Add a fix for initialize_in_field_declaration

The correction producer implements a fix. Pending discussion we might
choose to extend it to produce an assist, either in addition to the fix
(if we keep the lint) or instead of a fix (if we drop the lint).
Whatever we choose, I'd like to do that in a separate CL because I want
this to be available for testing purposes as soon as possible.

Change-Id: Icdd17ba0130149fc785c196be69e9ef990529800
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/503520
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analysis_server/lib/src/services/correction/dart/move_initialization_to_field_declaration.dart

@@ -0,0 +1,99 @@
+// Copyright (c) 2026, 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 'package:analysis_server/src/services/correction/fix.dart';
+import 'package:analysis_server_plugin/edit/dart/correction_producer.dart';
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/src/utilities/extensions/ast.dart';
+import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dart';
+import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
+import 'package:analyzer_plugin/utilities/range_factory.dart';
+
+class MoveInitializationToFieldDeclaration extends ResolvedCorrectionProducer {
+  MoveInitializationToFieldDeclaration({required super.context});
+
+  @override
+  CorrectionApplicability get applicability =>
+      CorrectionApplicability.automatically;
+
+  @override
+  FixKind get fixKind => DartFixKind.moveInitializationToFieldDeclaration;
+
+  @override
+  FixKind get multiFixKind =>
+      DartFixKind.moveInitializationToFieldDeclarationMulti;
+
+  @override
+  Future<void> compute(ChangeBuilder builder) async {
+    var node = this.node;
+    if (node is! SimpleIdentifier) {
+      return;
+    }
+    var initializer = node.parent;
+    if (initializer is! ConstructorFieldInitializer) {
+      return;
+    }
+    var field = node.element;
+    if (field is! FieldElement) {
+      return;
+    }
+    var constructorDeclaration = initializer.parent;
+    if (constructorDeclaration is! ClassMember) {
+      return;
+    }
+    var initializers = switch (constructorDeclaration) {
+      ConstructorDeclaration() => constructorDeclaration.initializers,
+      PrimaryConstructorBody() => constructorDeclaration.initializers,
+      _ => null,
+    };
+    if (initializers == null) {
+      return;
+    }
+    var fieldDeclaration = findFieldDeclaration(field, constructorDeclaration);
+    if (fieldDeclaration == null ||
+        fieldDeclaration.equals != null ||
+        fieldDeclaration.initializer != null) {
+      return;
+    }
+    var expression = utils.getRangeText(initializer.expression.sourceRange);
+
+    await builder.addDartFileEdit(file, (builder) {
+      builder.addDeletion(range.nodeInList(initializers, initializer));
+      builder.addSimpleInsertion(fieldDeclaration.end, ' = $expression');
+    });
+  }
+
+  /// Returns the declaration of the [field].
+  ///
+  /// Uses the [initializer] to find the container containing the field's
+  /// declaration.
+  ///
+  /// Returns `null` if the declaration can't be found.
+  VariableDeclaration? findFieldDeclaration(
+    FieldElement field,
+    ClassMember constructorDeclaration,
+  ) {
+    // TODO(brianwilkerson): When support for augmentations is added, this will
+    //  need to look at all of the fragments of the container in order to find
+    //  the declaration.
+    var container = constructorDeclaration.parent;
+    var members = switch (container) {
+      ClassBody() => container.members,
+      EnumBody() => container.members,
+      _ => null,
+    };
+    if (members == null) return null;
+    for (var member in members) {
+      if (member is FieldDeclaration) {
+        for (var fieldDeclarartion in member.fields.variables) {
+          if (fieldDeclarartion.declaredFieldElement == field) {
+            return fieldDeclarartion;
+          }
+        }
+      }
+    }
+    return null;
+  }
+}

pkg/analysis_server/lib/src/services/correction/error_fix_status.yaml

@@ -2165,7 +2165,7 @@ implicit_call_tearoffs:
 implicit_reopen:
   status: hasFix
 initialize_in_field_declaration:
-  status: needsFix
+  status: hasFix
 invalid_case_patterns:
   status: hasFix
 invalid_runtime_check_with_js_interop_types_catch_clause_js_interop_type:

pkg/analysis_server/lib/src/services/correction/fix.dart

@@ -1243,6 +1243,16 @@ abstract final class DartFixKind {
     DartFixKindPriority.standard,
     'Move this doc comment to a library directive',
   );
+  static const moveInitializationToFieldDeclaration = FixKind(
+    'dart.fix.moveInitializationToFieldDeclaration',
+    DartFixKindPriority.standard,
+    "Move the initialization to the field's declaration",
+  );
+  static const moveInitializationToFieldDeclarationMulti = FixKind(
+    'dart.fix.moveInitializationToFieldDeclaration.multi',
+    DartFixKindPriority.standard,
+    "Move initialization to the field's declaration everywhere",
+  );
   static const moveTypeArgumentsToClass = FixKind(
     'dart.fix.moveTypeArgumentsToClass',
     DartFixKindPriority.standard,

