Add a new prefer_mixin rule

analysis_options.yaml

@@ -99,6 +99,7 @@ linter:
     - prefer_is_empty
     - prefer_is_not_empty
 #    - prefer_iterable_whereType # under review (see #1068)
+    - prefer_mixin
     - prefer_single_quotes
 #    - prefer_typing_uninitialized_variables # under review (see #1068)
 #    - prefer_void_to_null # under review (see #1068)

example/all.yaml

@@ -96,6 +96,7 @@ linter:
     - prefer_is_empty
     - prefer_is_not_empty
     - prefer_iterable_whereType
+    - prefer_mixin
     - prefer_single_quotes
     - prefer_typing_uninitialized_variables
     - prefer_void_to_null

lib/src/rules.dart

@@ -96,6 +96,7 @@ import 'package:linter/src/rules/prefer_interpolation_to_compose_strings.dart';
 import 'package:linter/src/rules/prefer_is_empty.dart';
 import 'package:linter/src/rules/prefer_is_not_empty.dart';
 import 'package:linter/src/rules/prefer_iterable_whereType.dart';
+import 'package:linter/src/rules/prefer_mixin.dart';
 import 'package:linter/src/rules/prefer_single_quotes.dart';
 import 'package:linter/src/rules/prefer_typing_uninitialized_variables.dart';
 import 'package:linter/src/rules/prefer_void_to_null.dart';
@@ -225,10 +226,11 @@ void registerLintRules() {
     ..register(new PreferIterableWhereType())
     ..register(new PreferIsEmpty())
     ..register(new PreferIsNotEmpty())
-    ..register(new PublicMemberApiDocs())
+    ..register(new PreferMixin())
     ..register(new PreferSingleQuotes())
     ..register(new PreferTypingUninitializedVariables())
     ..register(new PreferVoidToNull())
+    ..register(new PublicMemberApiDocs())
     ..register(new PubPackageNames())
     ..register(new RecursiveGetters())
     ..registerDefault(new SlashForDocComments())

lib/src/rules/prefer_mixin.dart

@@ -0,0 +1,62 @@
+// Copyright (c) 2018, the Dart project authors. 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.
+
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:linter/src/analyzer.dart';
+
+const _desc = r'Prefer using mixins.';
+
+const _details = r'''
+
+Dart 2.1 introduced a new syntax for mixins that provides a safe way for a mixin
+to invoke inherited members using `super`. The new style of mixins should always
+be used for types that are to be mixed in. As a result, this lint will flag any
+uses of a class in a `with` clause.
+
+**BAD:**
+```
+class A {}
+class B extends Object with A {}
+```
+
+**OK:**
+```
+mixin M {}
+class C with M {}
+```
+
+''';
+
+class PreferMixin extends LintRule implements NodeLintRule {
+  PreferMixin()
+      : super(
+            name: 'prefer_mixin',
+            description: _desc,
+            details: _details,
+            group: Group.style);
+
+  @override
+  void registerNodeProcessors(NodeLintRegistry registry) {
+    final visitor = new _Visitor(this);
+    registry.addWithClause(this, visitor);
+  }
+}
+
+class _Visitor extends SimpleAstVisitor<void> {
+  final LintRule rule;
+
+  _Visitor(this.rule);
+
+  @override
+  void visitWithClause(WithClause node) {
+    for (TypeName type in node.mixinTypes) {
+      Element element = type.name.staticElement;
+      if (element is ClassElement && !element.isMixin) {
+        rule.reportLint(type);
+      }
+    }
+  }
+}

pubspec.yaml

@@ -10,7 +10,7 @@ environment:
   sdk: '>=2.0.0-dev <3.0.0'
 
 dependencies:
-  analyzer: ^0.32.5
+  analyzer: ^0.33.0-alpha.0
   args: '>=1.4.0 <2.0.0'
   glob: ^1.0.3
   meta: ^1.0.2

test/rules/prefer_mixin.dart

@@ -0,0 +1,11 @@
+// Copyright (c) 2018, the Dart project authors. 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.
+
+// test w/ `pub run test -N prefer_mixin`
+
+class A {}
+class B extends Object with A {} // LINT
+
+mixin M {}
+class C with M {} // OK