[dart2wasm] Make dart compile wasm compiled apps disable dart.library.ffi

This is a follow-up to [0]. That CL changed dart2wasm's modular
transformer to issue an error if `dart:ffi` is imported.

Users of packages that have specialized code for the VM (which supports
FFI) use conditional imports based on `dart.library.ffi`. We don't want
the VM-specific code to be used for web in dart2wasm (as dart2wasm
doesn't support the entirety of `dart:ffi`).

As a result we're going to make `dart.library.ffi` be false in
coditional imports (as well as in
`const bool.fromEnvironment('dart.library.ffi')`).

[0] https://dart-review.googlesource.com/c/sdk/+/368568

Issue https://github.com/dart-lang/sdk/issues/55948
Issue https://github.com/flutter/flutter/issues/149984

Change-Id: I70a775278ab701d1fd2596521e378cb6364edac2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/370580
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Srujan Gaddam <srujzs@google.com>

pkg/_js_interop_checks/lib/js_interop_checks.dart

@@ -85,6 +85,8 @@ class JsInteropChecks extends RecursiveVisitor {
   final ExportChecker exportChecker;
   final bool isDart2Wasm;
 
+  final List<String> _disallowedInteropLibrariesInDart2Wasm;
+
   /// Native tests to exclude from checks on external.
   // TODO(rileyporter): Use ExternalName from CFE to exclude native tests.
   static final List<Pattern> _allowedNativeTestPatterns = [
@@ -107,7 +109,7 @@ class JsInteropChecks extends RecursiveVisitor {
     // Negative lookahead to test the violation.
     RegExp(
         r'(?<!generated_)tests/lib/js/static_interop_test(?!/disallowed_interop_libraries_test.dart)'),
-    RegExp(r'(?<!generated_)tests/web/wasm'),
+    RegExp(r'(?<!generated_)tests/web/wasm/(?!ffi/).*'),
     // Flutter tests.
     RegExp(r'flutter/lib/web_ui/test'),
   ];
@@ -131,7 +133,7 @@ class JsInteropChecks extends RecursiveVisitor {
   ];
 
   /// Interop libraries that cannot be used in dart2wasm.
-  static const _disallowedInteropLibrariesInDart2Wasm = [
+  static const _disallowedInteropLibrariesInDart2WasmByDefault = [
     'package:js/js.dart',
     'package:js/js_util.dart',
     'dart:js_util',
@@ -163,12 +165,16 @@ class JsInteropChecks extends RecursiveVisitor {
 
   JsInteropChecks(this._coreTypes, ClassHierarchy hierarchy, this._reporter,
       this._nativeClasses,
-      {this.isDart2Wasm = false})
+      {this.isDart2Wasm = false, bool enableExperimentalFfi = false})
       : exportChecker = ExportChecker(_reporter, _coreTypes.objectClass),
         _functionToJSTarget = _coreTypes.index.getTopLevelProcedure(
             'dart:js_interop', 'FunctionToJSExportedDartFunction|get#toJS'),
         _staticTypeContext = StatefulStaticTypeContext.stacked(
-            TypeEnvironment(_coreTypes, hierarchy)) {
+            TypeEnvironment(_coreTypes, hierarchy)),
+        _disallowedInteropLibrariesInDart2Wasm = [
+          for (final entry in _disallowedInteropLibrariesInDart2WasmByDefault)
+            if (!(entry == 'dart:ffi' && enableExperimentalFfi)) entry
+        ] {
     extensionIndex =
         ExtensionIndex(_coreTypes, _staticTypeContext.typeEnvironment);
   }

pkg/dart2wasm/lib/compile.dart

@@ -81,7 +81,9 @@ Future<CompilerOutput?> compileToModule(compiler.WasmCompilerOptions options,
     mode = wasm.Mode.regular;
   }
   final WasmTarget target = WasmTarget(
-      removeAsserts: !options.translatorOptions.enableAsserts, mode: mode);
+      enableExperimentalFfi: options.translatorOptions.enableExperimentalFfi,
+      removeAsserts: !options.translatorOptions.enableAsserts,
+      mode: mode);
   CompilerOptions compilerOptions = CompilerOptions()
     ..target = target
     // This is a dummy directory that always exists. This option should be

pkg/dart2wasm/lib/dart2wasm.dart

@@ -88,6 +88,9 @@ final List<Option> options = [
   StringOption(
       "dump-kernel-after-tfa", (o, value) => o.dumpKernelAfterTfa = value,
       hide: true),
+  Flag("enable-experimental-ffi",
+      (o, value) => o.translatorOptions.enableExperimentalFfi = value,
+      defaultsTo: _d.translatorOptions.enableExperimentalFfi),
 ];
 
