Rewrite bootstrap code not using mirrors.

_test/test/goldens/generated_build_script.dart

@@ -2,18 +2,18 @@
 // ignore_for_file: directives_ordering
 // build_runner >=2.4.16
 // ignore_for_file: no_leading_underscores_for_library_prefixes
-import 'package:build_runner_core/build_runner_core.dart' as _i1;
-import 'package:provides_builder/builders.dart' as _i2;
-import 'package:build_web_compilers/builders.dart' as _i3;
-import 'package:build_test/builder.dart' as _i4;
+import 'dart:io' as _i11;
+import 'dart:isolate' as _i8;
+
+import 'package:build/build.dart' as _i7;
 import 'package:build_config/build_config.dart' as _i5;
 import 'package:build_modules/builders.dart' as _i6;
-import 'package:build/build.dart' as _i7;
-import 'dart:isolate' as _i8;
-import 'package:build_runner/src/build_script_generate/build_process_state.dart'
-    as _i9;
 import 'package:build_runner/build_runner.dart' as _i10;
-import 'dart:io' as _i11;
+import 'package:build_runner/src/bootstrapper/build_process_state.dart' as _i9;
+import 'package:build_runner_core/build_runner_core.dart' as _i1;
+import 'package:build_test/builder.dart' as _i4;
+import 'package:build_web_compilers/builders.dart' as _i3;
+import 'package:provides_builder/builders.dart' as _i2;
 
 final _builders = <_i1.BuilderApplication>[
   _i1.apply(

build/lib/src/library_cycle_graph/phased_reader.dart

@@ -34,6 +34,8 @@ abstract class PhasedReader {
   /// current phase.
   Future<String?> readAtPhase(AssetId id);
 
+  //
+
   /// Whether [id] is a generated asset that changes between [phase] and
   /// [comparedToPhase].
   ///

build_runner/bin/build_runner.dart

@@ -7,8 +7,8 @@ import 'dart:io';
 
 import 'package:args/args.dart';
 import 'package:args/command_runner.dart';
-import 'package:build_runner/src/build_script_generate/bootstrap.dart';
-import 'package:build_runner/src/build_script_generate/build_process_state.dart';
+import 'package:build_runner/src/bootstrapper/bootstrapper.dart';
+import 'package:build_runner/src/bootstrapper/build_process_state.dart';
 import 'package:build_runner/src/entrypoint/options.dart';
 import 'package:build_runner/src/entrypoint/runner.dart';
 import 'package:build_runner_core/build_runner_core.dart';
@@ -94,7 +94,7 @@ Future<void> main(List<String> args) async {
           b.mode = BuildLogMode.daemon;
         });
     }
-    while ((exitCode = await generateAndRun(args, experiments: experiments)) ==
-        ExitCode.tempFail.code) {}
+    exitCode = await Bootstrapper().run(args, experiments: experiments);
+    return;
   }
 }

