From 340a69147a9f1c6eea6adafeb4930e9dbe79395b Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 13:46:43 -0800
Subject: [PATCH 01/26] Initial commit: directory for header_guard_check.

---
 tools/header_guard_check/pubspec.yaml | 59 +++++++++++++++++++++++++++
 1 file changed, 59 insertions(+)
 create mode 100644 tools/header_guard_check/pubspec.yaml

diff --git a/tools/header_guard_check/pubspec.yaml b/tools/header_guard_check/pubspec.yaml
new file mode 100644
index 0000000000000..4f26b7c9771df
--- /dev/null
+++ b/tools/header_guard_check/pubspec.yaml
@@ -0,0 +1,59 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+name: header_guard_check
+publish_to: none
+
+# Do not add any dependencies that require more than what is provided in
+# //third_party/dart/pkg or //third_party/dart/third_party/pkg.
+# In particular, package:test is not usable here.
+
+# If you do add packages here, make sure you can run `pub get --offline`, and
+# check the .packages and .package_config to make sure all the paths are
+# relative to this directory into //third_party/dart
+
+environment:
+  sdk: '>=3.2.0-0 <4.0.0'
+
+dependencies: 
+  args: any
+  git_repo_tools: any
+  engine_repo_tools: any
+  meta: any
+  path: any
+
+dev_dependencies:
+  async_helper: any
+  expect: any
+  litetest: any
+  process_fakes: any
+  smith: any
+
+dependency_overrides:
+  args:
+    path: ../../../third_party/dart/third_party/pkg/args
+  async_helper:
+    path: ../../../third_party/dart/pkg/async_helper
+  engine_repo_tools:
+    path: ../pkg/engine_repo_tools
+  expect:
+    path: ../../../third_party/dart/pkg/expect
+  file:
+    path: ../../../third_party/dart/third_party/pkg/file/packages/file
+  git_repo_tools:
+    path: ../pkg/git_repo_tools
+  litetest:
+    path: ../../testing/litetest
+  path:
+    path: ../../../third_party/dart/third_party/pkg/path
+  platform:
+    path: ../../third_party/pkg/platform
+  process:
+    path: ../../third_party/pkg/process
+  process_fakes:
+    path: ../pkg/process_fakes
+  process_runner:
+    path: ../../third_party/pkg/process_runner
+  smith:
+    path: ../../../third_party/dart/pkg/smith

From 0058f60f9f7da6eec63d6949e39b6bedb13d1940 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 14:40:05 -0800
Subject: [PATCH 02/26] WIP.

---
 tools/header_guard_check/lib/src/header_file.dart | 15 +++++++++++++++
 1 file changed, 15 insertions(+)
 create mode 100644 tools/header_guard_check/lib/src/header_file.dart

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
new file mode 100644
index 0000000000000..6d2d8d88b72d0
--- /dev/null
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -0,0 +1,15 @@
+// Copyright 2013 The Flutter Authors. 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:meta/meta.dart';
+import 'package:path/path.dart' as p;
+
+/// Represents a C++ header file, i.e. a file on disk that ends in `.h`.
+@immutable
+final class HeaderFile {
+  /// Path to the file on disk.
+  final String path;
+
+  const HeaderFile(this.path);
+}

From 3aed665aa44fb766be7d7f83acecc99681df98d1 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 15:32:37 -0800
Subject: [PATCH 03/26] Add minimal library with some validation.

---
 .../lib/src/header_file.dart                  | 215 ++++++++++++++++-
 tools/header_guard_check/pubspec.yaml         |   7 +
 .../test/header_file_test.dart                | 217 ++++++++++++++++++
 3 files changed, 438 insertions(+), 1 deletion(-)
 create mode 100644 tools/header_guard_check/test/header_file_test.dart

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index 6d2d8d88b72d0..d6f01de3b8c87 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -2,14 +2,227 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:io' as io;
+
 import 'package:meta/meta.dart';
 import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
 
 /// Represents a C++ header file, i.e. a file on disk that ends in `.h`.
 @immutable
 final class HeaderFile {
+  /// Creates a new header file from the given [path].
+  const HeaderFile.from(
+    this.path, {
+    required this.guard,
+    required this.pragmaOnce,
+  });
+
+  /// Parses the given [path] as a header file.
+  ///
+  /// Throws an [ArgumentError] if the file does not exist.
+  factory HeaderFile.parse(String path) {
+    final io.File file = io.File(path);
+    if (!file.existsSync()) {
+      throw ArgumentError.value(path, 'path', 'File does not exist.');
+    }
+
+    final String contents = file.readAsStringSync();
+    final SourceFile sourceFile = SourceFile.fromString(contents, url: p.toUri(path));
+    return HeaderFile.from(
+      path,
+      guard: _parseGuard(sourceFile),
+      pragmaOnce: _parsePragmaOnce(sourceFile),
+    );
+  }
+
+  /// Parses the header guard of the given [sourceFile].
+  static HeaderGuardSpans? _parseGuard(SourceFile sourceFile) {
+    SourceSpan? ifndefSpan;
+    SourceSpan? defineSpan;
+    SourceSpan? endifSpan;
+
+    // Iterate over the lines in the file.
+    for (int i = 0; i < sourceFile.lines; i++) {
+      final int start = sourceFile.getOffset(i);
+      final int end = i == sourceFile.lines - 1
+          ? sourceFile.length
+          : sourceFile.getOffset(i + 1) - 1;
+      final String line = sourceFile.getText(start, end);
+
+      // Check if the line is a header guard directive.
+      if (line.startsWith('#ifndef')) {
+        ifndefSpan = sourceFile.span(start, end);
+      } else if (line.startsWith('#define')) {
+        defineSpan = sourceFile.span(start, end);
+        break;
+      }
+    }
+
+    // If we found no header guard, return null.
+    if (ifndefSpan == null) {
+      return null;
+    }
+
+    // Now iterate backwards to find the (last) #endif directive.
+    for (int i = sourceFile.lines - 1; i > 0; i--) {
+      final int start = sourceFile.getOffset(i);
+      final int end = i == sourceFile.lines - 1 ? 
+          sourceFile.length :
+          sourceFile.getOffset(i + 1) - 1;
+      final String line = sourceFile.getText(start, end);
+
+      // Check if the line is a header guard directive.
+      if (line.startsWith('#endif')) {
+        endifSpan = sourceFile.span(start, end);
+        break;
+      }
+    }
+
+    return HeaderGuardSpans(
+      ifndefSpan: ifndefSpan,
+      defineSpan: defineSpan,
+      endifSpan: endifSpan,
+    );
+  }
+
+  /// Parses the `#pragma once` directive of the given [sourceFile].
+  static SourceSpan? _parsePragmaOnce(SourceFile sourceFile) {
+    // Iterate over the lines in the file.
+    for (int i = 0; i < sourceFile.lines; i++) {
+      final int start = sourceFile.getOffset(i);
+      final int end = i == sourceFile.lines - 1
+          ? sourceFile.length
+          : sourceFile.getOffset(i + 1) - 1;
+      final String line = sourceFile.getText(start, end);
+
+      // Check if the line is a header guard directive.
+      if (line.startsWith('#pragma once')) {
+        return sourceFile.span(start, end);
+      }
+    }
+
+    return null;
+  }
+
   /// Path to the file on disk.
   final String path;
 
