[framework] dont allocate forgotten children set in profile/release mode (#94911)

dev/bots/analyze.dart

@@ -98,6 +98,9 @@ Future<void> run(List<String> arguments) async {
   print('$clock Integration test timeouts...');
   await verifyIntegrationTestTimeouts(flutterRoot);
 
+  print('$clock null initialized debug fields...');
+  await verifyNullInitializedDebugExpensiveFields(flutterRoot);
+
   // Ensure that all package dependencies are in sync.
   print('$clock Package dependencies...');
   await runCommand(flutter, <String>['update-packages', '--verify-only'],
@@ -785,7 +788,7 @@ Future<void> verifyIssueLinks(String workingDirectory) async {
   final Set<String> suggestions = <String>{};
   final List<File> files = await _gitFiles(workingDirectory);
   for (final File file in files) {
-    if (path.basename(file.path).endsWith('_test.dart'))
+    if (path.basename(file.path).endsWith('_test.dart') || path.basename(file.path) == 'analyze.dart')
       continue; // Skip tests, they're not public-facing.
     final Uint8List bytes = file.readAsBytesSync();
     // We allow invalid UTF-8 here so that binaries don't trip us up.
@@ -1492,6 +1495,40 @@ Future<void> _checkConsumerDependencies() async {
   }
 }
 
+const String _kDebugOnlyAnnotation = '@_debugOnly';
+final RegExp _nullInitializedField = RegExp(r'kDebugMode \? [\w\<\> ,{}()]+ : null;');
+
+Future<void> verifyNullInitializedDebugExpensiveFields(String workingDirectory, {int minimumMatches = 400}) async {
+  final String flutterLib = path.join(workingDirectory, 'packages', 'flutter', 'lib');
+  final List<File> files = await _allFiles(flutterLib, 'dart', minimumMatches: minimumMatches)
+    .toList();
+  final List<String> errors = <String>[];
+  for (final File file in files) {
+    final List<String> lines = file.readAsLinesSync();
+    for (int i = 0; i < lines.length; i += 1) {
+      final String line = lines[i];
+      if (!line.contains(_kDebugOnlyAnnotation)) {
+        continue;
+      }
+      final String nextLine = lines[i + 1];
+      if (_nullInitializedField.firstMatch(nextLine) == null) {
+        errors.add('${file.path} L$i');
+      }
+    }
+  }
+
+  if (errors.isNotEmpty) {
+    exitWithError(<String>[
+     '${bold}ERROR: ${red}fields annotated with @_debugOnly must null initialize.$reset',
+     'to ensure both the field and initializer are removed from profile/release mode.',
+     'These fields should be written as:\n',
+     'field = kDebugMode ? <DebugValue> : null;\n',
+     'Errors were found in the following files:',
+      ...errors,
+    ]);
+  }
+}
+
 Future<CommandResult> _runFlutterAnalyze(String workingDirectory, {
   List<String> options = const <String>[],
 }) async {

dev/bots/test/analyze-test-input/root/packages/flutter/lib/bar.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+class _DebugOnly {
+  const _DebugOnly();
+}
+
+const _DebugOnly _debugOnly = _DebugOnly();
+const bool kDebugMode = bool.fromEnvironment('test-only');
+
+class Foo {
+  @_debugOnly
+  final Map<String, String>? foo = kDebugMode ? <String, String>{} : null;
+
+  @_debugOnly
+  final Map<String, String>? bar = kDebugMode ? null : <String, String>{};
+}

dev/bots/test/analyze_test.dart

@@ -193,4 +193,14 @@ void main() {
       },
     ));
   });
+
+  test('analyze.dart - verifyNullInitializedDebugExpensiveFields', () async {
+    final String result = await capture(() => verifyNullInitializedDebugExpensiveFields(
+      testRootPath,
+      minimumMatches: 1,
+    ), exitCode: 1);
+
+    expect(result, contains('L15'));
+    expect(result, isNot(contains('L12')));
+  });
 }

packages/flutter/lib/src/widgets/framework.dart

@@ -37,6 +37,22 @@ export 'package:flutter/rendering.dart' show RenderObject, RenderBox, debugDumpR
 // abstract class FrogJar extends RenderObjectWidget { const FrogJar({Key? key}) : super(key: key); }
 // abstract class FrogJarParentData extends ParentData { late Size size; }
 
+// An annotation used by test_analysis package to verify patterns are followed
+// that allow for tree-shaking of both fields and their initializers. This
+// annotation has no impact on code by itself, but indicates the following pattern
+// should be followed for a given field:
+//
+// ```dart
+// class Foo {
+//   final bar = kDebugMode ? Object() : null;
+// }
+// ```
+class _DebugOnly {
+  const _DebugOnly();
+}
+
+const _DebugOnly _debugOnly = _DebugOnly();
+
 // KEYS
 
 /// A key that is only equal to itself.
@@ -2705,12 +2721,21 @@ class BuildOwner {
   }
 
   final Map<GlobalKey, Element> _globalKeyRegistry = <GlobalKey, Element>{};
-  final Set<Element> _debugIllFatedElements = HashSet<Element>();
+
+  // In Profile/Release mode this field is initialized to `null`. The Dart compiler can
+  // eliminate unused fields, but not their initializers.
+  @_debugOnly
+  final Set<Element>? _debugIllFatedElements = kDebugMode ? HashSet<Element>() : null;
+
   // This map keeps track which child reserves the global key with the parent.
   // Parent, child -> global key.
   // This provides us a way to remove old reservation while parent rebuilds the
   // child in the same slot.
-  final Map<Element, Map<Element, GlobalKey>> _debugGlobalKeyReservations = <Element, Map<Element, GlobalKey>>{};
+  //
+  // In Profile/Release mode this field is initialized to `null`. The Dart compiler can
+  // eliminate unused fields, but not their initializers.
+  @_debugOnly
+  final Map<Element, Map<Element, GlobalKey>>? _debugGlobalKeyReservations = kDebugMode ? <Element, Map<Element, GlobalKey>>{} : null;
 
