Use dart2wasm support-detection scripts (#3845)

* Use dart2wasm support expression

* Fix looking up support file

* Review feedback

---------

Co-authored-by: David Morgan <[email protected]>

Author: simolus3

build_web_compilers/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 4.1.2-wip
+
+- Use support-detection scripts emitted by `dart2wasm`.
+
 ## 4.1.1
 
 - Support 3.8.0 pre-release sdks.

build_web_compilers/lib/src/dart2wasm_bootstrap.dart

@@ -17,20 +17,42 @@ import 'web_entrypoint_builder.dart';
 
 final _resourcePool = Pool(maxWorkersPerTask);
 
+/// Result of invoking the `dart2wasm` compiler.
+final class Dart2WasmBootstrapResult {
+  /// Whether `dart2wasm` did compile the Dart program.
+  ///
+  /// This is typically false when the program transitively imports an
+  /// unsupported library, or if the compiler fails for another reason.
+  final bool didCompile;
+
+  /// An optional JavaScript expression that might be emitted by `dart2wasm`.
+  /// The expression evaluates to `true` if the current JavaScript environment
+  /// supports all the features expected by the generated WebAssembly module.
+  final String? supportExpression;
+
+  const Dart2WasmBootstrapResult.didNotCompile()
+      : didCompile = false,
+        supportExpression = null;
+
+  Dart2WasmBootstrapResult({
+    required this.supportExpression,
+  }) : didCompile = true;
+}
+
 /// Invokes `dart compile wasm` to compile the primary input of [buildStep].
 ///
 /// This only emits the `.wasm` and `.mjs` files produced by `dart2wasm`. An
 /// entrypoint loader needs to be emitted separately.
-Future<void> bootstrapDart2Wasm(
+Future<Dart2WasmBootstrapResult> bootstrapDart2Wasm(
   BuildStep buildStep,
   List<String> additionalArguments,
   String javaScriptModuleExtension,
 ) async {
-  await _resourcePool.withResource(() => _bootstrapDart2Wasm(
+  return await _resourcePool.withResource(() => _bootstrapDart2Wasm(
       buildStep, additionalArguments, javaScriptModuleExtension));
 }
 
-Future<void> _bootstrapDart2Wasm(
+Future<Dart2WasmBootstrapResult> _bootstrapDart2Wasm(
   BuildStep buildStep,
   List<String> additionalArguments,
   String javaScriptModuleExtension,
@@ -61,7 +83,7 @@ $librariesString
 
 https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-resolve-skipped-compiling-warnings
 ''');
-      return;
+      return const Dart2WasmBootstrapResult.didNotCompile();
     }
 
     var scratchSpace = await buildStep.fetchResource(scratchSpaceResource);
@@ -123,5 +145,15 @@ https://github.com/dart-lang/build/blob/master/docs/faq.md#how-can-i-resolve-ski
   } else {
     log.severe('ExitCode:${result.exitCode}\nStdOut:\n${result.stdout}\n'
         'StdErr:\n${result.stderr}');
+    return const Dart2WasmBootstrapResult.didNotCompile();
   }
+
+  var supportFile =
+      scratchSpace.fileFor(dartEntrypointId.changeExtension('.support.js'));
+  String? supportExpression;
+  if (await supportFile.exists()) {
+    supportExpression = await supportFile.readAsString();
+  }
+
+  return Dart2WasmBootstrapResult(supportExpression: supportExpression);
 }

build_web_compilers/lib/src/web_entrypoint_builder.dart

@@ -290,6 +290,7 @@ class WebEntrypointBuilder implements Builder {
     if (!isAppEntrypoint) return;
 
     final compilationSteps = <Future>[];
+    Dart2WasmBootstrapResult? dart2WasmResult;
 
     for (final compiler in options.compilers) {
       switch (compiler.compiler) {
@@ -311,17 +312,21 @@ class WebEntrypointBuilder implements Builder {
             entrypointExtension: compiler.extension,
           ));
         case WebCompiler.Dart2Wasm:
-          compilationSteps.add(bootstrapDart2Wasm(
-              buildStep, compiler.compilerArguments, compiler.extension));
+          compilationSteps.add(Future(() async {
+            dart2WasmResult = await bootstrapDart2Wasm(
+                buildStep, compiler.compilerArguments, compiler.extension);
+          }));
       }
     }
     await Future.wait(compilationSteps);
-    if (_generateLoader(buildStep.inputId) case (var id, var loader)?) {
+    if (_generateLoader(buildStep.inputId, dart2WasmResult)
+        case (var id, var loader)?) {
       await buildStep.writeAsString(id, loader);
     }
   }
 
-  (AssetId, String)? _generateLoader(AssetId input) {
+  (AssetId, String)? _generateLoader(
+      AssetId input, Dart2WasmBootstrapResult? dart2WasmResult) {
     var loaderExtension = options.loaderExtension;
     var wasmCompiler = options.optionsFor(WebCompiler.Dart2Wasm);
     if (loaderExtension == null || wasmCompiler == null) {
@@ -350,17 +355,13 @@ function relativeURL(ref) {
     // If we're compiling to JS, start a feature detection to prefer wasm but
     // fall back to JS if necessary.
     if (jsCompiler != null) {
-      loaderResult.writeln('''
-function supportsWasmGC() {
-  // This attempts to instantiate a wasm module that only will validate if the
-  // final WasmGC spec is implemented in the browser.
-  //
-  // Copied from https://github.com/GoogleChromeLabs/wasm-feature-detect/blob/main/src/detectors/gc/index.js
-  const bytes = [0, 97, 115, 109, 1, 0, 0, 0, 1, 5, 1, 95, 1, 120, 0];
-  return 'WebAssembly' in self && WebAssembly.validate(new Uint8Array(bytes));
-}
+      final supportCheck = dart2WasmResult?.supportExpression ??
+          "'WebAssembly' in self && "
+              'WebAssembly.validate(new Uint8Array('
+              '[0, 97, 115, 109, 1, 0, 0, 0, 1, 5, 1, 95, 1, 120, 0]));';
 
-if (supportsWasmGC()) {
+      loaderResult.writeln('''
+if ($supportCheck) {
 ''');
     }
 

build_web_compilers/pubspec.yaml

@@ -1,5 +1,5 @@
 name: build_web_compilers
-version: 4.1.1
+version: 4.1.2-wip
 description: Builder implementations wrapping the dart2js and DDC compilers.
 repository: https://github.com/dart-lang/build/tree/master/build_web_compilers
 resolution: workspace

build_web_compilers/test/dart2wasm_bootstrap_test.dart

@@ -69,9 +69,17 @@ void main() {
           'a|web/index.dart.js': decodedMatches(
             stringContainsInOrder(
               [
-                'if (supportsWasmGC())',
+                'if',
+                // Depending on whether dart2wasm emitted a .support.js file,
+                // this check either comes from dart2wasm or from our own script
+                // doing its own feature detection as a fallback. Which one is
+                // used depends on the SDK version, we can only assume that the
+                // check is guaranteed to include WebAssembly.validate to check
+                // for WASM features.
+                'WebAssembly.validate',
+                '{',
                 'compileStreaming',
-                'else',
+                '} else {',
                 'scriptTag.src = relativeURL("./index.dart2js.js");'
               ],
             ),