-  const HeaderFile(this.path);
+  /// The header guard span, if any.
+  ///
+  /// This is `null` if the file does not have a header guard.
+  final HeaderGuardSpans? guard;
+
+  /// The `#pragma once` directive, if any.
+  ///
+  /// This is `null` if the file does not have a `#pragma once` directive.
+  final SourceSpan? pragmaOnce;
+
+  @override
+  bool operator ==(Object other) {
+    return other is HeaderFile &&
+        path == other.path &&
+        guard == other.guard &&
+        pragmaOnce == other.pragmaOnce;
+  }
+
+  @override
+  int get hashCode => Object.hash(path, guard, pragmaOnce);
+
+  @override
+  String toString() {
+    return 'HeaderFile(\n'
+        '  path:       $path\n'
+        '  guard:      $guard\n'
+        '  pragmaOnce: $pragmaOnce\n'
+        ')';
+  }
+}
+
+/// Source elements that are part of a header guard.
+@immutable
+final class HeaderGuardSpans {
+  /// Collects the source spans of the header guard directives.
+  const HeaderGuardSpans({
+    required this.ifndefSpan,
+    required this.defineSpan,
+    required this.endifSpan,
+  });
+
+  /// Location of the `#ifndef` directive.
+  final SourceSpan? ifndefSpan;
+
+  /// Location of the `#define` directive.
+  final SourceSpan? defineSpan;
+
+  /// Location of the `#endif` directive.
+  final SourceSpan? endifSpan;
+
+  @override
+  bool operator ==(Object other) {
+    return other is HeaderGuardSpans &&
+        ifndefSpan == other.ifndefSpan &&
+        defineSpan == other.defineSpan &&
+        endifSpan == other.endifSpan;
+  }
+
+  @override
+  int get hashCode => Object.hash(ifndefSpan, defineSpan, endifSpan);
+
+  @override
+  String toString() {
+    return 'HeaderGuardSpans(\n'
+        '  #ifndef: $ifndefSpan\n'
+        '  #define: $defineSpan\n'
+        '  #endif:  $endifSpan\n'
+        ')';
+  }
+
+  /// Returns the value of the `#ifndef` directive.
+  ///
+  /// For example, `#ifndef FOO_H_`, this will return `FOO_H_`.
+  ///
+  /// If the span is not a valid `#ifndef` directive, `null` is returned.
+  String? get ifndefValue {
+    final String? value = ifndefSpan?.text;
+    if (value == null) {
+      return null;
+    }
+    if (!value.startsWith('#ifndef ')) {
+      return null;
+    }
+    return value.substring('#ifndef '.length);
+  }
+
+  /// Returns the value of the `#define` directive.
+  ///
+  /// For example, `#define FOO_H_`, this will return `FOO_H_`.
+  ///
+  /// If the span is not a valid `#define` directive, `null` is returned.
+  String? get defineValue {
+    final String? value = defineSpan?.text;
+    if (value == null) {
+      return null;
+    }
+    if (!value.startsWith('#define ')) {
+      return null;
+    }
+    return value.substring('#define '.length);
+  }
+
+  /// Returns the value of the `#endif` directive.
+  ///
+  /// For example, `#endif  // FOO_H_`, this will return `FOO_H_`.
+  ///
+  /// If the span is not a valid `#endif` directive, `null` is returned.
+  String? get endifValue {
+    final String? value = endifSpan?.text;
+    if (value == null) {
+      return null;
+    }
+    if (!value.startsWith('#endif  // ')) {
+      return null;
+    }
+    return value.substring('#endif  // '.length);
+  }
 }
diff --git a/tools/header_guard_check/pubspec.yaml b/tools/header_guard_check/pubspec.yaml
index 4f26b7c9771df..64e676d6b17fd 100644
--- a/tools/header_guard_check/pubspec.yaml
+++ b/tools/header_guard_check/pubspec.yaml
@@ -22,6 +22,7 @@ dependencies:
   engine_repo_tools: any
   meta: any
   path: any
+  source_span: any
 
 dev_dependencies:
   async_helper: any
@@ -35,6 +36,8 @@ dependency_overrides:
     path: ../../../third_party/dart/third_party/pkg/args
   async_helper:
     path: ../../../third_party/dart/pkg/async_helper
+  collection:
+    path: ../../../third_party/dart/third_party/pkg/collection
   engine_repo_tools:
     path: ../pkg/engine_repo_tools
   expect:
@@ -57,3 +60,7 @@ dependency_overrides:
     path: ../../third_party/pkg/process_runner
   smith:
     path: ../../../third_party/dart/pkg/smith
+  source_span:
+    path: ../../../third_party/dart/third_party/pkg/source_span
+  term_glyph:
+    path: ../../../third_party/dart/third_party/pkg/term_glyph
diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
new file mode 100644
index 0000000000000..a8abd41c02475
--- /dev/null
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -0,0 +1,217 @@
+// Copyright 2013 The Flutter Authors. 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' as io;
+
+import 'package:header_guard_check/src/header_file.dart';
+import 'package:litetest/litetest.dart';
+import 'package:path/path.dart' as p;
+import 'package:source_span/source_span.dart';
+
+Future<int> main(List<String> args) async {
+  void withTestFile(String path, String contents, void Function(io.File) fn) {
+    // Create a temporary file and delete it when we're done.
+    final io.Directory tempDir = io.Directory.systemTemp.createTempSync('header_guard_check_test');
+    final io.File file = io.File(p.join(tempDir.path, path));
+    file.writeAsStringSync(contents);
+    try {
+      fn(file);
+    } finally {
+      tempDir.deleteSync(recursive: true);
+    }
+  }
+
+  group('HeaderGuardSpans', () {
+    test('parses #ifndef', () {
+      const String input = '#ifndef FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+        defineSpan: null,
+        endifSpan: null,
+      );
+      expect(guard.ifndefValue, 'FOO_H_');
+    });
+
+    test('ignores #ifndef if omitted', () {
+      const HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: null,
+        endifSpan: null,
+      );
+      expect(guard.ifndefValue, isNull);
+    });
+
+    test('ignores #ifndef if invalid', () {
+      const String input = '#oops FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+        defineSpan: null,
+        endifSpan: null,
+      );
+      expect(guard.ifndefValue, isNull);
+    });
+
+    test('parses #define', () {
+      const String input = '#define FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+        endifSpan: null,
+      );
+      expect(guard.defineValue, 'FOO_H_');
+    });
+
+    test('ignores #define if omitted', () {
+      const HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: null,
+        endifSpan: null,
+      );
+      expect(guard.defineValue, isNull);
+    });
+
+    test('ignores #define if invalid', () {
+      const String input = '#oops FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+        endifSpan: null,
+      );
+      expect(guard.defineValue, isNull);
+    });
+
+    test('parses #endif', () {
+      const String input = '#endif  // FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: null,
+        endifSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+      );
+      expect(guard.endifValue, 'FOO_H_');
+    });
+
+    test('ignores #endif if omitted', () {
+      const HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: null,
+        endifSpan: null,
+      );
+      expect(guard.endifValue, isNull);
+    });
+
+    test('ignores #endif if invalid', () {
+      const String input = '#oops  // FOO_H_';
+      final HeaderGuardSpans guard = HeaderGuardSpans(
+        ifndefSpan: null,
+        defineSpan: null,
+        endifSpan: SourceSpanWithContext(
+          SourceLocation(0),
+          SourceLocation(input.length),
+          input,
+          input,
+        ),
+      );
+      expect(guard.endifValue, isNull);
+    });
+  });
+
+  group('HeaderFile', () {
+    test('parses a header file with a valid guard', () {
+      final String input = <String>[
+        '#ifndef FOO_H_',
+        '#define FOO_H_',
+        '',
+        '#endif  // FOO_H_',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.guard!.ifndefValue, 'FOO_H_');
+        expect(headerFile.guard!.defineValue, 'FOO_H_');
+        expect(headerFile.guard!.endifValue, 'FOO_H_');
+      });
+    });
+
+    test('parses a header file with an invalid #endif', () {
+      final String input = <String>[
+        '#ifndef FOO_H_',
+        '#define FOO_H_',
+        '',
+        // No comment after the #endif.
+        '#endif',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.guard!.ifndefValue, 'FOO_H_');
+        expect(headerFile.guard!.defineValue, 'FOO_H_');
+        expect(headerFile.guard!.endifValue, isNull);
+      });
+    });
+
+    test('parses a header file with a missing #define', () {
+      final String input = <String>[
+        '#ifndef FOO_H_',
+        // No #define.
+        '',
+        '#endif  // FOO_H_',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.guard!.ifndefValue, 'FOO_H_');
+        expect(headerFile.guard!.defineValue, isNull);
+        expect(headerFile.guard!.endifValue, 'FOO_H_');
+      });
+    });
+
+    test('parses a header file with a missing #ifndef', () {
+      final String input = <String>[
+        // No #ifndef.
+        '#define FOO_H_',
+        '',
+        '#endif  // FOO_H_',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.guard, isNull);
+      });
+    });
+
+    test('parses a header file with a #pragma once', () {
+      final String input = <String>[
+        '#pragma once',
+        '',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.pragmaOnce, isNotNull);
+      });
+    });
+  });
+
+  return 0;
+}

From 1784e68a0b0c3b9541a708bc15e172e54b5e5156 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 16:04:16 -0800
Subject: [PATCH 04/26] Working basic tool.

