[dyn_modules] Handle implicit uses from const constructors and field initializers of const classes

When const constructor is exposed through a dynamic interface,
dynamic module can create constants with that const constructor and
it can reference everything used in that constructor including default
values of parameters and field initializers. So dynamic interface
annotator should visit bodies of const constructors and add all
references to implicit uses.

Annotator should also visit initializers of instance fields of
classes which can be used in a constant (as they participate in
the constant evaluation too). Kernel trim tool should not remove such
field initializers.

TEST=pkg/dynamic_modules/test/data/const_constructor
Fixes b/418928636

Change-Id: Ie216f3a4257fe5a905b83af8c60dfe8a5b774ecf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/430002
Commit-Queue: Alexander Markov <[email protected]>
Reviewed-by: Sigmund Cherem <[email protected]>
Reviewed-by: Slava Egorov <[email protected]>

Author: alexmarkov

pkg/dynamic_modules/test/data/const_constructor/dynamic_interface.yaml

@@ -0,0 +1,6 @@
+# Copyright (c) 2025, 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.
+
+callable:
+  - library: 'shared/shared.dart'

pkg/dynamic_modules/test/data/const_constructor/main.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2025, 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.
+
+import '../../common/testing.dart' as helper;
+import 'package:expect/expect.dart';
+
+import 'shared/shared.dart'; // ignore: unused_import
+
+/// Verify that default values of const constructor parameters are retained.
+void main() async {
+  final result = (await helper.load('entry1.dart')) as String;
+  Expect.equals("Foo1:41 Foo2:42 Foo3:43", result);
+  helper.done();
+}

pkg/dynamic_modules/test/data/const_constructor/modules/entry1.dart

@@ -0,0 +1,10 @@
+// Copyright (c) 2025, 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.
+
+import '../shared/shared.dart';
+
+@pragma('dyn-module:entry-point')
+Object? dynamicModuleEntrypoint() {
+  return "${const Foo1()} ${const Foo2()} ${const Foo3()}";
+}

pkg/dynamic_modules/test/data/const_constructor/shared/shared.dart

@@ -0,0 +1,31 @@
+// Copyright (c) 2025, 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.
+
+int _defaultF1() => 41;
+int _defaultF2() => 42;
+int _defaultF3() => 43;
+
+class Foo1 {
+  final int Function() f1;
+  const Foo1({this.f1 = _defaultF1});
+
+  @override
+  String toString() => "Foo1:${f1()}";
+}
+
+class Foo2 {
+  final int Function() _f2 = _defaultF2;
+  const Foo2();
+
+  @override
+  String toString() => "Foo2:${_f2()}";
+}
+
+class Foo3 {
+  final int Function() _f3;
+  const Foo3() : _f3 = _defaultF3;
+
+  @override
+  String toString() => "Foo3:${_f3()}";
+}

pkg/front_end/lib/src/util/trim.dart

@@ -213,6 +213,7 @@ class Trimmer extends RecursiveVisitor {
   void visitField(Field node) {
     // Constant initializers are necessary for constant evaluation
     if (node.isConst) return;
+    if (!node.isStatic && node.enclosingClass!.hasConstConstructor) return;
 
     // Unfortunately a `null` initializer may be misinterpreted by the CFE or
     // the compiler. Ideally the kernel representation should have a sentinel

pkg/vm/lib/transformations/dynamic_interface_annotator.dart

@@ -377,6 +377,7 @@ class _ImplicitUsesAnnotator extends RecursiveVisitor {
     annotateMember(node);
     if (node.isConst) {
       annotateConstantClass(node.enclosingClass);
+      node.visitChildren(this);
     }
   }
 
@@ -431,6 +432,7 @@ class _ImplicitUsesAnnotator extends RecursiveVisitor {
       for (final f in node.fields) {
         if (f.isInstanceMember) {
           annotateMember(f);
+          f.initializer?.accept(this);
         }
       }
       final superclass = node.superclass;