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

@@ -131,6 +131,7 @@ import 'package:analysis_server/src/services/correction/dart/make_variable_nulla
 import 'package:analysis_server/src/services/correction/dart/merge_combinators.dart';
 import 'package:analysis_server/src/services/correction/dart/move_annotation_to_library_directive.dart';
 import 'package:analysis_server/src/services/correction/dart/move_doc_comment_to_library_directive.dart';
+import 'package:analysis_server/src/services/correction/dart/move_initialization_to_field_declaration.dart';
 import 'package:analysis_server/src/services/correction/dart/move_type_arguments_to_class.dart';
 import 'package:analysis_server/src/services/correction/dart/organize_imports.dart';
 import 'package:analysis_server/src/services/correction/dart/qualify_reference.dart';
@@ -341,6 +342,7 @@ final _builtInLintGenerators = <DiagnosticCode, List<ProducerGenerator>>{
   diag.hashAndEquals: [CreateMethod.equalityOrHashCode],
   diag.implicitCallTearoffs: [AddExplicitCall.new],
   diag.implicitReopen: [AddReopen.new],
+  diag.initializeInFieldDeclaration: [MoveInitializationToFieldDeclaration.new],
   diag.invalidCasePatterns: [AddConst.new],
   diag.leadingNewlinesInMultilineStrings: [AddLeadingNewlineToString.new],
   diag.libraryAnnotations: [MoveAnnotationToLibraryDirective.new],

pkg/analysis_server/test/src/services/correction/fix/move_initialization_to_field_declaration_test.dart

@@ -0,0 +1,96 @@
+// Copyright (c) 2026, 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 'package:analysis_server/src/services/correction/fix.dart';
+import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
+import 'package:test_reflective_loader/test_reflective_loader.dart';
+
+import 'fix_processor.dart';
+
+void main() {
+  defineReflectiveSuite(() {
+    defineReflectiveTests(MoveInitializationToFieldDeclarationTest);
+  });
+}
+
+@reflectiveTest
+class MoveInitializationToFieldDeclarationTest extends FixProcessorLintTest {
+  @override
+  FixKind get kind => DartFixKind.moveInitializationToFieldDeclaration;
+
+  @override
+  String get lintCode => LintNames.initialize_in_field_declaration;
+
+  Future<void> test_class_multipleFields() async {
+    await resolveTestCode('''
+class C(int x) {
+  this : y = x, z = 0;
+
+  int y, z;
+}
+''');
+    await assertHasFix('''
+class C(int x) {
+  this : z = 0;
+
+  int y = x, z;
+}
+''');
+  }
+
+  Future<void> test_class_multipleInitializers_first() async {
+    await resolveTestCode('''
+class C(int x) {
+  this : y = x, z = 0;
+
+  int y;
+  int z;
+}
+''');
+    await assertHasFix('''
+class C(int x) {
+  this : z = 0;
+
+  int y = x;
+  int z;
+}
+''');
+  }
+
+  Future<void> test_class_multipleInitializers_last() async {
+    await resolveTestCode('''
+class C(int x) {
+  this : y = 0, z = x;
+
+  int y;
+  int z;
+}
+''');
+    await assertHasFix('''
+class C(int x) {
+  this : y = 0;
+
+  int y;
+  int z = x;
+}
+''');
+  }
+
+  Future<void> test_class_singleInitializer() async {
+    await resolveTestCode('''
+class C(int x) {
+  this : y = x;
+
+  int y;
+}
+''');
+    await assertHasFix('''
+class C(int x) {
+  this;
+
+  int y = x;
+}
+''');
+  }
+}

pkg/analysis_server/test/src/services/correction/fix/test_all.dart

@@ -180,6 +180,8 @@ import 'move_annotation_to_library_directive_test.dart'
     as move_annotation_to_library_directive;
 import 'move_doc_comment_to_library_directive_test.dart'
     as move_doc_comment_to_library_directive;
+import 'move_initialization_to_field_declaration_test.dart'
+    as move_initialization_to_field_declaration;
 import 'move_type_arguments_to_class_test.dart' as move_type_arguments_to_class;
 import 'organize_imports_test.dart' as organize_imports;
 import 'pubspec/test_all.dart' as pubspec;
@@ -485,6 +487,7 @@ void main() {
     merge_combinators.main();
     move_annotation_to_library_directive.main();
     move_doc_comment_to_library_directive.main();
+    move_initialization_to_field_declaration.main();
     move_type_arguments_to_class.main();
     organize_imports.main();
     pubspec.main();