Add AsyncImporter.fromImport for Dart importers

CHANGELOG.md

@@ -9,6 +9,13 @@
 
 [import-only files]: https://sass-lang.com/documentation/at-rules/import#import-only-files
 
+### Dart API
+
+* Add an `Importer.fromImport` getter, which is `true` if the current
+  `Importer.canonicalize()` call comes from an `@import` rule and `false`
+  otherwise. Importers should only use this to determine whether to load
+  [import-only files].
+
 ## 1.32.13
 
 * **Potentially breaking bug fix:** Null values in `@use` and `@forward`

lib/src/importer/async.dart

@@ -4,7 +4,10 @@
 
 import 'dart:async';
 
+import 'package:meta/meta.dart';
+
 import 'result.dart';
+import 'utils.dart' as utils;
 
 /// An interface for importers that resolves URLs in `@import`s to the contents
 /// of Sass files.
@@ -20,6 +23,22 @@ import 'result.dart';
 ///
 /// Subclasses should extend [AsyncImporter], not implement it.
 abstract class AsyncImporter {
+  /// Whether the current [canonicalize] invocation comes from an `@import`
+  /// rule.
+  ///
+  /// When evaluating `@import` rules, URLs should canonicalize to an
+  /// [import-only file] if one exists for the URL being canonicalized.
+  /// Otherwise, canonicalization should be identical for `@import` and `@use`
+  /// rules.
+  ///
+  /// [import-only file]: https://sass-lang.com/documentation/at-rules/import#import-only-files
+  ///
+  /// Subclasses should only access this from within calls to [canonicalize].
+  /// Outside of that context, its value is undefined and subject to change.
+  @protected
+  @nonVirtual
+  bool get fromImport => utils.fromImport;
+
   /// If [url] is recognized by this importer, returns its canonical format.
   ///
   /// Note that canonical URLs *must* be absolute, including a scheme. Returning

lib/src/importer/utils.dart

@@ -2,6 +2,8 @@
 // MIT-style license that can be found in the LICENSE file or at
 // https://opensource.org/licenses/MIT.
 
+import 'dart:async';
+
 import 'package:path/path.dart' as p;
 
 import '../io.dart';
@@ -13,30 +15,12 @@ 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 _inImportRule = false;
-
-/// Runs [callback] in a context where [resolveImportPath] uses `@import`
-/// semantics rather than `@use` semantics.
-T inImportRule<T>(T callback()) {
-  var wasInImportRule = _inImportRule;
-  _inImportRule = true;
-  try {
-    return callback();
-  } finally {
-    _inImportRule = wasInImportRule;
-  }
-}
+bool get fromImport => Zone.current[#_inImportRule] as bool? ?? false;
 
-/// Like [inImportRule], but asynchronous.
-Future<T> inImportRuleAsync<T>(Future<T> callback()) async {
-  var wasInImportRule = _inImportRule;
-  _inImportRule = true;
-  try {
-    return await callback();
-  } finally {
-    _inImportRule = wasInImportRule;
-  }
-}
+/// Runs [callback] in a context where [inImportRule] returns `true` and
+/// [resolveImportPath] uses `@import` semantics rather than `@use` semantics.
+T inImportRule<T>(T callback()) =>
+    runZoned(callback, zoneValues: {#_inImportRule: true});
 
 /// Resolves an imported path using the same logic as the filesystem importer.
 ///
@@ -95,7 +79,7 @@ String? _exactlyOne(List<String> paths) {
       paths.map((path) => "  " + p.prettyUri(p.toUri(path))).join("\n");
 }
 
-/// If [_inImportRule] is `true`, invokes callback and returns the result.
+/// If [fromImport] is `true`, invokes callback and returns the result.
 ///
 /// Otherwise, returns `null`.
-T? _ifInImport<T>(T callback()) => _inImportRule ? callback() : null;
+T? _ifInImport<T>(T callback()) => fromImport ? callback() : null;

test/dart_api/from_import_importer.dart

@@ -0,0 +1,28 @@
+// Copyright 2017 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 'package:sass/sass.dart';
+import 'package:test/test.dart';
+
+/// An [Importer] whose [canonicalize] method asserts the value of
+/// [Importer.fromImport].
+class FromImportImporter extends Importer {
+  /// The expected value of [Importer.fromImport] in the call to [canonicalize].
+  final bool _expected;
+
+  /// The callback to call once [canonicalize] is called.
+  ///
+  /// This ensures that the test doesn't exit until [canonicalize] is called.
+  final void Function() _done;
+
+  FromImportImporter(this._expected) : _done = expectAsync0(() {});
+
+  Uri? canonicalize(Uri url) {
+    expect(fromImport, equals(_expected));
+    _done();
+    return Uri.parse('u:');
+  }
+
+  ImporterResult? load(Uri url) => ImporterResult("", syntax: Syntax.scss);
+}

test/dart_api/importer_test.dart

@@ -12,6 +12,7 @@ import 'package:test/test.dart';
 import 'package:sass/sass.dart';
 import 'package:sass/src/exception.dart';
 
+import 'from_import_importer.dart';
 import 'test_importer.dart';
 import '../utils.dart';
 
@@ -182,4 +183,23 @@ void main() {
       return true;
     })));
   });
+
+  group("currentLoadFromImport is", () {
+    test("true from an @import", () {
+      compileString('@import "foo"', importers: [FromImportImporter(true)]);
+    });
+
+    test("false from a @use", () {
+      compileString('@use "foo"', importers: [FromImportImporter(false)]);
+    });
+
+    test("false from a @forward", () {
+      compileString('@forward "foo"', importers: [FromImportImporter(false)]);
+    });
+
+    test("false from meta.load-css", () {
+      compileString('@use "sass:meta"; @include meta.load-css("")',
+          importers: [FromImportImporter(false)]);
+    });
+  });
 }