Fix mixin class generation for Dart 3.

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changelog
 
+# 8.6.1
+
+- Fix support for generating enum mixins for Dart 3. Instead of triggering
+  mixin generation with `abstract class TestEnumMixin = Object with
+  _$TestEnumMixin` it must now be triggered with `typedef TestEnumMixin =
+  _$TestEnumMixin`.
+
 # 8.6.0
 
 - Add support for value types with record fields. Serialization with records

built_value_generator/lib/src/enum_source_class.dart

@@ -15,6 +15,8 @@ import 'package:built_value_generator/src/strings.dart';
 import 'package:collection/collection.dart'
     show IterableExtension, IterableNullableExtension;
 
+import 'library_elements.dart';
+
 part 'enum_source_class.g.dart';
 
 abstract class EnumSourceClass
@@ -33,6 +35,14 @@ abstract class EnumSourceClass
   @memoized
   String get name => element.name;
 
+  /// Returns `mixin` if class modifiers are available, `abstract class`
+  /// otherwise.
+  ///
+  /// The two are equivalent as class modifiers change the meaning of `class`.
+  String get _mixin => LibraryElements.areClassMixinsEnabled(element.library)
+      ? 'mixin'
+      : 'abstract class';
+
   @memoized
   String get wireName => settings.wireName ?? name;
 
@@ -90,14 +100,10 @@ abstract class EnumSourceClass
   }
 
   @memoized
-  bool get usesMixin => element.library.getClass(name + 'Mixin') != null;
-
-  @memoized
-  String? get mixinDeclaration {
-    var mixinElement = element.library.getClass(name + 'Mixin');
-    if (mixinElement == null) return null;
-    return parsedLibrary.getElementDeclaration(mixinElement)!.node.toSource();
-  }
+  bool get usesMixin =>
+      element.library.getClass(name + 'Mixin') != null ||
+      element.library.definingCompilationUnit.typeAliases
+          .any((a) => a.name == name + 'Mixin');
 
   @memoized
   Iterable<String> get identifiers {
@@ -121,7 +127,6 @@ abstract class EnumSourceClass
       ..._checkConstructor(),
       ..._checkValuesGetter(),
       ..._checkValueOf(),
-      ..._checkMixin(),
     ];
   }
 
@@ -178,17 +183,6 @@ abstract class EnumSourceClass
     return result;
   }
 
-  Iterable<String> _checkMixin() {
-    if (usesMixin) {
-      final expectedCode =
-          'abstract class ${name}Mixin = Object with _\$${name}Mixin;';
-      if (!mixinDeclaration!.contains(expectedCode)) {
-        return ['Remove mixin or declare using exactly: $expectedCode'];
-      }
-    }
-    return [];
-  }
-
   String generateCode() {
     var result = StringBuffer();
 
@@ -244,7 +238,7 @@ abstract class EnumSourceClass
       ..writeln('$name valueOf(String name) => $valueOfIdentifier(name);')
       ..writeln('BuiltSet<$name> get values => $valuesIdentifier;')
       ..writeln('}')
-      ..writeln('abstract class _\$${name}Mixin {')
+      ..writeln('$_mixin _\$${name}Mixin {')
       ..writeln('  // ignore: non_constant_identifier_names')
       ..writeln('_\$${name}Meta get $name => const _\$${name}Meta();')
       ..writeln('}');

built_value_generator/test/enum_class_generator_test.dart

@@ -327,29 +327,6 @@ class TestEnum extends EnumClass {
 1. Add method: static TestEnum valueOf(String name) => _$valueOf(name)'''));
       });
 
-      test('with error on wrong mixin declaration', () async {
-        expect(await generate(r'''
-library test_enum;
-
-import 'package:built_value/built_value.dart';
-
-part 'test_enum.g.dart';
-
-class TestEnum extends EnumClass {
-  static const TestEnum yes = _$yes;
-
-  const TestEnum._(String name) : super(name);
-
-  static BuiltSet<TestEnum> get values => _$values;
-  static TestEnum valueOf(String name) => _$valueOf(name);
-}
-
-class TestEnumMixin = Object with _$TestEnumMixin;
-'''), endsWith(r'''Please make the following changes to use EnumClass:
-
-1. Remove mixin or declare using exactly: abstract class TestEnumMixin = Object with _$TestEnumMixin;'''));
-      });
-
       test('with error on abstract class', () async {
         expect(await generate(r'''
 library test_enum;

end_to_end_test/lib/enums_nnbd.dart

@@ -1,8 +1,6 @@
 // Copyright (c) 2017, Google Inc. Please see the AUTHORS file for details.
 // All rights reserved. Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
-// @dart=2.17
-
 library enums_nnbd;
 
 import 'package:built_collection/built_collection.dart';
@@ -24,7 +22,7 @@ class TestEnum extends EnumClass {
   static TestEnum valueOf(String name) => _$valueOf(name);
 }
 
-abstract class TestEnumMixin = Object with _$TestEnumMixin;
+typedef TestEnumMixin = _$TestEnumMixin;
 
 class SecondTestEnum extends EnumClass {
   static const SecondTestEnum yes = _$ys;

end_to_end_test/test/enums_nnbd_test.dart

@@ -7,7 +7,12 @@ import 'package:end_to_end_test/enums_nnbd.dart';
 import 'package:test/test.dart';
 
 void main() {
-  group('FallbackEnum', () {
+  group(TestEnumMixin, () {
+    test('mixin works as expected', () {
+      expect(UsesTestEnumMixin().toString(), 'yes no maybe');
+    });
+  });
+  group(FallbackEnum, () {
     test('valueOf works for non-fallback values', () {
       expect(FallbackEnum.valueOf('yes'), FallbackEnum.yes);
     });
@@ -18,3 +23,11 @@ void main() {
     });
   });
 }
+
+class UsesTestEnumMixin with TestEnumMixin {
+  // Refer to the constants to check they are available via the mixin from
+  // another class.
+  @override
+  String toString() =>
+      '${this.TestEnum.yes} ${this.TestEnum.no} ${this.TestEnum.maybe}';
+}