 Map<fe.ExperimentalFlag, bool> processFeExperimentalFlags(

pkg/dart2wasm/lib/target.dart

@@ -89,10 +89,14 @@ class ConstantResolver extends Transformer {
 }
 
 class WasmTarget extends Target {
-  WasmTarget({this.removeAsserts = false, this.mode = Mode.regular});
+  WasmTarget(
+      {this.enableExperimentalFfi = true,
+      this.removeAsserts = false,
+      this.mode = Mode.regular});
 
-  bool removeAsserts;
-  Mode mode;
+  final bool removeAsserts;
+  final Mode mode;
+  final bool enableExperimentalFfi;
   Class? _growableList;
   Class? _immutableList;
   Class? _wasmDefaultMap;
@@ -206,7 +210,7 @@ class WasmTarget extends Target {
         diagnosticReporter as DiagnosticReporter<Message, LocatedMessage>);
     final jsInteropChecks = JsInteropChecks(
         coreTypes, hierarchy, jsInteropReporter, _nativeClasses!,
-        isDart2Wasm: true);
+        isDart2Wasm: true, enableExperimentalFfi: enableExperimentalFfi);
     // Process and validate first before doing anything with exports.
     for (Library library in interopDependentLibraries) {
       jsInteropChecks.visitLibrary(library);
@@ -497,6 +501,10 @@ class WasmTarget extends Target {
   @override
   Class concreteDoubleLiteralClass(CoreTypes coreTypes, double value) =>
       _boxedDouble ??= coreTypes.index.getClass("dart:core", "_BoxedDouble");
+
+  @override
+  DartLibrarySupport get dartLibrarySupport => CustomizedDartLibrarySupport(
+      unsupported: {if (!enableExperimentalFfi) 'ffi'});
 }
 
 class WasmVerification extends Verification {

pkg/dart2wasm/lib/translator.dart

@@ -44,6 +44,7 @@ class TranslatorOptions {
   bool minify = false;
   bool verifyTypeChecks = false;
   bool verbose = false;
+  bool enableExperimentalFfi = false;
   int inliningLimit = 0;
   int? sharedMemoryMaxPages;
   List<int> watchPoints = [];

tests/web/wasm/ffi/disabled_helper.dart

@@ -0,0 +1,5 @@
+// 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.
+
+String tryAccessFfi() => 'Have no dart:ffi support';

tests/web/wasm/ffi/disabled_test.dart

@@ -0,0 +1,12 @@
+// 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.
+
+import 'package:expect/expect.dart';
+
+import 'disabled_helper.dart' if (dart.library.ffi) 'enabled_helper.dart';
+
+void main() {
+  Expect.isFalse(const bool.fromEnvironment('dart.library.ffi'));
+  Expect.equals('Have no dart:ffi support', tryAccessFfi());
+}

tests/web/wasm/ffi/enabled_helper.dart

@@ -0,0 +1,8 @@
+// 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.
+
+import 'dart:ffi';
+
+String tryAccessFfi() =>
+    'Have dart:ffi support (${Pointer<Int8>.fromAddress(int.parse('10')).address})';

tests/web/wasm/ffi/enabled_test.dart

@@ -0,0 +1,14 @@
+// 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.
+
+// dart2wasmOptions=--extra-compiler-option=--enable-experimental-ffi
+
+import 'package:expect/expect.dart';
+
+import 'disabled_helper.dart' if (dart.library.ffi) 'enabled_helper.dart';
+
+void main() {
+  Expect.isTrue(const bool.fromEnvironment('dart.library.ffi'));
+  Expect.equals('Have dart:ffi support (10)', tryAccessFfi());
+}

tests/web/wasm/ffi/ffi_native_test.dart

@@ -2,6 +2,7 @@
 // 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.
 
+// dart2wasmOptions=--extra-compiler-option=--enable-experimental-ffi
 // SharedObjects=ffi_native_test_module
 
 import 'dart:ffi';

utils/dart2wasm/BUILD.gn

@@ -14,7 +14,7 @@ template("wasm_module") {
       "compiled_action",
       rebase_path("//third_party/emsdk/upstream/emscripten/emcc"),
       "--no-entry",
-      rebase_path("//tests/web/wasm/${invoker.module_name}.c"),
+      rebase_path("//tests/web/wasm/ffi/${invoker.module_name}.c"),
       "-o",
       rebase_path("$root_out_dir/wasm/${invoker.module_name}.wasm"),
       "-O",