[const_finder] Ignore constructor invocations from generated tear-off declarations (flutter#38131)

* fix

* ignore tear off declarations

* add TODO comment

* clean up

* fix analysis

Author: christopherfujino

tools/const_finder/lib/const_finder.dart

@@ -32,6 +32,33 @@ class _ConstVisitor extends RecursiveVisitor<void> {
 
   bool inIgnoredClass = false;
 
+  /// Whether or not we are currently within the declaration of the target class.
+  ///
+  /// We use this to determine when to skip tracking non-constant
+  /// [ConstructorInvocation]s. This is because, in web builds, a static
+  /// method is always created called _#new#tearOff() which returns the result
+  /// of a non-constant invocation of the unnamed constructor.
+  ///
+  /// For the following Dart class "FooBar":
+  ///
+  /// class FooBar {
+  ///   const FooBar();
+  /// }
+  ///
+  /// The following kernel structure is generated:
+  ///
+  /// class FooBar extends core::Object /*hasConstConstructor*/  {
+  ///   const constructor •() → min::FooBar
+  ///     : super core::Object::•()
+  ///     ;
+  ///   static method _#new#tearOff() → min::FooBar
+  ///     return new min::FooBar::•(); /* this is a non-const constructor invocation */
+  ///   method noOp() → void {}
+  /// }
+  bool inTargetClass = false;
+
+  bool inTargetTearOff = false;
+
   /// The name of the name of the class of the annotation marking classes
   /// whose constant references should be ignored.
   final String? annotationClassName;
@@ -58,6 +85,18 @@ class _ConstVisitor extends RecursiveVisitor<void> {
   // Avoid visiting the same constant more than once.
   final Set<Constant> _cache = LinkedHashSet<Constant>.identity();
 
+  @override
+  void visitProcedure(Procedure node) {
+    final bool isTearOff = node.isStatic &&
+        node.kind == ProcedureKind.Method &&
+        node.name.text == '_#new#tearOff';
+    if (inTargetClass && isTearOff) {
+      inTargetTearOff = true;
+    }
+    super.visitProcedure(node);
+    inTargetTearOff = false;
+  }
+
   @override
   void defaultConstant(Constant node) {
     if (_cache.add(node)) {
@@ -73,7 +112,7 @@ class _ConstVisitor extends RecursiveVisitor<void> {
   @override
   void visitConstructorInvocation(ConstructorInvocation node) {
     final Class parentClass = node.target.parent! as Class;
-    if (_matches(parentClass)) {
+    if (!inTargetTearOff && _matches(parentClass)) {
       final Location location = node.location!;
       nonConstantLocations.add(<String, dynamic>{
         'file': location.file.toString(),
@@ -86,9 +125,11 @@ class _ConstVisitor extends RecursiveVisitor<void> {
 
   @override
   void visitClass(Class node) {
+    inTargetClass = _matches(node);
     // check if this is a class that we should ignore
     inIgnoredClass = _classShouldBeIgnored(node);
     super.visitClass(node);
+    inTargetClass = false;
     inIgnoredClass = false;
   }
 

tools/const_finder/pubspec.yaml

@@ -6,7 +6,7 @@ name: const_finder
 publish_to: none
 
 environment:
-  sdk: ">=2.17.0 <3.0.0"
+  sdk: ">=2.17.0 <4.0.0"
 
 # Do not add any dependencies that require more than what is provided in
 # //third_party/dart/pkg or //third_party/dart/third_party/pkg.

tools/const_finder/test/const_finder_test.dart

@@ -31,10 +31,6 @@ void expectInstances(dynamic value, dynamic expected, Compiler compiler) {
 
   final Equality<Object?> equality;
   if (compiler == Compiler.dart2js) {
-    // Ignore comparing nonConstantLocations in web dills because it will have
-    // extra fields.
-    (value as Map<String, Object?>).remove('nonConstantLocations');
-    (expected as Map<String, Object?>).remove('nonConstantLocations');
     equality = const Dart2JSDeepCollectionEquality();
   } else {
     equality = const DeepCollectionEquality();
@@ -69,9 +65,7 @@ void _checkConsts(String dillPath, Compiler compiler) {
     classLibraryUri: 'package:const_finder_fixtures/target.dart',
     className: 'Target',
   );
-  expectInstances(
-    finder.findInstances(),
-    <String, dynamic>{
+  final Map<String, Object?> expectation = <String, dynamic>{
       'constantInstances': <Map<String, dynamic>>[
         <String, dynamic>{'stringValue': '100', 'intValue': 100, 'targetValue': null},
         <String, dynamic>{'stringValue': '102', 'intValue': 102, 'targetValue': null},
@@ -95,7 +89,27 @@ void _checkConsts(String dillPath, Compiler compiler) {
         <String, dynamic>{'stringValue': 'package', 'intValue':-1, 'targetValue': null},
       ],
       'nonConstantLocations': <dynamic>[],
-    },
+    };
+  if (compiler == Compiler.aot) {
+    expectation['nonConstantLocations'] = <Object?>[];
+  } else {
+    final String fixturesUrl = Platform.isWindows
+      ? '/$fixtures'.replaceAll(Platform.pathSeparator, '/')
+      : fixtures;
+
+    // Without true tree-shaking, there is a non-const reference in a
+    // never-invoked function that will be present in the dill.
+    expectation['nonConstantLocations'] = <Object?>[
+      <String, dynamic>{
+        'file': 'file://$fixturesUrl/pkg/package.dart',
+        'line': 14,
+        'column': 25,
+      },
+    ];
+  }
+  expectInstances(
+    finder.findInstances(),
+    expectation,
     compiler,
   );
 