---
 tools/header_guard_check/README.md            |  27 ++++
 tools/header_guard_check/bin/main.dart        |  15 ++
 .../lib/header_guard_check.dart               | 128 ++++++++++++++++++
 3 files changed, 170 insertions(+)
 create mode 100644 tools/header_guard_check/README.md
 create mode 100644 tools/header_guard_check/bin/main.dart
 create mode 100644 tools/header_guard_check/lib/header_guard_check.dart

diff --git a/tools/header_guard_check/README.md b/tools/header_guard_check/README.md
new file mode 100644
index 0000000000000..e0b7333458570
--- /dev/null
+++ b/tools/header_guard_check/README.md
@@ -0,0 +1,27 @@
+# header_guard_check
+
+A tool to check that C++ header guards are used consistently in the engine.
+
+```shell
+# Assuming you are in the `flutter` root of the engine repo.
+dart ./tools/header_guard_check/bin/main.dart
+```
+
+The tool checks _all_ header files for the following pattern:
+
+```h
+// path/to/file.h
+
+#ifndef PATH_TO_FILE_H_
+#define PATH_TO_FILE_H_
+...
+#endif  // PATH_TO_FILE_H_
+```
+
+If the header file does not follow this pattern, the tool will print an error
+message and exit with a non-zero exit code. For more information about why we
+use this pattern, see [the Google C++ style guide](https://google.github.io/styleguide/cppguide.html#The__define_Guard).
+
+> [!IMPORTANT]
+> This is a prototype tool and is not yet integrated into the engine's CI, nor
+> provides a way to fix the header guards automatically.
diff --git a/tools/header_guard_check/bin/main.dart b/tools/header_guard_check/bin/main.dart
new file mode 100644
index 0000000000000..014b51c2d839d
--- /dev/null
+++ b/tools/header_guard_check/bin/main.dart
@@ -0,0 +1,15 @@
+// Copyright 2013 The Flutter Authors. 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' as io;
+
+import 'package:header_guard_check/header_guard_check.dart';
+
+Future<int> main(List<String> arguments) async {
+  final int result = await HeaderGuardCheck.fromCommandLine(arguments).run();
+  if (result != 0) {
+    io.exit(result);
+  }
+  return result;
+}
diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
new file mode 100644
index 0000000000000..5fb6e8ee1922b
--- /dev/null
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -0,0 +1,128 @@
+// Copyright 2013 The Flutter Authors. 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' as io;
+
+import 'package:args/args.dart';
+import 'package:engine_repo_tools/engine_repo_tools.dart';
+import 'package:meta/meta.dart';
+import 'package:path/path.dart' as p;
+
+import 'src/header_file.dart';
+
+/// Checks C++ header files for header guards.
+@immutable
+final class HeaderGuardCheck {
+  /// Creates a new header guard checker.
+  const HeaderGuardCheck({
+    required this.source,
+    required this.exclude,
+  });
+
+  /// Parses the command line arguments and creates a new header guard checker.
+  factory HeaderGuardCheck.fromCommandLine(List<String> arguments) {
+    final ArgResults argResults = _parser.parse(arguments);
+    return HeaderGuardCheck(
+      source: Engine.fromSrcPath(argResults['root'] as String),
+      exclude: argResults['exclude'] as List<String>,
+    );
+  }
+
+  /// Engine source root.
+  final Engine source;
+
+  /// Path directories to exclude from the check.
+  final List<String> exclude;
+
+  /// Runs the header guard check.
+  Future<int> run() async {
+    final List<io.File> files = <io.File>[];
+    
+    // Recursive search for header files.
+  final io.Directory dir = source.flutterDir;
+  await for (final io.FileSystemEntity entity in dir.list(recursive: true)) {
+    if (entity is io.File && entity.path.endsWith('.h')) {
+      // Check that the file is not excluded.
+      bool excluded = false;
+      for (final String excludePath in exclude) {
+        if (p.isWithin(excludePath, entity.path)) {
+          excluded = true;
+          break;
+        }
+      }
+      if (!excluded) {
+        files.add(entity);
+      }
+    }
+  }
+
+    // Check each file.
+    final List<HeaderFile> badFiles = <HeaderFile>[];
+    for (final io.File file in files) {
+      final HeaderFile headerFile = HeaderFile.parse(file.path);
+      if (headerFile.pragmaOnce != null) {
+        io.stderr.writeln(headerFile.pragmaOnce!.message('Unexpected #pragma once'));
+        badFiles.add(headerFile);
+        continue;
+      }
+      if (headerFile.guard == null) {
+        io.stderr.writeln('Missing header guard in ${headerFile.path}');
+        badFiles.add(headerFile);
+        continue;
+      }
+
+      // Determine what the expected guard should be.
+      // Find the relative path from the engine root to the file.
+      final String relativePath = p.relative(file.path, from: source.flutterDir.path);
+      final String underscoredRelativePath = relativePath.replaceAll(p.separator, '_');
+      final String expectedGuard = 'FLUTTER_${p.withoutExtension(underscoredRelativePath).toUpperCase().replaceAll('.', '_')}_H_';
+      if (headerFile.guard!.ifndefValue != expectedGuard) {
+        io.stderr.writeln(headerFile.guard!.ifndefSpan!.message('Expected #ifndef $expectedGuard'));
+        badFiles.add(headerFile);
+        continue;
+      }
+      if (headerFile.guard!.defineValue != expectedGuard) {
+        io.stderr.writeln(headerFile.guard!.defineSpan!.message('Expected #define $expectedGuard'));
+        badFiles.add(headerFile);
+        continue;
+      }
+      if (headerFile.guard!.endifValue != expectedGuard) {
+        io.stderr.writeln(headerFile.guard!.endifSpan!.message('Expected #endif // $expectedGuard'));
+        badFiles.add(headerFile);
+        continue;
+      }
+    }
+
+    if (badFiles.isNotEmpty) {
+      io.stdout.writeln('The following ${badFiles.length} files have invalid header guards:');
+      for (final HeaderFile headerFile in badFiles) {
+        io.stdout.writeln('  ${headerFile.path}');
+      }
+      return 1;
+    }
+
+    return 0;
+  }
+}
+
+final Engine? _engine = Engine.tryFindWithin(p.dirname(p.fromUri(io.Platform.script)));
+
+final ArgParser _parser = ArgParser()
+  ..addOption(
+    'root',
+    abbr: 'r',
+    help: 'Path to the engine source root.',
+    valueHelp: 'path/to/engine/src',
+    defaultsTo: _engine?.srcDir.path,
+  )
+  ..addMultiOption(
+    'exclude',
+    abbr: 'e',
+    help: 'Path directories to exclude from the check.',
+    valueHelp: 'path/to/dir',
+    defaultsTo: _engine != null ? <String>[
+      p.join(_engine!.flutterDir.path, 'build'),
+      p.join(_engine!.flutterDir.path, 'third_party'),
+    ] : null,
+  );

From 90892fad66b6437485cea5d7d307c8072d3b424c Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 16:06:15 -0800
Subject: [PATCH 05/26] WS.

---
 tools/header_guard_check/lib/header_guard_check.dart | 4 ++--
 tools/header_guard_check/lib/src/header_file.dart    | 2 +-
 tools/header_guard_check/pubspec.yaml                | 2 +-
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 5fb6e8ee1922b..62d1ba1dac203 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -38,8 +38,8 @@ final class HeaderGuardCheck {
   /// Runs the header guard check.
   Future<int> run() async {
     final List<io.File> files = <io.File>[];
-    
-    // Recursive search for header files.
+
+  // Recursive search for header files.
   final io.Directory dir = source.flutterDir;
   await for (final io.FileSystemEntity entity in dir.list(recursive: true)) {
     if (entity is io.File && entity.path.endsWith('.h')) {
diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index d6f01de3b8c87..d5da4a9db2de7 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -67,7 +67,7 @@ final class HeaderFile {
     // Now iterate backwards to find the (last) #endif directive.
     for (int i = sourceFile.lines - 1; i > 0; i--) {
       final int start = sourceFile.getOffset(i);
-      final int end = i == sourceFile.lines - 1 ? 
+      final int end = i == sourceFile.lines - 1 ?
           sourceFile.length :
           sourceFile.getOffset(i + 1) - 1;
       final String line = sourceFile.getText(start, end);
diff --git a/tools/header_guard_check/pubspec.yaml b/tools/header_guard_check/pubspec.yaml
index 64e676d6b17fd..2796069a76580 100644
--- a/tools/header_guard_check/pubspec.yaml
+++ b/tools/header_guard_check/pubspec.yaml
@@ -16,7 +16,7 @@ publish_to: none
 environment:
   sdk: '>=3.2.0-0 <4.0.0'
 
-dependencies: 
+dependencies:
   args: any
   git_repo_tools: any
   engine_repo_tools: any

From 5b21cc1b316d9e022c1bf8564a5759ac9317c6d4 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Mon, 11 Dec 2023 16:10:21 -0800
Subject: [PATCH 06/26] Tweak.

