update DEAD_CODE analysis for local wildcard functions

See: #55680

Change-Id: Ib65be42206c6bd66ddaaf54224490870acf4a2bd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/368941
Auto-Submit: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

Author: pq

pkg/analyzer/lib/src/dart/analysis/library_analyzer.dart

@@ -437,7 +437,7 @@ class LibraryAnalyzer {
 
     UnicodeTextVerifier(errorReporter).verify(unit, unitAnalysis.file.content);
 
-    unit.accept(DeadCodeVerifier(errorReporter));
+    unit.accept(DeadCodeVerifier(errorReporter, _libraryElement));
 
     unit.accept(
       BestPracticesVerifier(

pkg/analyzer/lib/src/error/dead_code_verifier.dart

@@ -2,6 +2,7 @@
 // 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/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ -31,7 +32,12 @@ class DeadCodeVerifier extends RecursiveAstVisitor<void> {
   /// The object used to track the usage of labels within a given label scope.
   _LabelTracker? _labelTracker;
 
-  DeadCodeVerifier(this._errorReporter);
+  /// Whether the `wildcard_variables` feature is enabled.
+  final bool _wildCardVariablesEnabled;
+
+  DeadCodeVerifier(this._errorReporter, LibraryElement library)
+      : _wildCardVariablesEnabled =
+            library.featureSet.isEnabled(Feature.wildcard_variables);
 
   @override
   void visitBreakStatement(BreakStatement node) {
@@ -58,6 +64,19 @@ class DeadCodeVerifier extends RecursiveAstVisitor<void> {
     super.visitExportDirective(node);
   }
 
+  @override
+  void visitFunctionDeclaration(FunctionDeclaration node) {
+    var element = node.declaredElement;
+    // TODO(pq): ask the FunctionElement once implemented
+    if (_wildCardVariablesEnabled &&
+        element is FunctionElement &&
+        element.isLocal &&
+        element.name == '_') {
+      _errorReporter.atNode(node, WarningCode.DEAD_CODE);
+    }
+    super.visitFunctionDeclaration(node);
+  }
+
   @override
   void visitImportDirective(ImportDirective node) {
     var importElement = node.element;
@@ -545,6 +564,11 @@ class _LabelTracker {
   }
 }
 
+extension on FunctionElement {
+  bool get isLocal =>
+      enclosingElement is FunctionElement || enclosingElement is MethodElement;
+}
+
 extension DoStatementExtension on DoStatement {
   bool get hasBreakStatement {
     var visitor = _BreakDoStatementVisitor(this);

pkg/analyzer/test/src/diagnostics/dead_code_test.dart

@@ -10,6 +10,7 @@ import '../dart/resolution/context_collection_resolution.dart';
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(DeadCodeTest);
+    defineReflectiveTests(DeadCodeWildcardVariablesTest);
     defineReflectiveTests(DeadCodeTest_Language219);
   });
 }
@@ -137,6 +138,17 @@ void f(int a) {
     ]);
   }
 
+  test_localFunction_wildcard() async {
+    await assertErrorsInCode(r'''
+void f() {
+  _(){}
+}
+''', [
+      // No dead code.
+      error(WarningCode.UNUSED_ELEMENT, 13, 1),
+    ]);
+  }
+
   test_objectPattern_neverTypedGetter() async {
     await assertErrorsInCode(r'''
 class A {
@@ -1744,3 +1756,19 @@ Iterable<int> f() sync* {
     ]);
   }
 }
+
+// TODO(pq): inline this test once we've sorted out how we want to use language overrides for experiment testing
+@reflectiveTest
+class DeadCodeWildcardVariablesTest extends DeadCodeTest
+    with WithWildCardVariablesMixin {
+  @override
+  test_localFunction_wildcard() async {
+    await assertErrorsInCode(r'''
+void f() {
+  _(){}
+}
+''', [
+      error(WarningCode.DEAD_CODE, 13, 5),
+    ]);
+  }
+}

pkg/analyzer/test/src/diagnostics/unused_element_test.dart

@@ -2929,21 +2929,27 @@ class UnusedElementWildCardVariablesTest extends UnusedElementTest
     with WithWildCardVariablesMixin {
   @override
   test_localFunction_inFunction_wildcard() async {
-    await assertNoErrorsInCode(r'''
+    await assertErrorsInCode(r'''
 m() {
   _(){}
 }
-''');
+''', [
+      // Code is dead but not unused.
+      error(WarningCode.DEAD_CODE, 8, 5),
+    ]);
   }
 
   @override
   test_localFunction_inMethod_wildcard() async {
-    await assertNoErrorsInCode(r'''
+    await assertErrorsInCode(r'''
 class C {
   m() {
     _(){}
   }
 }
-''');
+''', [
+      // Code is dead but not unused.
+      error(WarningCode.DEAD_CODE, 22, 5),
+    ]);
   }
 }