Add @widgetFactory annotation (#117455)

* Add `@widgetFactory` annotation

* Simplify and fix example

* Specify `TargetKind`s for `widgetFactory`

* Explain why `library_private_types_in_public_api` is ignored.

* Trigger CI

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

@@ -23,6 +23,7 @@ import 'dart:ui' as ui
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
+import 'package:meta/meta_meta.dart';
 
 import 'app.dart';
 import 'basic.dart';
@@ -3684,3 +3685,68 @@ class InspectorSerializationDelegate implements DiagnosticsSerializationDelegate
     );
   }
 }
+
+@Target(<TargetKind>{TargetKind.method})
+class _WidgetFactory {
+  const _WidgetFactory();
+}
+
+/// Annotation which marks a function as a widget factory for the purpose of
+/// widget creation tracking.
+///
+/// When widget creation tracking is enabled, the framework tracks the source
+/// code location of the constructor call for each widget instance. This
+/// information is used by the DevTools to provide an improved developer
+/// experience. For example, it allows the Flutter inspector to present the
+/// widget tree in a manner similar to how the UI was defined in your source
+/// code.
+///
+/// [Widget] constructors are automatically instrumented to track the source
+/// code location of constructor calls. However, there are cases where
+/// a function acts as a sort of a constructor for a widget and a call to such
+/// a function should be considered as the creation location for the returned
+/// widget instance.
+///
+/// Annotating a function with this annotation marks the function as a widget
+/// factory. The framework will then instrument that function in the same way
+/// as it does for [Widget] constructors.
+///
+/// Note that the function **must not** have optional positional parameters for
+/// tracking to work correctly.
+///
+/// Currently this annotation is only supported on extension methods.
+///
+/// {@tool snippet}
+///
+/// This example shows how to use the [widgetFactory] annotation to mark an
+/// extension method as a widget factory:
+///
+/// ```dart
+/// extension PaddingModifier on Widget {
+///   @widgetFactory
+///   Widget padding(EdgeInsetsGeometry padding) {
+///     return Padding(padding: padding, child: this);
+///   }
+/// }
+/// ```
+///
+/// When using the above extension method, the framework will track the
+/// creation location of the [Padding] widget instance as the source code
+/// location where the `padding` extension method was called:
+///
+/// ```dart
+/// // continuing from previous example...
+/// const Text('Hello World!')
+///     .padding(const EdgeInsets.all(8));
+/// ```
+///
+/// {@end-tool}
+///
+/// See also:
+///
+/// * the documentation for [Track widget creation](https://docs.flutter.dev/development/tools/devtools/inspector#track-widget-creation).
+// The below ignore is needed because the static type of the annotation is used
+// by the CFE kernel transformer that implements the instrumentation to
+// recognize the annotation.
+// ignore: library_private_types_in_public_api
+const _WidgetFactory widgetFactory = _WidgetFactory();

packages/flutter/test/widgets/widget_inspector_test.dart

@@ -238,6 +238,13 @@ int getChildLayerCount(OffsetLayer layer) {
   return count;
 }
 
+extension TextFromString on String {
+  @widgetFactory
+  Widget text() {
+    return Text(this);
+  }
+}
+
 void main() {
   _TestWidgetInspectorService.runTests();
 }
@@ -944,19 +951,20 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
 
     testWidgets('WidgetInspectorService creationLocation', (WidgetTester tester) async {
       await tester.pumpWidget(
-        const Directionality(
+        Directionality(
           textDirection: TextDirection.ltr,
           child: Stack(
             children: <Widget>[
-              Text('a'),
-              Text('b', textDirection: TextDirection.ltr),
-              Text('c', textDirection: TextDirection.ltr),
+              const Text('a'),
+              const Text('b', textDirection: TextDirection.ltr),
+              'c'.text(),
             ],
           ),
         ),
       );
       final Element elementA = find.text('a').evaluate().first;
       final Element elementB = find.text('b').evaluate().first;
+      final Element elementC = find.text('c').evaluate().first;
 
       service.disposeAllGroups();
       service.resetPubRootDirectories();
@@ -979,14 +987,28 @@ class _TestWidgetInspectorService extends TestWidgetInspectorService {
       final int columnB = creationLocationB['column']! as int;
       final String? nameB = creationLocationB['name'] as String?;
       expect(nameB, equals('Text'));
+
+      service.setSelection(elementC, 'my-group');
+      final Map<String, Object?> jsonC = json.decode(service.getSelectedWidget(null, 'my-group')) as Map<String, Object?>;
+      final Map<String, Object?> creationLocationC = jsonC['creationLocation']! as Map<String, Object?>;
+      expect(creationLocationC, isNotNull);
+      final String fileC = creationLocationC['file']! as String;
+      final int lineC = creationLocationC['line']! as int;
+      final int columnC = creationLocationC['column']! as int;
+      final String? nameC = creationLocationC['name'] as String?;
+      expect(nameC, equals('TextFromString|text'));
+
       expect(fileA, endsWith('widget_inspector_test.dart'));
       expect(fileA, equals(fileB));
+      expect(fileA, equals(fileC));
       // We don't hardcode the actual lines the widgets are created on as that
       // would make this test fragile.
       expect(lineA + 1, equals(lineB));
+      expect(lineB + 1, equals(lineC));
       // Column numbers are more stable than line numbers.
-      expect(columnA, equals(15));
+      expect(columnA, equals(21));
       expect(columnA, equals(columnB));
+      expect(columnC, equals(19));
     }, skip: !WidgetInspectorService.instance.isWidgetCreationTracked()); // [intended] Test requires --track-widget-creation flag.
 
   testWidgets('WidgetInspectorService setSelection notifiers for an Element',