---
 tools/header_guard_check/lib/header_guard_check.dart | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 62d1ba1dac203..05057bf0673c7 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -123,6 +123,7 @@ final ArgParser _parser = ArgParser()
     valueHelp: 'path/to/dir',
     defaultsTo: _engine != null ? <String>[
       p.join(_engine!.flutterDir.path, 'build'),
+      p.join(_engine!.flutterDir.path, 'prebuilts'),
       p.join(_engine!.flutterDir.path, 'third_party'),
     ] : null,
   );

From 98e3307ccd8ca25be2364916f149d52431bab394 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:06:06 -0800
Subject: [PATCH 07/26] Tweak.

---
 .../lib/header_guard_check.dart               | 62 +++++++++++++------
 1 file changed, 44 insertions(+), 18 deletions(-)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 05057bf0673c7..32c942ddb8b95 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -18,6 +18,7 @@ final class HeaderGuardCheck {
   const HeaderGuardCheck({
     required this.source,
     required this.exclude,
+    this.include = const <String>[],
   });
 
   /// Parses the command line arguments and creates a new header guard checker.
@@ -25,6 +26,7 @@ final class HeaderGuardCheck {
     final ArgResults argResults = _parser.parse(arguments);
     return HeaderGuardCheck(
       source: Engine.fromSrcPath(argResults['root'] as String),
+      include: argResults['include'] as List<String>,
       exclude: argResults['exclude'] as List<String>,
     );
   }
@@ -32,6 +34,9 @@ final class HeaderGuardCheck {
   /// Engine source root.
   final Engine source;
 
+  /// Path directories to include in the check.
+  final List<String> include;
+
   /// Path directories to exclude from the check.
   final List<String> exclude;
 
@@ -39,23 +44,37 @@ final class HeaderGuardCheck {
   Future<int> run() async {
     final List<io.File> files = <io.File>[];
 
-  // Recursive search for header files.
-  final io.Directory dir = source.flutterDir;
-  await for (final io.FileSystemEntity entity in dir.list(recursive: true)) {
-    if (entity is io.File && entity.path.endsWith('.h')) {
-      // Check that the file is not excluded.
-      bool excluded = false;
-      for (final String excludePath in exclude) {
-        if (p.isWithin(excludePath, entity.path)) {
-          excluded = true;
-          break;
+    // Recursive search for header files.
+    final io.Directory dir = source.flutterDir;
+    await for (final io.FileSystemEntity entity in dir.list(recursive: true)) {
+      if (entity is io.File && entity.path.endsWith('.h')) {
+        // Check that the file is included.
+        bool included = include.isEmpty;
+        for (final String includePath in include) {
+          final String relativePath = p.relative(includePath, from: source.flutterDir.path);
+          if (p.isWithin(relativePath, entity.path)) {
+            included = true;
+            break;
+          }
+        }
+        if (!included) {
+          continue;
+        }
+
+        // Check that the file is not excluded.
+        bool excluded = false;
+        for (final String excludePath in exclude) {
+          final String relativePath = p.relative(excludePath, from: source.flutterDir.path);
+          if (p.isWithin(relativePath, entity.path)) {
+            excluded = true;
+            break;
+          }
+        }
+        if (!excluded) {
+          files.add(entity);
         }
-      }
-      if (!excluded) {
-        files.add(entity);
       }
     }
-  }
 
     // Check each file.
     final List<HeaderFile> badFiles = <HeaderFile>[];
@@ -116,14 +135,21 @@ final ArgParser _parser = ArgParser()
     valueHelp: 'path/to/engine/src',
     defaultsTo: _engine?.srcDir.path,
   )
+  ..addMultiOption(
+    'include',
+    abbr: 'i',
+    help: 'Path directories to include in the check.',
+    valueHelp: 'path/to/dir (relative to the engine root)',
+    defaultsTo: <String>[],
+  )
   ..addMultiOption(
     'exclude',
     abbr: 'e',
     help: 'Path directories to exclude from the check.',
-    valueHelp: 'path/to/dir',
+    valueHelp: 'path/to/dir (relative to the engine root)',
     defaultsTo: _engine != null ? <String>[
-      p.join(_engine!.flutterDir.path, 'build'),
-      p.join(_engine!.flutterDir.path, 'prebuilts'),
-      p.join(_engine!.flutterDir.path, 'third_party'),
+      'build',
+      'prebuilts',
+      'third_party',
     ] : null,
   );

From 3afe63c1665f305b0d6fc46e8407ec94f1bf9584 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:15:52 -0800
Subject: [PATCH 08/26] ++

---
 tools/header_guard_check/README.md            | 12 +++
 .../lib/header_guard_check.dart               |  6 +-
 .../lib/src/header_file.dart                  | 88 ++++++++++++++++++-
 3 files changed, 97 insertions(+), 9 deletions(-)

diff --git a/tools/header_guard_check/README.md b/tools/header_guard_check/README.md
index e0b7333458570..964ca65bc77f6 100644
--- a/tools/header_guard_check/README.md
+++ b/tools/header_guard_check/README.md
@@ -25,3 +25,15 @@ use this pattern, see [the Google C++ style guide](https://google.github.io/styl
 > [!IMPORTANT]
 > This is a prototype tool and is not yet integrated into the engine's CI, nor
 > provides a way to fix the header guards automatically.
+
+## Advanced usage
+
+### Restricting the files to check
+
+By default, the tool checks all header files in the engine. You can restrict the
+files to check by passing a relative (to the `engine/src/flutter` root) paths to
+include:
+
+```shell
+dart ./tools/header_guard_check/bin/main.dart --include impeller
+```
diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 32c942ddb8b95..910430cf3eca0 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -91,11 +91,7 @@ final class HeaderGuardCheck {
         continue;
       }
 
-      // Determine what the expected guard should be.
-      // Find the relative path from the engine root to the file.
-      final String relativePath = p.relative(file.path, from: source.flutterDir.path);
-      final String underscoredRelativePath = relativePath.replaceAll(p.separator, '_');
-      final String expectedGuard = 'FLUTTER_${p.withoutExtension(underscoredRelativePath).toUpperCase().replaceAll('.', '_')}_H_';
+      final String expectedGuard = headerFile.expectedName(engineRoot: source.flutterDir.path);
       if (headerFile.guard!.ifndefValue != expectedGuard) {
         io.stderr.writeln(headerFile.guard!.ifndefSpan!.message('Expected #ifndef $expectedGuard'));
         badFiles.add(headerFile);
diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index d5da4a9db2de7..e4036da026ea4 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -28,7 +28,8 @@ final class HeaderFile {
     }
 
     final String contents = file.readAsStringSync();
-    final SourceFile sourceFile = SourceFile.fromString(contents, url: p.toUri(path));
+    final SourceFile sourceFile =
+        SourceFile.fromString(contents, url: p.toUri(path));
     return HeaderFile.from(
       path,
       guard: _parseGuard(sourceFile),
@@ -67,9 +68,9 @@ final class HeaderFile {
     // Now iterate backwards to find the (last) #endif directive.
     for (int i = sourceFile.lines - 1; i > 0; i--) {
       final int start = sourceFile.getOffset(i);
-      final int end = i == sourceFile.lines - 1 ?
-          sourceFile.length :
-          sourceFile.getOffset(i + 1) - 1;
+      final int end = i == sourceFile.lines - 1
+          ? sourceFile.length
+          : sourceFile.getOffset(i + 1) - 1;
       final String line = sourceFile.getText(start, end);
 
       // Check if the line is a header guard directive.
@@ -118,6 +119,85 @@ final class HeaderFile {
   /// This is `null` if the file does not have a `#pragma once` directive.
   final SourceSpan? pragmaOnce;
 