@@ -125,8 +139,9 @@ void _checkAnnotation(String dillPath, Compiler compiler) {
     annotationClassName: 'StaticIconProvider',
     annotationClassLibraryUri: 'package:const_finder_fixtures/static_icon_provider.dart',
   );
+  final Map<String, dynamic> instances = finder.findInstances();
   expectInstances(
-    finder.findInstances(),
+    instances,
     <String, dynamic>{
       'constantInstances': <Map<String, Object?>>[
         <String, Object?>{
@@ -140,6 +155,8 @@ void _checkAnnotation(String dillPath, Compiler compiler) {
           'targetValue': null,
         },
       ],
+      // TODO(fujino): This should have non-constant locations from the use of
+      // a tear-off, see https://github.com/flutter/flutter/issues/116797
       'nonConstantLocations': <Object?>[],
     },
     compiler,
@@ -212,6 +229,9 @@ void _checkNonConstsWeb(String dillPath, Compiler compiler) {
     className: 'Target',
   );
 
+  final String fixturesUrl = Platform.isWindows
+    ? '/$fixtures'.replaceAll(Platform.pathSeparator, '/')
+    : fixtures;
   expectInstances(
     finder.findInstances(),
     <String, dynamic>{
@@ -225,7 +245,33 @@ void _checkNonConstsWeb(String dillPath, Compiler compiler) {
         <String, dynamic>{'stringValue': '7', 'intValue': 7, 'targetValue': null},
         <String, dynamic>{'stringValue': 'package', 'intValue': -1, 'targetValue': null},
       ],
-      'nonConstantLocations': <dynamic>[]
+      'nonConstantLocations': <dynamic>[
+        <String, dynamic>{
+          'file': 'file://$fixturesUrl/lib/consts_and_non.dart',
+          'line': 14,
+          'column': 26,
+        },
+        <String, dynamic>{
+          'file': 'file://$fixturesUrl/lib/consts_and_non.dart',
+          'line': 16,
+          'column': 26,
+        },
+        <String, dynamic>{
+          'file': 'file://$fixturesUrl/lib/consts_and_non.dart',
+          'line': 16,
+          'column': 41,
+        },
+        <String, dynamic>{
+          'file': 'file://$fixturesUrl/lib/consts_and_non.dart',
+          'line': 17,
+          'column': 26,
+        },
+        <String, dynamic>{
+          'file': 'file://$fixturesUrl/pkg/package.dart',
+          'line': 14,
+          'column': 25,
+        }
+      ],
     },
     compiler,
   );

tools/const_finder/test/fixtures/.dart_tool/package_config.json

@@ -4,12 +4,12 @@
         {
             "name": "const_finder_fixtures",
             "rootUri": "../lib/",
-            "languageVersion": "2.12"
+            "languageVersion": "2.17"
         },
         {
             "name": "const_finder_fixtures_package",
             "rootUri": "../pkg/",
-            "languageVersion": "2.12"
+            "languageVersion": "2.17"
         }
     ]
 }

tools/const_finder/test/fixtures/lib/static_icon_provider.dart

@@ -7,6 +7,12 @@ import 'target.dart';
 void main() {
   Targets.used1.hit();
   Targets.used2.hit();
+  final Target nonConstUsed3 = helper(Target.new);
+  nonConstUsed3.hit();
+}
+
+Target helper(Target Function(String, int, Target?) tearOff) {
+  return tearOff('from tear-off', 3, null);
 }
 
 @staticIconProvider

tools/const_finder/test/fixtures/lib/target.dart

@@ -15,8 +15,7 @@ class Target {
 }
 
 class ExtendsTarget extends Target {
-  const ExtendsTarget(String stringValue, int intValue, Target? targetValue)
-      : super(stringValue, intValue, targetValue);
+  const ExtendsTarget(super.stringValue, super.intValue, super.targetValue);
 }
 
 class ImplementsTarget implements Target {