build_runner/lib/src/bootstrapper/bootstrapper.dart

@@ -0,0 +1,138 @@
+// Copyright (c) 2025, 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:io';
+
+import 'package:build_runner_core/build_runner_core.dart';
+import 'package:io/io.dart';
+import 'package:package_config/package_config.dart';
+
+import '../build_script_generate/build_script_generate.dart';
+import 'compiler.dart';
+import 'depfiles.dart';
+import 'runner.dart';
+
+// DO NOT SUBMIT
+// ignore_for_file: deprecated_member_use
+// ignore_for_file: only_throw_errors
+
+class Bootstrapper {
+  final Depfile buildRunnerDepfile = Depfile(
+    depfilePath: '.dart_tool/build/entrypoint/build_runner.deps',
+    digestPath: '.dart_tool/build/entrypoint/build_runner.digest',
+    output: null,
+    inputs: [],
+  );
+  final Depfile buildSourceDepfile = Depfile(
+    depfilePath: '.dart_tool/build/entrypoint/build.dart.deps',
+    digestPath: '.dart_tool/build/entrypoint/build.dart.digest',
+    output: '.dart_tool/build/entrypoint/build.dart',
+    inputs: [],
+  );
+  final Depfile dillDepfile = Depfile(
+    depfilePath: '.dart_tool/build/entrypoint/build.dart.dill.deps',
+    digestPath: '.dart_tool/build/entrypoint/build.dart.dill.digest',
+    output: '.dart_tool/build/entrypoint/build.dart.dill',
+    inputs: [],
+  );
+
+  PackageConfig? config;
+  bool? buildRunnerHasChanged;
+  bool? buildYamlHasChanged;
+  bool? buildDillHasChanged;
+
+  bool runningFromBuildScript() {
+    return StackTrace.current.toString().contains(
+      '.dart_tool/build/entrypoint/build.dart',
+    );
+  }
+
+  Future<bool> needsRebuild() async {
+    if (!runningFromBuildScript()) return false;
+    buildLog.debug(Platform.script.toString());
+    // TODO(davidmorgan): fix for workspace, error handling.
+    config ??= (await findPackageConfig(Directory.current, recurse: true))!;
+
+    // TODO(davidmorgan): is this the right thing to check?
+    buildLog.debug('needsRebuild?');
+    final r1 = buildRunnerDepfile.outputIsUpToDate();
+    final r2 = buildSourceDepfile.outputIsUpToDate();
+    final r3 = dillDepfile.outputIsUpToDate();
+    final result = !r1 || !r2 || !r3;
+    buildLog.debug('needsRebuild? $r1 $r2 $r3 --> $result');
+    return result;
+  }
+
+  Future<int> run(List<String> args, {List<String>? experiments}) async {
+    buildRunnerDepfile.output = Platform.script.path;
+    while (true) {
+      // TODO(davidmorgan): fix for workspace, error handling.
+      config = (await findPackageConfig(Directory.current, recurse: true))!;
+
+      _checkBuildRunner();
+      await _checkBuildSource(force: buildRunnerHasChanged!);
+      await _checkBuildDill();
+
+      final exitCode = await Runner().run(args);
+      if (exitCode != ExitCode.tempFail.code) {
+        return exitCode;
+      }
+    }
+  }
+
+  void _checkBuildRunner() {
+    buildLog.debug('Check build_runner.');
+    final package = config!['build_runner']!.packageUriRoot;
+    final script = package.resolve('../bin/build_runner.dart');
+    if (buildRunnerDepfile.outputIsUpToDate()) {
+      buildLog.debug('build runner is up to date');
+      buildRunnerHasChanged = false;
+    } else {
+      buildLog.debug('build runner update');
+      buildRunnerHasChanged = true;
+      buildRunnerDepfile.clear();
+      buildRunnerDepfile.addScriptDeps(
+        scriptPath: script.path,
+        packageConfig: config!,
+      );
+      buildRunnerDepfile.write();
+    }
+  }
+
+  Future<void> _checkBuildSource({required bool force}) async {
+    if (!force && buildSourceDepfile.outputIsUpToDate()) {
+      buildLog.debug('build script up to date');
+      buildYamlHasChanged = false;
+    } else {
+      buildLog.debug('build script update (force: $force)');
+      buildYamlHasChanged = true;
+      final buildScript = await generateBuildScript();
+      File(buildSourceDepfile.output!).writeAsStringSync(buildScript.content);
+      buildSourceDepfile.clear();
+      buildSourceDepfile.addDeps(buildScript.inputs);
+      buildSourceDepfile.addScriptDeps(
+        scriptPath: buildSourceDepfile.output!,
+        packageConfig: config!,
+      );
+      buildSourceDepfile.write();
+    }
+  }
+
+  Future<void> _checkBuildDill() async {
+    final compiler = Compiler();
+    if (dillDepfile.outputIsUpToDate()) {
+      buildLog.debug('dill up to date');
+      buildDillHasChanged = false;
+    } else {
+      buildLog.debug('dill update');
+      buildDillHasChanged = true;
+
+      final result = await compiler.compile();
+      if (!result.succeeded) throw 'failed';
+      // TODO(davidmorgan): this is weird.
+      dillDepfile.loadDeps();
+      dillDepfile.write();
+    }
+  }
+}

build_runner/lib/src/bootstrapper/compiler.dart

@@ -0,0 +1,36 @@
+// Copyright (c) 2025, 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:io';
+
+import 'package:build_runner_core/build_runner_core.dart';
+
+class CompileResult {
+  final String? messages;
+
+  CompileResult({required this.messages});
+
+  bool get succeeded => messages == null;
+}
+
+// TODO(davidmorgan): experiments.
+class Compiler {
+  Future<CompileResult> compile() async {
+    buildLog.doing('Compiling the build script.');
+    final result = await Process.run('dart', [
+      'compile',
+      'kernel',
+      '.dart_tool/build/entrypoint/build.dart',
+      '--output',
+      '.dart_tool/build/entrypoint/build.dart.dill',
+      '--depfile',
+      '.dart_tool/build/entrypoint/build.dart.dill.deps',
+    ]);
+    if (result.exitCode != 0) {
+      print('Compile failed: ${result.stdout} ${result.stderr}');
+      return CompileResult(messages: result.stderr as String);
+    }
+    return CompileResult(messages: null);
+  }
+}