+  /// Returns the expected header guard for this file, relative to [engineRoot].
+  ///
+  /// For example, if the file is `foo/bar/baz.h`, this will return `FLUTTER_FOO_BAR_BAZ_H_`.
+  String expectedName({required String engineRoot}) {
+    final String relativePath = p.relative(path, from: engineRoot);
+    final String underscoredRelativePath = relativePath.replaceAll(p.separator, '_');
+    return 'FLUTTER_${p.withoutExtension(underscoredRelativePath).toUpperCase().replaceAll('.', '_')}_H_';
+  }
+
+  /// Updates the file at [path] to have the expected header guard.
+  bool fix({required String engineRoot}) {
+    // Check if the file already has a valid header guard.
+    if (guard != null) {
+      final String expectedGuard = expectedName(engineRoot: engineRoot);
+      if (guard!.ifndefValue == expectedGuard &&
+          guard!.defineValue == expectedGuard &&
+          guard!.endifValue == expectedGuard) {
+        return false;
+      }
+    }
+
+    // Get the contents of the file.
+    final String oldContents = io.File(path).readAsStringSync();
+
+    // If we're using pragma once, replace it with an ifndef/define, and
+    // append an endif and a newline at the end of the file.
+    if (pragmaOnce != null) {
+      final String expectedGuard = expectedName(engineRoot: engineRoot);
+      
+      // Append the endif and newline.
+      String newContents = '$oldContents\n#endif  // $expectedGuard\n';
+
+      // Replace the span with the ifndef/define.
+      newContents = newContents.replaceRange(
+        pragmaOnce!.start.offset,
+        pragmaOnce!.end.offset,
+        '#ifndef $expectedGuard\n'
+        '#define $expectedGuard\n'
+      );
+
+      // Write the new contents to the file.
+      io.File(path).writeAsStringSync(newContents);
+
+      return true;
+    }
+
+    // If we're not using pragma once, replace the header guard with the
+    // expected header guard.
+    if (guard != null) {
+      final String expectedGuard = expectedName(engineRoot: engineRoot);
+      
+      // Replace endif:
+      String newContents = oldContents.replaceRange(
+        guard!.endifSpan!.start.offset,
+        guard!.endifSpan!.end.offset,
+        '#endif  // $expectedGuard\n'
+      );
+
+      // Replace define:
+      newContents = newContents.replaceRange(
+        guard!.defineSpan!.start.offset,
+        guard!.defineSpan!.end.offset,
+        '#define $expectedGuard\n'
+      );
+
+      // Replace ifndef:
+      newContents = newContents.replaceRange(
+        guard!.ifndefSpan!.start.offset,
+        guard!.ifndefSpan!.end.offset,
+        '#ifndef $expectedGuard\n'
+      );
+
+      // Write the new contents to the file.
+      io.File(path).writeAsStringSync(newContents);
+
+      return true;
+    }
+  }
+
   @override
   bool operator ==(Object other) {
     return other is HeaderFile &&

From 59e98e6bd696f2736654ec78535d127e0d48dc65 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:17:17 -0800
Subject: [PATCH 09/26] ++

---
 .../lib/header_guard_check.dart               | 20 +++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 910430cf3eca0..13bdd10c99179 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -19,6 +19,7 @@ final class HeaderGuardCheck {
     required this.source,
     required this.exclude,
     this.include = const <String>[],
+    this.fix = false,
   });
 
   /// Parses the command line arguments and creates a new header guard checker.
@@ -28,12 +29,16 @@ final class HeaderGuardCheck {
       source: Engine.fromSrcPath(argResults['root'] as String),
       include: argResults['include'] as List<String>,
       exclude: argResults['exclude'] as List<String>,
+      fix: argResults['fix'] as bool,
     );
   }
 
   /// Engine source root.
   final Engine source;
 
+  /// Whether to automatically fix most header guards.
+  final bool fix;
+
   /// Path directories to include in the check.
   final List<String> include;
 
@@ -114,6 +119,17 @@ final class HeaderGuardCheck {
       for (final HeaderFile headerFile in badFiles) {
         io.stdout.writeln('  ${headerFile.path}');
       }
+
+      // If we're fixing, fix the files.
+      if (fix) {
+        for (final HeaderFile headerFile in badFiles) {
+          headerFile.fix(engineRoot: source.flutterDir.path);
+        }
+
+        io.stdout.writeln('Fixed ${badFiles.length} files.');
+        return 0;
+      }
+
       return 1;
     }
 
@@ -124,6 +140,10 @@ final class HeaderGuardCheck {
 final Engine? _engine = Engine.tryFindWithin(p.dirname(p.fromUri(io.Platform.script)));
 
 final ArgParser _parser = ArgParser()
+  ..addFlag(
+    'fix',
+    help: 'Automatically fixes most header guards.',
+  )
   ..addOption(
     'root',
     abbr: 'r',

From acc939ac8781079c47e337f0f0aa71e30e5c3489 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:17:57 -0800
Subject: [PATCH 10/26] ++

---
 tools/header_guard_check/lib/src/header_file.dart | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index e4036da026ea4..4dc3dbc4a3cfd 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -196,6 +196,8 @@ final class HeaderFile {
 
       return true;
     }
+
+    return false;
   }
 
   @override

From 8d3b0f9e941401fe5b400afc0fe300f9ae3958ca Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:20:22 -0800
Subject: [PATCH 11/26] ++

---
 tools/pub_get_offline.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/pub_get_offline.py b/tools/pub_get_offline.py
index 931db7319b8f9..f5e41ba3245e1 100644
--- a/tools/pub_get_offline.py
+++ b/tools/pub_get_offline.py
@@ -40,6 +40,7 @@
     os.path.join(ENGINE_DIR, 'tools', 'const_finder'),
     os.path.join(ENGINE_DIR, 'tools', 'gen_web_locale_keymap'),
     os.path.join(ENGINE_DIR, 'tools', 'githooks'),
+    os.path.join(ENGINE_DIR, 'tools', 'header_guard_check'),
     os.path.join(ENGINE_DIR, 'tools', 'licenses'),
     os.path.join(ENGINE_DIR, 'tools', 'path_ops', 'dart'),
     os.path.join(ENGINE_DIR, 'tools', 'pkg', 'engine_build_configs'),

From 1c8fa0ebbe3d97bf2ebf9fde2c87a7a2318ca720 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:24:11 -0800
Subject: [PATCH 12/26] ++

---
 .../header_guard_check/lib/header_guard_check.dart  | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 13bdd10c99179..34d27262d7126 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -57,7 +57,7 @@ final class HeaderGuardCheck {
         bool included = include.isEmpty;
         for (final String includePath in include) {
           final String relativePath = p.relative(includePath, from: source.flutterDir.path);
-          if (p.isWithin(relativePath, entity.path)) {
+          if (p.isWithin(relativePath, entity.path) || p.equals(relativePath, entity.path)) {
             included = true;
             break;
           }
@@ -70,7 +70,7 @@ final class HeaderGuardCheck {
         bool excluded = false;
         for (final String excludePath in exclude) {
           final String relativePath = p.relative(excludePath, from: source.flutterDir.path);
-          if (p.isWithin(relativePath, entity.path)) {
+          if (p.isWithin(relativePath, entity.path) || p.equals(relativePath, entity.path)) {
             excluded = true;
             break;
           }
@@ -154,17 +154,18 @@ final ArgParser _parser = ArgParser()
   ..addMultiOption(
     'include',
     abbr: 'i',
-    help: 'Path directories to include in the check.',
-    valueHelp: 'path/to/dir (relative to the engine root)',
+    help: 'Paths to include in the check.',
+    valueHelp: 'path/to/dir/or/file (relative to the engine root)',
     defaultsTo: <String>[],
   )
   ..addMultiOption(
     'exclude',
     abbr: 'e',
-    help: 'Path directories to exclude from the check.',
-    valueHelp: 'path/to/dir (relative to the engine root)',
+    help: 'Paths to exclude from the check.',
+    valueHelp: 'path/to/dir/or/file (relative to the engine root)',
     defaultsTo: _engine != null ? <String>[
       'build',
+      'impeller/compiler/code_gen_template.h',
       'prebuilts',
       'third_party',
     ] : null,

From 7d1a6218161bb20dd52c7c0bd781ecd84497fe36 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:30:20 -0800
Subject: [PATCH 13/26] Change WS.

---
 tools/header_guard_check/lib/src/header_file.dart | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index 4dc3dbc4a3cfd..026d8bb324a5e 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -156,7 +156,7 @@ final class HeaderFile {
         pragmaOnce!.start.offset,
         pragmaOnce!.end.offset,
         '#ifndef $expectedGuard\n'
-        '#define $expectedGuard\n'
+        '#define $expectedGuard'
       );
 
       // Write the new contents to the file.
@@ -181,7 +181,7 @@ final class HeaderFile {
       newContents = newContents.replaceRange(
         guard!.defineSpan!.start.offset,
         guard!.defineSpan!.end.offset,
-        '#define $expectedGuard\n'
+        '#define $expectedGuard'
       );
 
       // Replace ifndef:

From 68ce106c59d3759383d55168d05fa541b40362d6 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Tue, 12 Dec 2023 17:59:26 -0800
Subject: [PATCH 14/26] ++

---
 tools/header_guard_check/lib/src/header_file.dart | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index 026d8bb324a5e..b51779865e5c9 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -147,7 +147,7 @@ final class HeaderFile {
     // append an endif and a newline at the end of the file.
     if (pragmaOnce != null) {
       final String expectedGuard = expectedName(engineRoot: engineRoot);
-      
+
       // Append the endif and newline.
       String newContents = '$oldContents\n#endif  // $expectedGuard\n';
 
@@ -169,7 +169,7 @@ final class HeaderFile {
     // expected header guard.
     if (guard != null) {
       final String expectedGuard = expectedName(engineRoot: engineRoot);
-      
+
       // Replace endif:
       String newContents = oldContents.replaceRange(
         guard!.endifSpan!.start.offset,

From 5212adee2514e31365e81f03278640e43506e576 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Wed, 13 Dec 2023 16:54:07 -0800
Subject: [PATCH 15/26] Tweak WS.

