[ffigen] Return structs by value from ObjC methods

[liamappelbe]

Returns structs by value from ObjC methods by allocating their backing storage using calloc, converting that pointer to a TypedData with calloc.nativeFree as its finalizer, then converting that to the struct using Struct.create. For example, these are some bindings from method_test.dart:

/// twiddleVec4Components:
Vec4 twiddleVec4Components_(Vec4 v) {
  final _ptr = pkg_ffi.calloc<Vec4>();
  final _data = _ptr
      .cast<ffi.Uint8>()
      .asTypedList(ffi.sizeOf<Vec4>(), finalizer: pkg_ffi.calloc.nativeFree);
  objc.useMsgSendVariants
      ? _objc_msgSend_5Stret(
          _ptr, this.ref.pointer, _sel_twiddleVec4Components_, v)
      : _ptr.ref =
          _objc_msgSend_5(this.ref.pointer, _sel_twiddleVec4Components_, v);
  return ffi.Struct.create<Vec4>(_data);
}

Fixes #1480

[github-actions]

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// Copyright (c) 2024, 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.

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pkgs/ffigen/example/libclang-example/generated_bindings.dart
pkgs/ffigen/example/shared_bindings/generate.dart
pkgs/ffigen/example/shared_bindings/lib/generated/a_gen.dart
pkgs/ffigen/example/shared_bindings/lib/generated/a_shared_b_gen.dart
pkgs/ffigen/example/shared_bindings/lib/generated/base_gen.dart
pkgs/ffigen/example/simple/generated_bindings.dart
pkgs/ffigen/lib/src/header_parser/clang_bindings/clang_bindings.dart
pkgs/ffigen/test/collision_tests/expected_bindings/_expected_decl_decl_collision_bindings.dart
pkgs/ffigen/test/collision_tests/expected_bindings/_expected_decl_symbol_address_collision_bindings.dart
pkgs/ffigen/test/collision_tests/expected_bindings/_expected_decl_type_name_collision_bindings.dart
pkgs/ffigen/test/collision_tests/expected_bindings/_expected_reserved_keyword_collision_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_comment_markup_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_dart_handle_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_enum_int_mimic_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_forward_decl_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_functions_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_imported_types_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_native_func_typedef_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_opaque_dependencies_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_packed_structs_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_regress_384_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_struct_fptr_fields_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_typedef_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_unions_bindings.dart
pkgs/ffigen/test/header_parser_tests/expected_bindings/_expected_varargs_bindings.dart
pkgs/ffigen/test/large_integration_tests/_expected_cjson_bindings.dart
pkgs/ffigen/test/large_integration_tests/_expected_libclang_bindings.dart
pkgs/ffigen/test/large_integration_tests/_expected_sqlite_bindings.dart
pkgs/ffigen/test/native_test/_expected_native_test_bindings.dart
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/jni/lib/src/third_party/global_env_extensions.dart
pkgs/jni/lib/src/third_party/jni_bindings_generated.dart
pkgs/jnigen/android_test_runner/lib/main.dart
pkgs/jnigen/example/in_app_java/lib/android_utils.dart
pkgs/jnigen/example/kotlin_plugin/example/lib/main.dart
pkgs/jnigen/example/kotlin_plugin/lib/kotlin_bindings.dart
pkgs/jnigen/example/kotlin_plugin/lib/kotlin_plugin.dart
pkgs/jnigen/example/pdfbox_plugin/lib/pdfbox_plugin.dart
pkgs/jnigen/example/pdfbox_plugin/lib/src/third_party/org/apache/pdfbox/pdmodel/PDDocument.dart
pkgs/jnigen/example/pdfbox_plugin/lib/src/third_party/org/apache/pdfbox/pdmodel/PDDocumentInformation.dart
pkgs/jnigen/example/pdfbox_plugin/lib/src/third_party/org/apache/pdfbox/pdmodel/_package.dart
pkgs/jnigen/example/pdfbox_plugin/lib/src/third_party/org/apache/pdfbox/text/PDFTextStripper.dart
pkgs/jnigen/example/pdfbox_plugin/lib/src/third_party/org/apache/pdfbox/text/_package.dart
pkgs/jnigen/lib/src/bindings/descriptor.dart
pkgs/jnigen/lib/src/elements/elements.g.dart
pkgs/jnigen/test/jackson_core_test/third_party/bindings/com/fasterxml/jackson/core/_package.dart
pkgs/jnigen/tool/command_runner.dart
pkgs/native_assets_builder/test_data/native_dynamic_linking/bin/native_dynamic_linking.dart
pkgs/swift2objc/lib/src/config.dart
pkgs/swift2objc/lib/src/generate_wrapper.dart
pkgs/swift2objc/lib/src/generator/_core/utils.dart
pkgs/swift2objc/lib/src/generator/generator.dart
pkgs/swift2objc/lib/src/generator/generators/class_generator.dart
pkgs/swift2objc/lib/src/parser/parsers/declaration_parsers/parse_initializer_declaration.dart
pkgs/swift2objc/lib/src/parser/parsers/declaration_parsers/parse_property_declaration.dart
pkgs/swift2objc/lib/src/transformer/_core/unique_namer.dart
pkgs/swift2objc/lib/src/transformer/_core/utils.dart
pkgs/swift2objc/lib/src/transformer/transformers/transform_property.dart
pkgs/swift2objc/test/unit/parse_initializer_param_test.dart

Package publish validation ✔️

Package	Version	Status
package:ffi	2.1.3	already published at pub.dev
package:ffigen	15.0.0-wip	WIP (no publish necessary)
package:jni	0.12.0-wip	WIP (no publish necessary)
package:jnigen	0.12.0-wip	WIP (no publish necessary)
package:native_assets_cli	0.9.0-wip	WIP (no publish necessary)
package:objective_c	2.1.0-wip	WIP (no publish necessary)
package:swift2objc	0.0.1-wip	WIP (no publish necessary)
package:swiftgen	0.0.1-wip	WIP (no publish necessary)

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[coveralls]

coverage: 92.094% (+0.1%) from 91.993%
when pulling 2d8a282 on stret
into f9a3888 on main.

[dcharkes]

It should instead be

  final _ptr = pkg_ffi.calloc<Vec4>();
  objc.useMsgSendVariants
      ? _objc_msgSend_5Stret(
          _ptr, this.ref.pointer, _sel_twiddleVec4Components_, v)
      : _ptr.ref =
          _objc_msgSend_5(this.ref.pointer, _sel_twiddleVec4Components_, v);
  final _data = _ptr
      .cast<ffi.Uint8>()
      .asTypedList(ffi.sizeOf<Vec4>(), finalizer: pkg_ffi.calloc.nativeFree);
  return ffi.Struct.create<Vec4>(_data);

Otherwise, if the method is inlined the finalizer will still run before before the native call.

[dcharkes]

lgtm once code is reordered to prevent finalizer from running early on inlining.