[wildcards] support for no_leading_underscores_for_library_prefixes

Fixes: https://github.com/dart-lang/linter/issues/5039

Change-Id: I71c3af855c8744877711921ab0e7b27293a53c01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/378570
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/linter/lib/src/rules/no_leading_underscores_for_library_prefixes.dart

@@ -2,8 +2,10 @@
 // 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/analysis/features.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:analyzer/dart/element/element.dart';
 
 import '../analyzer.dart';
 import '../util/ascii_utils.dart';
@@ -48,22 +50,29 @@ class NoLeadingUnderscoresForLibraryPrefixes extends LintRule {
   @override
   void registerNodeProcessors(
       NodeLintRegistry registry, LinterContext context) {
-    var visitor = _Visitor(this);
+    var visitor = _Visitor(this, context.libraryElement);
     registry.addImportDirective(this, visitor);
   }
 }
 
 class _Visitor extends SimpleAstVisitor<void> {
+  /// Whether the `wildcard_variables` feature is enabled.
+  final bool _wildCardVariablesEnabled;
+
   final LintRule rule;
 
-  _Visitor(this.rule);
+  _Visitor(this.rule, LibraryElement? library)
+      : _wildCardVariablesEnabled =
+            library?.featureSet.isEnabled(Feature.wildcard_variables) ?? false;
 
   void checkIdentifier(SimpleIdentifier? id) {
-    if (id == null) {
-      return;
-    }
+    if (id == null) return;
+
+    var name = id.name;
+
+    if (_wildCardVariablesEnabled && name == '_') return;
 
-    if (id.name.hasLeadingUnderscore) {
+    if (name.hasLeadingUnderscore) {
       rule.reportLint(id, arguments: [id.name]);
     }
   }

pkg/linter/test/rules/no_leading_underscores_for_library_prefixes_test.dart

@@ -33,6 +33,20 @@ import 'dart:async' as _async;
   test_snakeCase() async {
     await assertNoDiagnostics(r'''
 import 'dart:async' as dart_async;
+''');
+  }
+
+  test_underscores() async {
+    await assertDiagnostics(r'''
+import 'dart:async' as __;
+''', [
+      lint(23, 2),
+    ]);
+  }
+
+  test_wildcard() async {
+    await assertNoDiagnostics(r'''
+import 'dart:async' as _;
 ''');
   }
 }