---
 tools/header_guard_check/lib/src/header_file.dart | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index b51779865e5c9..df20212cd3eaa 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -174,7 +174,7 @@ final class HeaderFile {
       String newContents = oldContents.replaceRange(
         guard!.endifSpan!.start.offset,
         guard!.endifSpan!.end.offset,
-        '#endif  // $expectedGuard\n'
+        '#endif  // $expectedGuard'
       );
 
       // Replace define:
@@ -188,7 +188,7 @@ final class HeaderFile {
       newContents = newContents.replaceRange(
         guard!.ifndefSpan!.start.offset,
         guard!.ifndefSpan!.end.offset,
-        '#ifndef $expectedGuard\n'
+        '#ifndef $expectedGuard'
       );
 
       // Write the new contents to the file.

From f9bfce155c1af691990231718e3300cc71a4f84f Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 14 Dec 2023 13:56:49 -0800
Subject: [PATCH 16/26] Tweak expected name.

---
 .../lib/src/header_file.dart                  |  9 +++++----
 .../test/header_file_test.dart                | 19 +++++++++++++++++++
 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index df20212cd3eaa..a2b0a90e0af2f 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -28,8 +28,7 @@ final class HeaderFile {
     }
 
     final String contents = file.readAsStringSync();
-    final SourceFile sourceFile =
-        SourceFile.fromString(contents, url: p.toUri(path));
+    final SourceFile sourceFile = SourceFile.fromString(contents, url: p.toUri(path));
     return HeaderFile.from(
       path,
       guard: _parseGuard(sourceFile),
@@ -119,13 +118,15 @@ final class HeaderFile {
   /// This is `null` if the file does not have a `#pragma once` directive.
   final SourceSpan? pragmaOnce;
 
+  static final RegExp _nonAlphaNumeric = RegExp(r'[^a-zA-Z0-9]');
+
   /// Returns the expected header guard for this file, relative to [engineRoot].
   ///
   /// For example, if the file is `foo/bar/baz.h`, this will return `FLUTTER_FOO_BAR_BAZ_H_`.
   String expectedName({required String engineRoot}) {
     final String relativePath = p.relative(path, from: engineRoot);
-    final String underscoredRelativePath = relativePath.replaceAll(p.separator, '_');
-    return 'FLUTTER_${p.withoutExtension(underscoredRelativePath).toUpperCase().replaceAll('.', '_')}_H_';
+    final String underscoredRelativePath = p.withoutExtension(relativePath).replaceAll(_nonAlphaNumeric, '_');
+    return 'FLUTTER_${underscoredRelativePath.toUpperCase()}_H_';
   }
 
   /// Updates the file at [path] to have the expected header guard.
diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
index a8abd41c02475..cda4748952fe2 100644
--- a/tools/header_guard_check/test/header_file_test.dart
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -142,6 +142,25 @@ Future<int> main(List<String> args) async {
   });
 
   group('HeaderFile', () {
+    test('produces a valid header guard name from various file names', () {
+      // All of these should produce the name `FOO_BAR_BAZ_H_`.
+      const List<String> inputs = <String>[
+        'foo_bar_baz.h',
+        'foo-bar-baz.h',
+        'foo_bar-baz.h',
+        'foo-bar_baz.h',
+        'foo+bar+baz.h',
+      ];
+      for (final String input in inputs) {
+        final HeaderFile headerFile = HeaderFile.from(
+          input, 
+          guard: null,
+          pragmaOnce: null,
+        );
+        expect(headerFile.expectedName(engineRoot: ''), endsWith('FOO_BAR_BAZ_H_'));
+      }
+    });
+
     test('parses a header file with a valid guard', () {
       final String input = <String>[
         '#ifndef FOO_H_',

From 1b6ea20d40e8217ffc59d31b241f645ca61ecdf7 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 14 Dec 2023 13:58:10 -0800
Subject: [PATCH 17/26] ++

---
 tools/header_guard_check/test/header_file_test.dart | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
index cda4748952fe2..ff7999e08c89f 100644
--- a/tools/header_guard_check/test/header_file_test.dart
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -153,7 +153,7 @@ Future<int> main(List<String> args) async {
       ];
       for (final String input in inputs) {
         final HeaderFile headerFile = HeaderFile.from(
-          input, 
+          input,
           guard: null,
           pragmaOnce: null,
         );

From de041721a717ac84616e427c50fe58c7da720da5 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 14 Dec 2023 14:00:55 -0800
Subject: [PATCH 18/26] ++

---
 tools/header_guard_check/README.md | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/tools/header_guard_check/README.md b/tools/header_guard_check/README.md
index 964ca65bc77f6..a8354b7ee5b40 100644
--- a/tools/header_guard_check/README.md
+++ b/tools/header_guard_check/README.md
@@ -23,8 +23,7 @@ message and exit with a non-zero exit code. For more information about why we
 use this pattern, see [the Google C++ style guide](https://google.github.io/styleguide/cppguide.html#The__define_Guard).
 
 > [!IMPORTANT]
-> This is a prototype tool and is not yet integrated into the engine's CI, nor
-> provides a way to fix the header guards automatically.
+> This is a prototype tool and is not yet integrated into the engine's CI.
 
 ## Advanced usage
 

From dd2b5b713baf89dc6e302f0c98a0628744225d32 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 14 Dec 2023 14:04:38 -0800
Subject: [PATCH 19/26] ++

---
 tools/header_guard_check/README.md | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tools/header_guard_check/README.md b/tools/header_guard_check/README.md
index a8354b7ee5b40..20ec093564f0e 100644
--- a/tools/header_guard_check/README.md
+++ b/tools/header_guard_check/README.md
@@ -25,6 +25,14 @@ use this pattern, see [the Google C++ style guide](https://google.github.io/styl
 > [!IMPORTANT]
 > This is a prototype tool and is not yet integrated into the engine's CI.
 
+## Automatic fixes
+
+The tool can automatically fix header files that do not follow the pattern:
+
+```shell
+dart ./tools/header_guard_check/bin/main.dart --fix
+```
+
 ## Advanced usage
 
 ### Restricting the files to check

From 713416dec8022949f053c49cfeaad7a8b6996f5e Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 14 Dec 2023 14:51:06 -0800
Subject: [PATCH 20/26] ++

---
 tools/header_guard_check/pubspec.yaml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/header_guard_check/pubspec.yaml b/tools/header_guard_check/pubspec.yaml
index 2796069a76580..52e00456a9548 100644
--- a/tools/header_guard_check/pubspec.yaml
+++ b/tools/header_guard_check/pubspec.yaml
@@ -48,6 +48,8 @@ dependency_overrides:
     path: ../pkg/git_repo_tools
   litetest:
     path: ../../testing/litetest
+  meta:
+    path: ../../../third_party/dart/pkg/meta
   path:
     path: ../../../third_party/dart/third_party/pkg/path
   platform:

From 03b909905a3c6811298085d6249ca368d9ce1bd5 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 25 Jan 2024 17:30:46 -0800
Subject: [PATCH 21/26] Tweak rules.

---
 .../lib/src/header_file.dart                  | 25 ++++--
 .../test/header_file_test.dart                | 76 +++++++++++++++++++
 2 files changed, 93 insertions(+), 8 deletions(-)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index a2b0a90e0af2f..b3dfd44ee4fd9 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -131,9 +131,10 @@ final class HeaderFile {
 
   /// Updates the file at [path] to have the expected header guard.
   bool fix({required String engineRoot}) {
+    final String expectedGuard = expectedName(engineRoot: engineRoot);
+
     // Check if the file already has a valid header guard.
     if (guard != null) {
-      final String expectedGuard = expectedName(engineRoot: engineRoot);
       if (guard!.ifndefValue == expectedGuard &&
           guard!.defineValue == expectedGuard &&
           guard!.endifValue == expectedGuard) {
@@ -147,8 +148,6 @@ final class HeaderFile {
     // If we're using pragma once, replace it with an ifndef/define, and
     // append an endif and a newline at the end of the file.
     if (pragmaOnce != null) {
-      final String expectedGuard = expectedName(engineRoot: engineRoot);
-
       // Append the endif and newline.
       String newContents = '$oldContents\n#endif  // $expectedGuard\n';
 
@@ -162,14 +161,12 @@ final class HeaderFile {
 
       // Write the new contents to the file.
       io.File(path).writeAsStringSync(newContents);
-
       return true;
     }
 
