Issue 29393. Consider 'v ??= doSomething()' as usage of 'v'.

pkg/analyzer/test/generated/hint_code_test.dart

 // Copyright (c) 2016, 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.
 
 library analyzer.test.generated.hint_code_test;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/parser.dart';
@@ skipping 1989 matching lines @@
 @immutable
 class A {
   int x;
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.MUST_BE_IMMUTABLE]);
     verify([source]);
   }
 
-  test_mustBeImmutable_instance() async {
-    Source source = addSource(r'''
-import 'package:meta/meta.dart';
-@immutable
-class A {
-  static int x;
-}
-''');
-    await computeAnalysisResult(source);
-    assertErrors(source, []);
-    verify([source]);
-  }
-
   test_mustBeImmutable_extends() async {
     Source source = addSource(r'''
 import 'package:meta/meta.dart';
 @immutable
 class A {}
 class B extends A {
   int x;
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.MUST_BE_IMMUTABLE]);
     verify([source]);
   }
 
   test_mustBeImmutable_fromMixin() async {
     Source source = addSource(r'''
 import 'package:meta/meta.dart';
 @immutable
 class A {}
 class B {
   int x;
 }
 class C extends A with B {}
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.MUST_BE_IMMUTABLE]);
     verify([source]);
   }
 
+  test_mustBeImmutable_instance() async {
+    Source source = addSource(r'''
+import 'package:meta/meta.dart';
+@immutable
+class A {
+  static int x;
+}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(source, []);
+    verify([source]);
+  }
+
   test_mustCallSuper() async {
     Source source = addSource(r'''
 import 'package:meta/meta.dart';
 class A {
   @mustCallSuper
   void a() {}
 }
 class B extends A {
   @override
   void a()
@@ skipping 1673 matching lines @@
     Source source = addSource(r'''
 class A {
   int _f;
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.UNUSED_FIELD]);
     verify([source]);
   }
 
+  test_unusedField_notUsed_nullAssign() async {
+    enableUnusedElement = true;
+    Source source = addSource(r'''
+class A {
+  var _f;
+  m() {
+    _f ??= doSomething();
+  }
+}
+doSomething() => 0;
+''');
+    await computeAnalysisResult(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
   test_unusedField_notUsed_postfixExpr() async {
     enableUnusedElement = true;
     Source source = addSource(r'''
 class A {
   int _f = 0;
   main() {
     _f++;
   }
 }''');
     await computeAnalysisResult(source);
@@ skipping 252 matching lines @@
 typedef Foo();
 main() {
   Foo foo;
   foo();
 }''');
     await computeAnalysisResult(source);
     assertErrors(source);
     verify([source]);
   }
 
+  test_unusedLocalVariable_isNullAssigned() async {
+    enableUnusedLocalVariable = true;
+    Source source = addSource(r'''
+typedef Foo();
+main() {
+  var v;
+  v ??= doSomething();
+}
+doSomething() => 42;
+''');
+    await computeAnalysisResult(source);
+    assertErrors(source);
+    verify([source]);
+  }
+
   test_unusedLocalVariable_isRead_notUsed_compoundAssign() async {
     enableUnusedLocalVariable = true;
     Source source = addSource(r'''
 main() {
   var v = 1;
   v += 2;
 }''');
     await computeAnalysisResult(source);
     assertErrors(source, [HintCode.UNUSED_LOCAL_VARIABLE]);
     verify([source]);
@@ skipping 208 matching lines @@
   n() {
     var a = m(), b = m();
   }
 }''');
     await computeAnalysisResult(source);
     assertErrors(
         source, [HintCode.USE_OF_VOID_RESULT, HintCode.USE_OF_VOID_RESULT]);
     verify([source]);
   }
 }