[ Linter ] Don't mark functions annotated with @Preview() as unreachable from main

Fixes flutter/flutter#167195

Change-Id: Ie98a0fd5c1b69771b7e9e9cb54c33bfa403381e2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/422520
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Ben Konyi <[email protected]>

Author: bkonyi

pkg/analyzer_utilities/lib/test/mock_packages/package_content/flutter/lib/src/widget_previews/widget_previews.dart

@@ -0,0 +1,28 @@
+// 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.
+
+import 'package:flutter/material.dart' show Brightness;
+import 'package:flutter/widgets.dart';
+
+base class Preview {
+  const Preview({
+    this.name,
+    this.width,
+    this.height,
+    this.textScaleFactor,
+    this.wrapper,
+    this.theme,
+    this.brightness,
+  });
+
+  final String? name;
+  final double? width;
+  final double? height;
+  final double? textScaleFactor;
+  final Widget Function(Widget)? wrapper;
+  final PreviewThemeData Function()? theme;
+  final Brightness? brightness;
+}
+
+base class PreviewThemeData {}

pkg/analyzer_utilities/lib/test/mock_packages/package_content/flutter/lib/widget_previews.dart

@@ -0,0 +1,5 @@
+// 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.
+
+export 'src/widget_previews/widget_previews.dart';

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

@@ -523,7 +523,8 @@ class _Visitor extends SimpleAstVisitor<void> {
           var element = declaration.declaredFragment?.element;
           return element != null &&
               element.isPublic &&
-              !element.hasVisibleForTesting;
+              !element.hasVisibleForTesting &&
+              !element.hasWidgetPreview;
         }).toList();
 
     for (var member in unusedMembers) {
@@ -556,10 +557,18 @@ class _Visitor extends SimpleAstVisitor<void> {
 
   bool _isEntryPoint(Declaration e) =>
       e is FunctionDeclaration &&
-      (e.name.lexeme == 'main' || e.metadata.any(_isPragmaVmEntry));
+      (e.name.lexeme == 'main' || e.metadata.any(_isExemptingAnnotation));
 
-  bool _isPragmaVmEntry(Annotation annotation) {
-    if (!annotation.isPragma) return false;
+  bool _isExemptingAnnotation(Annotation annotation) {
+    if (annotation.isPragma) {
+      return _isValidVmEntryPoint(annotation);
+    } else if (annotation.isWidgetPreview) {
+      return true;
+    }
+    return false;
+  }
+
+  bool _isValidVmEntryPoint(Annotation annotation) {
     var value = annotation.elementAnnotation?.computeConstantValue();
     if (value == null) return false;
     var name = value.getField('name');
@@ -569,27 +578,91 @@ class _Visitor extends SimpleAstVisitor<void> {
   }
 }
 
+extension on Metadata {
+  bool get hasWidgetPreview {
+    var annotations = this.annotations;
+    for (var i = 0; i < annotations.length; i++) {
+      var annotation = annotations[i];
+      if (annotation.isWidgetPreview) {
+        return true;
+      }
+    }
+    return false;
+  }
+}
+
+extension on ElementAnnotation {
+  /// The URI of the Flutter widget previews library.
+  static final Uri _flutterWidgetPreviewLibraryUri = Uri.parse(
+    'package:flutter/src/widget_previews/widget_previews.dart',
+  );
+
+  bool get isWidgetPreview {
+    var element2 = this.element2;
+    return element2 is ConstructorElement2 &&
+        element2.enclosingElement2.name3 == 'Preview' &&
+        element2.library2.uri == _flutterWidgetPreviewLibraryUri;
+  }
+}
+
+extension on LibraryElement2 {
+  bool get isWidgetPreviews =>
+      uri ==
+      Uri.parse('package:flutter/src/widget_previews/widget_previews.dart');
+}
+
 extension on Element2 {
   bool get hasVisibleForTesting => switch (this) {
     Annotatable(:var metadata2) => metadata2.hasVisibleForTesting,
     _ => false,
   };
+  bool get hasWidgetPreview => switch (this) {
+    Annotatable(:var metadata2) =>
+      // Widget previews can be applied to public:
+      //   - Constructors (generative and factory)
+      //   - Top-level functions
+      //   - Static member functions
+      (this is ConstructorElement2 ||
+              this is TopLevelFunctionElement ||
+              (this is ExecutableElement2 &&
+                  (this as ExecutableElement2).isStatic)) &&
+          !isPrivate &&
+          metadata2.hasWidgetPreview,
+    _ => false,
+  };
   bool get isPragma => (library2?.isDartCore ?? false) && name3 == 'pragma';
+  bool get isWidgetPreview =>
+      (library2?.isWidgetPreviews ?? false) && name3 == 'Preview';
 }
 