     // If we're not using pragma once, replace the header guard with the
     // expected header guard.
     if (guard != null) {
-      final String expectedGuard = expectedName(engineRoot: engineRoot);
 
       // Replace endif:
       String newContents = oldContents.replaceRange(
@@ -193,12 +190,24 @@ final class HeaderFile {
       );
 
       // Write the new contents to the file.
-      io.File(path).writeAsStringSync(newContents);
-
+      io.File(path).writeAsStringSync('$newContents\n');
       return true;
     }
 
-    return false;
+    // If we're missing a guard entirely, add one. The rules are:
+    // 1. Add a newline, #endif at the end of the file.
+    // 2. Add a newline, #ifndef, #define after the first non-comment line.
+    String newContents = oldContents;
+    newContents += '\n#endif  // $expectedGuard\n';
+    newContents = newContents.replaceFirst(
+      RegExp(r'^(?!//)', multiLine: true),
+      '\n#ifndef $expectedGuard\n'
+      '#define $expectedGuard\n'
+    );
+
+    // Write the new contents to the file.
+    io.File(path).writeAsStringSync(newContents);
+    return true;
   }
 
   @override
diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
index ff7999e08c89f..d6c5660779203 100644
--- a/tools/header_guard_check/test/header_file_test.dart
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -230,6 +230,82 @@ Future<int> main(List<String> args) async {
         expect(headerFile.pragmaOnce, isNotNull);
       });
     });
+
+    test('fixes a file that uses #pragma once', () {
+      final String input = <String>[
+        '#pragma once',
+        '',
+        '// ...',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.fix(engineRoot: p.dirname(file.path)), isTrue);
+        expect(file.readAsStringSync(), <String>[
+          '#ifndef FLUTTER_FOO_H_',
+          '#define FLUTTER_FOO_H_',
+          '',
+          '// ...',
+          '#endif  // FLUTTER_FOO_H_',
+          '',
+        ].join('\n'));
+      });
+    });
+
+    test('fixes a file with an incorrect header guard', () {
+      final String input = <String>[
+        '#ifndef FOO_H_',
+        '#define FOO_H_',
+        '',
+        '#endif  // FOO_H_',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.fix(engineRoot: p.dirname(file.path)), isTrue);
+        expect(file.readAsStringSync(), <String>[
+          '#ifndef FLUTTER_FOO_H_',
+          '#define FLUTTER_FOO_H_',
+          '',
+          '#endif  // FLUTTER_FOO_H_',
+          '',
+        ].join('\n'));
+      });
+    });
+
+    test('fixes a file with no header guard', () {
+      final String input = <String>[
+        '// 1.',
+        '// 2.',
+        '// 3.',
+        '',
+        "#import 'flutter/shell/platform/darwin/Flutter.h'",
+        '',
+        '@protocl Flutter',
+        '',
+        '@end',
+        '',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.fix(engineRoot: p.dirname(file.path)), isTrue);
+        expect(file.readAsStringSync(), <String>[
+          '// 1.',
+          '// 2.',
+          '// 3.',
+          '',
+          '#ifndef FLUTTER_FOO_H_',
+          '#define FLUTTER_FOO_H_',
+          '',
+          "#import 'flutter/shell/platform/darwin/Flutter.h'",
+          '',
+          '@protocl Flutter',
+          '',
+          '@end',
+          '',
+          '#endif  // FLUTTER_FOO_H_',
+          '',
+        ].join('\n'));
+      });
+    });
   });
 
   return 0;

From 6593be63f2f21dd55fb4ebc4addb0e018c564e61 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Thu, 25 Jan 2024 17:51:54 -0800
Subject: [PATCH 22/26] ++

---
 .../lib/src/header_file.dart                  |  4 ++++
 .../test/header_file_test.dart                | 20 +++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index b3dfd44ee4fd9..82cb6bb1abca5 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -54,6 +54,10 @@ final class HeaderFile {
       if (line.startsWith('#ifndef')) {
         ifndefSpan = sourceFile.span(start, end);
       } else if (line.startsWith('#define')) {
+        // If we find a define preceding an ifndef, it is not a header guard.
+        if (ifndefSpan == null) {
+          continue;
+        }
         defineSpan = sourceFile.span(start, end);
         break;
       }
diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
index d6c5660779203..11bb11ede0c70 100644
--- a/tools/header_guard_check/test/header_file_test.dart
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -306,6 +306,26 @@ Future<int> main(List<String> args) async {
         ].join('\n'));
       });
     });
+
+    test('does not touch a file with an existing guard and another #define', () {
+      final String input = <String>[
+        '// 1.',
+        '// 2.',
+        '// 3.',
+        '',
+        '#define FML_USED_ON_EMBEDDER',
+        '',
+        '#ifndef FLUTTER_FOO_H_',
+        '#define FLUTTER_FOO_H_',
+        '',
+        '#endif  // FLUTTER_FOO_H_',
+        '',
+      ].join('\n');
+      withTestFile('foo.h', input, (io.File file) {
+        final HeaderFile headerFile = HeaderFile.parse(file.path);
+        expect(headerFile.fix(engineRoot: p.dirname(file.path)), isFalse);
+      });
+    });
   });
 
   return 0;

From 26fa5288de3b8112a9262ce0dccb2f1519399eca Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Fri, 26 Jan 2024 11:37:50 -0800
Subject: [PATCH 23/26] Address feedback.

---
 testing/run_tests.py                          |   1 +
 .../lib/header_guard_check.dart               | 131 +++++++++---------
 .../lib/src/header_file.dart                  |   6 +-
 .../test/header_file_test.dart                |   2 +-
 4 files changed, 73 insertions(+), 67 deletions(-)

diff --git a/testing/run_tests.py b/testing/run_tests.py
index 2517c091123ce..b829dfd0dc447 100755
--- a/testing/run_tests.py
+++ b/testing/run_tests.py
@@ -1004,6 +1004,7 @@ def build_dart_host_test_list(build_dir):
       (os.path.join('flutter', 'tools', 'pkg', 'engine_build_configs'), []),
       (os.path.join('flutter', 'tools', 'pkg', 'engine_repo_tools'), []),
       (os.path.join('flutter', 'tools', 'pkg', 'git_repo_tools'), []),
+      (os.path.join('flutter', 'tools', 'pkg', 'header_guard_check'), []),
   ]
   if not is_asan(build_dir):
     dart_host_tests += [
diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 34d27262d7126..396d586890537 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -47,93 +47,96 @@ final class HeaderGuardCheck {
 
   /// Runs the header guard check.
   Future<int> run() async {
-    final List<io.File> files = <io.File>[];
+    final List<HeaderFile> badFiles = _checkFiles(_findIncludedHeaderFiles()).toList();
 
-    // Recursive search for header files.
-    final io.Directory dir = source.flutterDir;
-    await for (final io.FileSystemEntity entity in dir.list(recursive: true)) {
-      if (entity is io.File && entity.path.endsWith('.h')) {
-        // Check that the file is included.
-        bool included = include.isEmpty;
-        for (final String includePath in include) {
-          final String relativePath = p.relative(includePath, from: source.flutterDir.path);
-          if (p.isWithin(relativePath, entity.path) || p.equals(relativePath, entity.path)) {
-            included = true;
-            break;
-          }
-        }
-        if (!included) {
-          continue;
-        }
+    if (badFiles.isNotEmpty) {
+      io.stdout.writeln('The following ${badFiles.length} files have invalid header guards:');
+      for (final HeaderFile headerFile in badFiles) {
+        io.stdout.writeln('  ${headerFile.path}');
+      }
 
-        // Check that the file is not excluded.
-        bool excluded = false;
-        for (final String excludePath in exclude) {
-          final String relativePath = p.relative(excludePath, from: source.flutterDir.path);
-          if (p.isWithin(relativePath, entity.path) || p.equals(relativePath, entity.path)) {
-            excluded = true;
-            break;
-          }
-        }
-        if (!excluded) {
-          files.add(entity);
+      // If we're fixing, fix the files.
+      if (fix) {
+        for (final HeaderFile headerFile in badFiles) {
+          headerFile.fix(engineRoot: source.flutterDir.path);
         }
+
+        io.stdout.writeln('Fixed ${badFiles.length} files.');
+        return 0;
+      }
+
+      return 1;
+    }
+
+    return 0;
+  }
+  
+  Iterable<io.File> _findIncludedHeaderFiles() sync* {
+    final io.Directory dir = source.flutterDir;
+    for (final io.FileSystemEntity entity in dir.listSync(recursive: true)) {
+      if (entity is! io.File) {
+        continue;
+      }
+
+      if (!entity.path.endsWith('.h')) {
+        continue;
+      }
+
+      if (!_isIncluded(entity.path) || _isExcluded(entity.path)) {
+        continue;
+      }
+
+      yield entity;
+    }
+  }
+
+  bool _isIncluded(String path) {
+    for (final String includePath in include) {
+      final String relativePath = p.relative(includePath, from: source.flutterDir.path);
+      if (p.isWithin(relativePath, path) || p.equals(relativePath, path)) {
+        return true;
       }
     }
+    return include.isEmpty;
+  }
 