build_runner/lib/src/bootstrapper/depfiles.dart

@@ -0,0 +1,135 @@
+// Copyright (c) 2025, 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:convert';
+import 'dart:io';
+
+import 'package:analyzer/dart/analysis/utilities.dart';
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:convert/convert.dart';
+import 'package:crypto/crypto.dart';
+import 'package:package_config/package_config.dart';
+import 'package:path/path.dart' as p;
+
+class Depfile {
+  final String depfilePath;
+  final String digestPath;
+  String? output;
+  List<String> inputs;
+
+  Depfile({
+    required this.depfilePath,
+    required this.digestPath,
+    required this.output,
+    required this.inputs,
+  });
+
+  void addDeps(Iterable<String> inputs) {
+    this.inputs.addAll(inputs);
+  }
+
+  void addScriptDeps({
+    required String scriptPath,
+    required PackageConfig packageConfig,
+  }) {
+    final seenPaths = <String>{scriptPath};
+    final nextPaths = [scriptPath];
+
+    while (nextPaths.isNotEmpty) {
+      final nextPath = nextPaths.removeLast();
+      final dirname = p.dirname(nextPath);
+
+      final file = File(nextPath);
+      final content = file.existsSync() ? file.readAsStringSync() : null;
+      if (content == null) continue;
+      final parsed =
+          parseString(content: content, throwIfDiagnostics: false).unit;
+      for (final directive in parsed.directives) {
+        if (directive is! UriBasedDirective) continue;
+        final uri = directive.uri.stringValue;
+        if (uri == null) continue;
+        final parsedUri = Uri.parse(uri);
+        if (parsedUri.scheme == 'dart') continue;
+        final path =
+            parsedUri.scheme == 'package'
+                ? packageConfig.resolve(parsedUri)!.toFilePath()
+                : p.canonicalize(p.join(dirname, parsedUri.path));
+        if (seenPaths.add(path)) nextPaths.add(path);
+      }
+    }
+
+    inputs = seenPaths.toList()..sort();
+  }
+
+  bool outputIsUpToDate() {
+    final depsFile = File(depfilePath);
+    if (!depsFile.existsSync()) return false;
+    final digestFile = File(digestPath);
+    if (!digestFile.existsSync()) return false;
+    final digests = digestFile.readAsStringSync();
+    final expectedDigests = computeDigests();
+    return digests == expectedDigests;
+  }
+
+  void loadDeps() {
+    inputs = _loadDeps();
+  }
+
+  String _loadOutput() {
+    final depsFile = File(depfilePath);
+    final deps = depsFile.readAsStringSync();
+    // TODO(davidmorgan): unescape spaces.
+    var result = deps.split(' ').first;
+    // Strip off trailing ':'.
+    result = result.substring(0, result.length - 1);
+    return result;
+  }
+
+  List<String> _loadDeps() {
+    final depsFile = File(depfilePath);
+    final deps = depsFile.readAsStringSync();
+    // TODO(davidmorgan): unescape spaces.
+    final result = deps.split(' ').skip(1).toList();
+    // File ends in a newline.
+    result.last = result.last.substring(0, result.last.length - 1);
+    return result;
+  }
+
+  void clear() {
+    inputs.clear();
+  }
+
+  void write() {
+    File(depfilePath)
+      ..createSync(recursive: true)
+      ..writeAsStringSync(
+        '$output: '
+        // TODO(davidmorgan): escaping.
+        '${inputs.join(' ')}'
+        '\n',
+      );
+    File(digestPath).writeAsStringSync(computeDigests());
+  }
+
+  String computeDigests() => '''
+inputs digest: ${_computeDigest(_loadDeps())}
+output digest: ${_computeDigest([output ?? _loadOutput()])}
+''';
+  //////////
+  String _computeDigest(Iterable<String> deps) {
+    final digestSink = AccumulatorSink<Digest>();
+    final result = md5.startChunkedConversion(digestSink);
+    for (final dep in deps) {
+      final file = File(dep);
+      if (file.existsSync()) {
+        result.add([1]);
+        result.add(File(dep).readAsBytesSync());
+      } else {
+        result.add([0]);
+      }
+    }
+    result.close();
+    return base64.encode(digestSink.events.first.bytes);
+  }
+}

build_runner/lib/src/bootstrapper/runner.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2025, 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:io';
+
+// TODO(davidmorgan): pass state.
+// TODO(davidmorgan): handle uncaught errors--in `run` method?
+class Runner {
+  Future<int> run(List<String> arguments) async {
+    final process = await Process.start('dart', [
+      '.dart_tool/build/entrypoint/build.dart.dill',
+      ...arguments,
+    ], mode: ProcessStartMode.inheritStdio);
+    return process.exitCode;
+  }
+}