 extension on Annotation {
   bool get isPragma {
+    DartType? type = _elementType;
+    if (type == null) {
+      // Dunno what this is.
+      return false;
+    }
+    return type is InterfaceType && type.element3.isPragma;
+  }
+
+  bool get isWidgetPreview {
+    DartType? type = _elementType;
+    if (type == null) {
+      // Dunno what this is.
+      return false;
+    }
+    return type is InterfaceType && type.element3.isWidgetPreview;
+  }
+
+  DartType? get _elementType {
     var element = elementAnnotation?.element2;
-    DartType type;
+    DartType? type;
     if (element is ConstructorElement2) {
       type = element.returnType;
     } else if (element is GetterElement) {
       type = element.returnType;
-    } else {
-      // Dunno what this is.
-      return false;
     }
-    return type is InterfaceType && type.element3.isPragma;
+    return type;
   }
 }
 

pkg/linter/test/rules/unreachable_from_main_test.dart

@@ -14,6 +14,9 @@ void main() {
 
 @reflectiveTest
 class UnreachableFromMainTest extends LintRuleTest {
+  @override
+  bool get addFlutterPackageDep => true;
+
   @override
   bool get addMetaPackageDep => true;
 
@@ -1411,13 +1414,17 @@ void f() {}
 
   test_topLevelFunction_vmEntryPoint() async {
     await assertNoDiagnostics(r'''
+void main() {}
+
 @pragma('vm:entry-point')
 void f6() {}
 ''');
   }
 
   test_topLevelFunction_vmEntryPoint_const() async {
     await assertNoDiagnostics(r'''
+void main() {}
+
 const entryPoint = pragma('vm:entry-point');
 @entryPoint
 void f6() {}
@@ -1630,4 +1637,143 @@ typedef T = String;
       [lint(24, 1)],
     );
   }
+
+  test_widgetPreview_classInstanceMethod() async {
+    await assertDiagnostics(
+      r'''
+import 'package:flutter/widgets.dart';
+import 'package:flutter/widget_previews.dart';
+
+void main() {}
+
+class B {
+  // Widget previews can't be defined with instance methods.
+  @Preview()
+  Widget foo() => Text('');
+}
+''',
+      [lint(109, 1), lint(196, 3)],
+    );
+  }
+
+  test_widgetPreview_classStaticMethod() async {
+    await assertDiagnostics(
+      r'''
+import 'package:flutter/widgets.dart';
+import 'package:flutter/widget_previews.dart';
+
+void main() {}
+
+class B {
+  @Preview()
+  static Widget foo() => Text('');
+}
+''',
+      [lint(109, 1)],
+    );
+  }
+
+  test_widgetPreview_constructor() async {
+    await assertDiagnostics(
+      r'''
+import 'package:flutter/widget_previews.dart';
+
+void main() {}
+
+class B {
+  @Preview()
+  const B();
+}
+''',
+      [lint(70, 1)],
+    );
+  }
+
+  test_widgetPreview_factoryConstructor() async {
+    await assertDiagnostics(
+      r'''
+import 'package:flutter/widget_previews.dart';
+
+void main() {}
+
+class B {
+  @Preview()
+  factory B.foo() => B();
+
+  const B();
+}
+''',
+      [lint(70, 1), lint(122, 1)],
+    );
+  }
+
+  test_widgetPreview_privatePreview() async {
+    await assertDiagnostics(
+      r'''
+// Widget previews can't be defined with private functions.
+import 'package:flutter/widget_previews.dart';
+import 'package:flutter/widgets.dart';
+void main() {}
+
+class B {
+  @Preview()
+  factory B._foo() => B._();
+
+  @Preview()
+  B._();
+
+  @Preview()
+  static Widget _bar() => Text('');
+}
+
+@Preview()
+void _f6() {}
+''',
+      [
+        lint(168, 1),
+        error(WarningCode.UNUSED_ELEMENT, 197, 4),
+        error(WarningCode.UNUSED_ELEMENT, 267, 4),
+        error(WarningCode.UNUSED_ELEMENT, 306, 3),
+      ],
+    );
+  }
+
+  test_widgetPreview_topLevelFunction() async {
+    await assertNoDiagnostics(r'''
+import 'package:flutter/widget_previews.dart';
+
+void main() {}
+
+@Preview()
+void f6() {}
+''');
+  }
+
+  test_widgetPreview_topLevelFunction_const() async {
+    await assertNoDiagnostics(r'''
+import 'package:flutter/widget_previews.dart';
+void main() {}
+
+const preview = Preview();
+@preview
+void f6() {}
+''');
+  }
+
+  test_widgetPreview_topLevelFunction_customPreviewClass() async {
+    await assertDiagnostics(
+      r'''
+void main() {}
+
+class Preview {
+  const Preview();
+}
+
+// This isn't from package:flutter/widget_previews.dart and shouldn't be exempt.
+@Preview()
+void f6() {}
+''',
+      [lint(22, 7), lint(40, 7), lint(151, 2)],
+    );
+  }
 }