-    // Check each file.
-    final List<HeaderFile> badFiles = <HeaderFile>[];
-    for (final io.File file in files) {
-      final HeaderFile headerFile = HeaderFile.parse(file.path);
+  bool _isExcluded(String path) {
+    for (final String excludePath in exclude) {
+      final String relativePath = p.relative(excludePath, from: source.flutterDir.path);
+      if (p.isWithin(relativePath, path) || p.equals(relativePath, path)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  Iterable<HeaderFile> _checkFiles(Iterable<io.File> headers) sync* {
+    for (final io.File header in headers) {
+      final HeaderFile headerFile = HeaderFile.parse(header.path);
       if (headerFile.pragmaOnce != null) {
         io.stderr.writeln(headerFile.pragmaOnce!.message('Unexpected #pragma once'));
-        badFiles.add(headerFile);
-        continue;
+        yield headerFile;
       }
+
       if (headerFile.guard == null) {
         io.stderr.writeln('Missing header guard in ${headerFile.path}');
-        badFiles.add(headerFile);
-        continue;
+        yield headerFile;
       }
 
-      final String expectedGuard = headerFile.expectedName(engineRoot: source.flutterDir.path);
+      final String expectedGuard = headerFile.computeExpectedName(engineRoot: source.flutterDir.path);
       if (headerFile.guard!.ifndefValue != expectedGuard) {
         io.stderr.writeln(headerFile.guard!.ifndefSpan!.message('Expected #ifndef $expectedGuard'));
-        badFiles.add(headerFile);
-        continue;
+        yield headerFile;
       }
       if (headerFile.guard!.defineValue != expectedGuard) {
         io.stderr.writeln(headerFile.guard!.defineSpan!.message('Expected #define $expectedGuard'));
-        badFiles.add(headerFile);
-        continue;
+        yield headerFile;
       }
       if (headerFile.guard!.endifValue != expectedGuard) {
         io.stderr.writeln(headerFile.guard!.endifSpan!.message('Expected #endif // $expectedGuard'));
-        badFiles.add(headerFile);
-        continue;
-      }
-    }
-
-    if (badFiles.isNotEmpty) {
-      io.stdout.writeln('The following ${badFiles.length} files have invalid header guards:');
-      for (final HeaderFile headerFile in badFiles) {
-        io.stdout.writeln('  ${headerFile.path}');
+        yield headerFile;
       }
-
-      // If we're fixing, fix the files.
-      if (fix) {
-        for (final HeaderFile headerFile in badFiles) {
-          headerFile.fix(engineRoot: source.flutterDir.path);
-        }
-
-        io.stdout.writeln('Fixed ${badFiles.length} files.');
-        return 0;
-      }
-
-      return 1;
     }
-
-    return 0;
   }
 }
 
diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index 82cb6bb1abca5..16cd45302ded4 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -127,15 +127,17 @@ final class HeaderFile {
   /// Returns the expected header guard for this file, relative to [engineRoot].
   ///
   /// For example, if the file is `foo/bar/baz.h`, this will return `FLUTTER_FOO_BAR_BAZ_H_`.
-  String expectedName({required String engineRoot}) {
+  String computeExpectedName({required String engineRoot}) {
     final String relativePath = p.relative(path, from: engineRoot);
     final String underscoredRelativePath = p.withoutExtension(relativePath).replaceAll(_nonAlphaNumeric, '_');
     return 'FLUTTER_${underscoredRelativePath.toUpperCase()}_H_';
   }
 
   /// Updates the file at [path] to have the expected header guard.
+  /// 
+  /// Returns `true` if the file was modified, `false` otherwise.
   bool fix({required String engineRoot}) {
-    final String expectedGuard = expectedName(engineRoot: engineRoot);
+    final String expectedGuard = computeExpectedName(engineRoot: engineRoot);
 
     // Check if the file already has a valid header guard.
     if (guard != null) {
diff --git a/tools/header_guard_check/test/header_file_test.dart b/tools/header_guard_check/test/header_file_test.dart
index 11bb11ede0c70..4342ba3ed40b5 100644
--- a/tools/header_guard_check/test/header_file_test.dart
+++ b/tools/header_guard_check/test/header_file_test.dart
@@ -157,7 +157,7 @@ Future<int> main(List<String> args) async {
           guard: null,
           pragmaOnce: null,
         );
-        expect(headerFile.expectedName(engineRoot: ''), endsWith('FOO_BAR_BAZ_H_'));
+        expect(headerFile.computeExpectedName(engineRoot: ''), endsWith('FOO_BAR_BAZ_H_'));
       }
     });
 

From 1d1627a3bbf29a7a598929ff9ec4db993e9b1669 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Fri, 26 Jan 2024 11:38:12 -0800
Subject: [PATCH 24/26] ++

---
 tools/header_guard_check/lib/header_guard_check.dart | 2 +-
 tools/header_guard_check/lib/src/header_file.dart    | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/header_guard_check/lib/header_guard_check.dart b/tools/header_guard_check/lib/header_guard_check.dart
index 396d586890537..f7880c7fe562d 100644
--- a/tools/header_guard_check/lib/header_guard_check.dart
+++ b/tools/header_guard_check/lib/header_guard_check.dart
@@ -70,7 +70,7 @@ final class HeaderGuardCheck {
 
     return 0;
   }
-  
+
   Iterable<io.File> _findIncludedHeaderFiles() sync* {
     final io.Directory dir = source.flutterDir;
     for (final io.FileSystemEntity entity in dir.listSync(recursive: true)) {
diff --git a/tools/header_guard_check/lib/src/header_file.dart b/tools/header_guard_check/lib/src/header_file.dart
index 16cd45302ded4..9ab0b6318b015 100644
--- a/tools/header_guard_check/lib/src/header_file.dart
+++ b/tools/header_guard_check/lib/src/header_file.dart
@@ -134,7 +134,7 @@ final class HeaderFile {
   }
 
   /// Updates the file at [path] to have the expected header guard.
-  /// 
+  ///
   /// Returns `true` if the file was modified, `false` otherwise.
   bool fix({required String engineRoot}) {
     final String expectedGuard = computeExpectedName(engineRoot: engineRoot);

From f91b3fda1ff43958f18f45f718132c7f06e7846f Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Fri, 26 Jan 2024 12:01:03 -0800
Subject: [PATCH 25/26] ++

---
 testing/run_tests.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/testing/run_tests.py b/testing/run_tests.py
index b829dfd0dc447..bc5fa72269889 100755
--- a/testing/run_tests.py
+++ b/testing/run_tests.py
@@ -1001,10 +1001,10 @@ def build_dart_host_test_list(build_dir):
           ],
       ),
       (os.path.join('flutter', 'tools', 'githooks'), []),
+      (os.path.join('flutter', 'tools', 'header_guard_check'), []),
       (os.path.join('flutter', 'tools', 'pkg', 'engine_build_configs'), []),
       (os.path.join('flutter', 'tools', 'pkg', 'engine_repo_tools'), []),
       (os.path.join('flutter', 'tools', 'pkg', 'git_repo_tools'), []),
-      (os.path.join('flutter', 'tools', 'pkg', 'header_guard_check'), []),
   ]
   if not is_asan(build_dir):
     dart_host_tests += [

From 7107ef1beff4299237b9a65b4afce6cc3069f6a7 Mon Sep 17 00:00:00 2001
From: Matan Lurey <matan@lurey.org>
Date: Fri, 26 Jan 2024 12:43:49 -0800
Subject: [PATCH 26/26] ++

---
 tools/header_guard_check/pubspec.yaml | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/header_guard_check/pubspec.yaml b/tools/header_guard_check/pubspec.yaml
index 52e00456a9548..d775e403853e2 100644
--- a/tools/header_guard_check/pubspec.yaml
+++ b/tools/header_guard_check/pubspec.yaml
@@ -34,6 +34,8 @@ dev_dependencies:
 dependency_overrides:
   args:
     path: ../../../third_party/dart/third_party/pkg/args
+  async:
+    path: ../../../third_party/dart/third_party/pkg/async
   async_helper:
     path: ../../../third_party/dart/pkg/async_helper
   collection: