/js.dart';
 
+import '../importer/canonicalize_context.dart';
 import 'url.dart';
 
 @JS()
@@ -15,15 +16,6 @@ class JSImporter {
   external Object? get nonCanonicalScheme;
 }
 
-@JS()
-@anonymous
-class CanonicalizeContext {
-  external bool get fromImport;
-  external JSUrl? get containingUrl;
-
-  external factory CanonicalizeContext({bool fromImport, JSUrl? containingUrl});
-}
-
 @JS()
 @anonymous
 class JSImporterResult {

lib/src/js/importer/canonicalize_context.dart

@@ -0,0 +1,16 @@
+// Copyright 2014 Google Inc. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import '../../importer/canonicalize_context.dart';
+import '../../util/nullable.dart';
+import '../reflection.dart';
+import '../utils.dart';
+
+/// Adds JS members to Dart's `CanonicalizeContext` class.
+void updateCanonicalizeContextPrototype() =>
+    getJSClass(CanonicalizeContext(null, false)).defineGetters({
+      'fromImport': (CanonicalizeContext self) => self.fromImport,
+      'containingUrl': (CanonicalizeContext self) =>
+          self.containingUrl.andThen(dartToJSUrl),
+    });