Implement null_safety rule (#2773)

* Implement enable_null_safety rule

* year

* Update tests

* Use `reflective_test_loader`

* `all.dart`

CHANGELOG.md

@@ -38,6 +38,7 @@
   - `prefer_const_constructors`
 - new lint: `implicit_call_tearoffs`
 - new lint: `unnecessary_library_directive`
+- new lint: `enable_null_safety`
 
 # 1.28.0
 

example/all.yaml

@@ -73,6 +73,7 @@ linter:
     - empty_catches
     - empty_constructor_bodies
     - empty_statements
+    - enable_null_safety
     - eol_at_end_of_file
     - exhaustive_cases
     - file_names

lib/src/rules.dart

@@ -75,6 +75,7 @@ import 'rules/do_not_use_environment.dart';
 import 'rules/empty_catches.dart';
 import 'rules/empty_constructor_bodies.dart';
 import 'rules/empty_statements.dart';
+import 'rules/enable_null_safety.dart';
 import 'rules/eol_at_end_of_file.dart';
 import 'rules/exhaustive_cases.dart';
 import 'rules/file_names.dart';
@@ -297,6 +298,7 @@ void registerLintRules({bool inTestMode = false}) {
     ..register(EmptyCatches())
     ..register(EmptyConstructorBodies())
     ..register(EmptyStatements())
+    ..register(EnableNullSafety())
     ..register(EolAtEndOfFile())
     ..register(ExhaustiveCases())
     ..register(FileNames())

lib/src/rules/enable_null_safety.dart

@@ -0,0 +1,74 @@
+// Copyright (c) 2022, 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/token.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+
+import '../analyzer.dart';
+
+const _desc = r'Do use sound null safety.';
+
+const _details = r'''
+**DO** use sound null safety, by not specifying a dart version lower than `2.12`.
+
+**BAD:**
+```dart
+// @dart=2.8
+a() {
+}
+```
+
+**GOOD:**
+```dart
+b() {
+}
+''';
+
+class EnableNullSafety extends LintRule implements NodeLintRule {
+  EnableNullSafety()
+      : super(
+            name: 'enable_null_safety',
+            description: _desc,
+            details: _details,
+            group: Group.style);
+
+  @override
+  void registerNodeProcessors(
+      NodeLintRegistry registry, LinterContext context) {
+    var visitor = _Visitor(this, context);
+    registry.addCompilationUnit(this, visitor);
+  }
+}
+
+class _Visitor extends SimpleAstVisitor<void> {
+  // to be kept in sync with LanguageVersionOverrideVerifier (added groups for the version)
+  static final regExp = RegExp(r'^\s*//\s*@dart\s*=\s*(\d+)\.(\d+)');
+  final LintRule rule;
+  final LinterContext context;
+
+  _Visitor(this.rule, this.context);
+
+  @override
+  void visitCompilationUnit(CompilationUnit node) {
+    var beginToken = node.beginToken;
+    if (beginToken.type == TokenType.SCRIPT_TAG) {
+      beginToken = beginToken.next!;
+    }
+    CommentToken? comment = beginToken.precedingComments;
+    while (comment != null) {
+      var match = regExp.firstMatch(comment.lexeme);
+      if (match != null && match.groupCount == 2) {
+        var major = int.parse(match.group(1)!);
+        var minor = int.parse(match.group(2)!);
+        if (major == 1 || (major == 2 && minor < 12)) {
+          rule.reportLintForToken(comment);
+        }
+      }
+
+      var next = comment.next;
+      comment = next is CommentToken ? next : null;
+    }
+  }
+}

test/rules/all.dart

@@ -34,6 +34,7 @@ import 'dangling_library_doc_comments_test.dart'
     as dangling_library_doc_comments;
 import 'deprecated_consistency_test.dart' as deprecated_consistency;
 import 'discarded_futures_test.dart' as discarded_futures;
+import 'enable_null_safety_test.dart' as enable_null_safety;
 import 'file_names_test.dart' as file_names;
 import 'flutter_style_todos_test.dart' as flutter_style_todos;
 import 'hash_and_equals_test.dart' as hash_and_equals;
@@ -114,6 +115,7 @@ void main() {
   dangling_library_doc_comments.main();
   deprecated_consistency.main();
   discarded_futures.main();
+  enable_null_safety.main();
   file_names.main();
   flutter_style_todos.main();
   hash_and_equals.main();

test/rules/enable_null_safety_test.dart

@@ -0,0 +1,58 @@
+// Copyright (c) 2022, 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:test_reflective_loader/test_reflective_loader.dart';
+
+import '../rule_test_support.dart';
+
+main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(EnableNullSafetyTest);
+  });
+}
+
+@reflectiveTest
+class EnableNullSafetyTest extends LintRuleTest {
+  @override
+  String get lintRule => 'enable_null_safety';
+
+  test_2_11() async {
+    await assertDiagnostics(r'''
+// @dart=2.11
+f() {
+}
+''', [
+      lint(0, 13),
+    ]);
+  }
+
+  test_2_12() async {
+    await assertNoDiagnostics(r'''
+// @dart=2.12
+f() {
+}
+''');
+  }
+
+  test_2_8() async {
+    await assertDiagnostics(r'''
+// @dart=2.8
+f() {
+}
+''', [
+      lint(0, 12),
+    ]);
+  }
+
+  test_2_8_shebang() async {
+    await assertDiagnostics(r'''
+#!/usr/bin/dart
+// @dart=2.8
+f() {
+}
+''', [
+      lint(16, 12),
+    ]);
+  }
+}