   /// The number of [GlobalKey] instances that are currently associated with
   /// [Element]s that have been built by this build owner.
@@ -2720,7 +2745,7 @@ class BuildOwner {
     assert(() {
       assert(parent != null);
       assert(child != null);
-      _debugGlobalKeyReservations[parent]?.remove(child);
+      _debugGlobalKeyReservations?[parent]?.remove(child);
       return true;
     }());
   }
@@ -2732,7 +2757,7 @@ class BuildOwner {
         final Element oldElement = _globalKeyRegistry[key]!;
         assert(oldElement.widget != null);
         assert(element.widget.runtimeType != oldElement.widget.runtimeType);
-        _debugIllFatedElements.add(oldElement);
+        _debugIllFatedElements?.add(oldElement);
       }
       return true;
     }());
@@ -2757,8 +2782,8 @@ class BuildOwner {
     assert(() {
       assert(parent != null);
       assert(child != null);
-      _debugGlobalKeyReservations[parent] ??= <Element, GlobalKey>{};
-      _debugGlobalKeyReservations[parent]![child] = key;
+      _debugGlobalKeyReservations?[parent] ??= <Element, GlobalKey>{};
+      _debugGlobalKeyReservations?[parent]![child] = key;
       return true;
     }());
   }
@@ -2766,7 +2791,7 @@ class BuildOwner {
   void _debugVerifyGlobalKeyReservation() {
     assert(() {
       final Map<GlobalKey, Element> keyToParent = <GlobalKey, Element>{};
-      _debugGlobalKeyReservations.forEach((Element parent, Map<Element, GlobalKey> childToKey) {
+      _debugGlobalKeyReservations?.forEach((Element parent, Map<Element, GlobalKey> childToKey) {
         // We ignore parent that are unmounted or detached.
         if (parent._lifecycleState == _ElementLifecycle.defunct || parent.renderObject?.attached == false)
           return;
@@ -2829,7 +2854,7 @@ class BuildOwner {
           }
         });
       });
-      _debugGlobalKeyReservations.clear();
+      _debugGlobalKeyReservations?.clear();
       return true;
     }());
   }
@@ -2837,7 +2862,7 @@ class BuildOwner {
   void _debugVerifyIllFatedPopulation() {
     assert(() {
       Map<GlobalKey, Set<Element>>? duplicates;
-      for (final Element element in _debugIllFatedElements) {
+      for (final Element element in _debugIllFatedElements!) {
         if (element._lifecycleState != _ElementLifecycle.defunct) {
           assert(element != null);
           assert(element.widget != null);
@@ -2851,7 +2876,7 @@ class BuildOwner {
           elements.add(_globalKeyRegistry[key]!);
         }
       }
-      _debugIllFatedElements.clear();
+      _debugIllFatedElements?.clear();
       if (duplicates != null) {
         final List<DiagnosticsNode> information = <DiagnosticsNode>[];
         information.add(ErrorSummary('Multiple widgets used the same GlobalKey.'));
@@ -2887,9 +2912,7 @@ class BuildOwner {
       Timeline.startSync('FINALIZE TREE', arguments: timelineArgumentsIndicatingLandmarkEvent);
     }
     try {
-      lockState(() {
-        _inactiveElements._unmountAll(); // this unregisters the GlobalKeys
-      });
+      lockState(_inactiveElements._unmountAll); // this unregisters the GlobalKeys
       assert(() {
         try {
           _debugVerifyGlobalKeyReservation();
@@ -3588,8 +3611,8 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
     // never updates (the forgotten children are not removed from the tree
     // until the call to update happens)
     assert(() {
-      _debugForgottenChildrenWithGlobalKey.forEach(_debugRemoveGlobalKeyReservation);
-      _debugForgottenChildrenWithGlobalKey.clear();
+      _debugForgottenChildrenWithGlobalKey?.forEach(_debugRemoveGlobalKeyReservation);
+      _debugForgottenChildrenWithGlobalKey?.clear();
       return true;
     }());
     _widget = newWidget;
@@ -3795,7 +3818,11 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
 
   // The children that have been forgotten by forgetChild. This will be used in
   // [update] to remove the global key reservations of forgotten children.
-  final Set<Element> _debugForgottenChildrenWithGlobalKey = HashSet<Element>();
+  //
+  // In Profile/Release mode this field is initialized to `null`. The Dart compiler can
+  // eliminate unused fields, but not their initializers.
+  @_debugOnly
+  final Set<Element>? _debugForgottenChildrenWithGlobalKey = kDebugMode ? HashSet<Element>() : null;
 
   /// Remove the given child from the element's child list, in preparation for
   /// the child being reused elsewhere in the element tree.
@@ -3821,7 +3848,7 @@ abstract class Element extends DiagnosticableTree implements BuildContext {
     // key duplication that we need to catch.
     assert(() {
       if (child.widget.key is GlobalKey)
-        _debugForgottenChildrenWithGlobalKey.add(child);
+        _debugForgottenChildrenWithGlobalKey?.add(child);
       return true;
     }());
   }