[analyzer/ffi] Reject Struct-constrained mixin applications

Closes #63849

GitOrigin-RevId: 55f80e7
Change-Id: I99887afa22e833297ef639e8402be4645302cf99
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/525581
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Daco Harkes <dacoharkes@google.com>

Author: Yusufihsangorgel

BUILD.gn

@@ -312,6 +312,7 @@ if (is_fuchsia) {
     "tests/ffi/static_checks/regress_60250_test.dart",
     "tests/ffi/static_checks/regress_62693_test.dart",
     "tests/ffi/static_checks/regress_62716_test.dart",
+    "tests/ffi/static_checks/regress_63388_test.dart",
     "tests/ffi/static_checks/regress_flutter_188175_test.dart",
     "tests/ffi/static_checks/vmspecific_function_callbacks_negative_test.dart",
     "tests/ffi/static_checks/vmspecific_regress_38993_test.dart",

pkg/analyzer/lib/src/generated/ffi_verifier.dart

@@ -2799,7 +2799,7 @@ extension on NamedType {
   /// Return `true` if this represents a subtype of `Struct` or `Union`.
   bool get isCompoundSubtype {
     var element = this.element;
-    if (element is ClassElement) {
+    if (element is InterfaceElement) {
       return element.allSupertypes.any((e) => e.isCompound);
     }
     return false;

pkg/analyzer/test/src/diagnostics/subtype_of_struct_class_test.dart

@@ -63,6 +63,25 @@ final class AbiSpecificInteger4 implements AbiSpecificInteger1 {
 ''');
   }
 
+  test_implements_mixin_constrained_to_struct() async {
+    await resolveTestCodeWithDiagnostics(r'''
+import 'dart:ffi';
+
+base mixin HeaderFields on Struct {
+  @Int32()
+  external int fieldA;
+}
+
+final class ExampleStruct implements HeaderFields {
+//                                   ^^^^^^^^^^^^
+// [diag.baseClassImplementedOutsideOfLibrary] The class 'Struct' can't be implemented outside of its library because it's a base class.
+// [diag.subtypeOfStructClassInImplements] The class 'ExampleStruct' can't implement 'HeaderFields' because 'HeaderFields' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.
+  @override
+  int fieldA = 0;
+}
+''');
+  }
+
   test_implements_struct() async {
     await resolveTestCodeWithDiagnostics(r'''
 import 'dart:ffi';
@@ -107,6 +126,26 @@ final class C implements S {}
 
 @reflectiveTest
 class SubtypeOfStructClassInWithTest extends PubPackageResolutionTest {
+  test_with_mixin_constrained_to_struct() async {
+    await resolveTestCodeWithDiagnostics(r'''
+import 'dart:ffi';
+
+base mixin HeaderFields on Struct {
+  @Int32()
+  external int fieldA;
+
+  external Pointer<Void> fieldB;
+}
+
+final class ExampleStruct extends Struct with HeaderFields {
+//                                            ^^^^^^^^^^^^
+// [diag.subtypeOfStructClassInWith] The class 'ExampleStruct' can't mix in 'HeaderFields' because 'HeaderFields' is a subtype of 'Struct', 'Union', or 'AbiSpecificInteger'.
+  @Uint32()
+  external int fieldC;
+}
+''');
+  }
+
   test_with_struct() async {
     await resolveTestCodeWithDiagnostics(r'''
 import 'dart:ffi';

tests/ffi/static_checks/regress_63388_test.dart

@@ -0,0 +1,52 @@
+// 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.
+
+// Regression test for https://github.com/dart-lang/sdk/issues/63388
+
+import 'dart:ffi';
+
+base mixin HeaderFields on Struct {
+  @Int32()
+  external int fieldA;
+
+  external Pointer<Void> fieldB;
+}
+
+final class ExampleStruct extends Struct with HeaderFields {
+  //        ^^^^^^^^^^^^^
+  // [cfe] Class 'Struct with HeaderFields' cannot be extended or implemented.
+  //                                          ^^^^^^^^^^^^
+  // [analyzer] COMPILE_TIME_ERROR.SUBTYPE_OF_STRUCT_CLASS
+  @Uint32()
+  external int fieldC;
+}
+
+// An unconstrained mixin cannot be applied to a struct either: the mixin
+// application class is itself treated as a struct, so the CFE rejects it.
+mixin ExtraGetters {
+  int get extra => 4;
+}
+
+final class GetterStruct extends Struct with ExtraGetters {
+  //        ^^^^^^^^^^^^
+  // [cfe] Class 'Struct with ExtraGetters' cannot be extended or implemented.
+  // [cfe] Struct 'Struct with ExtraGetters' is empty. Empty structs and unions are undefined behavior.
+  @Int32()
+  external int fieldD;
+}
+
+// Sharing members through a plain interface is allowed.
+abstract interface class HasChecksum {
+  int get checksum;
+}
+
+final class ChecksumStruct extends Struct implements HasChecksum {
+  @Int32()
+  external int payload;
+
+  @override
+  int get checksum => payload ^ 0xFF;
+}
+
+void main() {}