utils: ipc: Update mojo

Update mojo from commit

9be4263648d7d1a04bb78be75df53f56449a5e3a "Updating trunk VERSION from 6225.0 to 6226.0"

from the Chromium repository.

The update-mojo.sh script was used for this update.

Bug: https://bugs.libcamera.org/show_bug.cgi?id=206
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Milan Zamazal <mzamazal@redhat.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

35 files changed, 2049 insertions, 686 deletions

diff --git a/utils/ipc/mojo/public/tools/mojom/BUILD.gn b/utils/ipc/mojo/public/tools/mojom/BUILD.gn
new file mode 100644
index 00000000..eafb95a1
--- /dev/null
+++ b/utils/ipc/mojo/public/tools/mojom/BUILD.gn
@@ -0,0 +1,18 @@
+# Copyright 2022 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+group("tests") {
+  data = [
+    "check_stable_mojom_compatibility_unittest.py",
+    "check_stable_mojom_compatibility.py",
+    "const_unittest.py",
+    "enum_unittest.py",
+    "feature_unittest.py",
+    "mojom_parser_test_case.py",
+    "mojom_parser_unittest.py",
+    "mojom_parser.py",
+    "stable_attribute_unittest.py",
+    "version_compatibility_unittest.py",
+  ]
+}
diff --git a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
index 08bd672f..35cd1cfd 100755
--- a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
+++ b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility.py
@@ -1,5 +1,5 @@
-#!/usr/bin/env python
-# Copyright 2020 The Chromium Authors. All rights reserved.
+#!/usr/bin/env python3
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Verifies backward-compatibility of mojom type changes.
@@ -12,20 +12,18 @@ This can be used e.g. by a presubmit check to prevent developers from making
 breaking changes to stable mojoms."""
 
 import argparse
-import errno
 import io
 import json
 import os
 import os.path
-import shutil
-import six
 import sys
-import tempfile
 
 from mojom.generate import module
 from mojom.generate import translate
 from mojom.parse import parser
 
+# pylint: disable=raise-missing-from
+
 
 class ParseError(Exception):
   pass
@@ -41,6 +39,8 @@ def _ValidateDelta(root, delta):
   transitive closure of a mojom's input dependencies all at once.
   """
 
+  translate.is_running_backwards_compatibility_check_hack = True
+
   # First build a map of all files covered by the delta
   affected_files = set()
   old_files = {}
@@ -73,11 +73,35 @@ def _ValidateDelta(root, delta):
     try:
       ast = parser.Parse(contents, mojom)
     except Exception as e:
-      six.reraise(
-          ParseError,
-          'encountered exception {0} while parsing {1}'.format(e, mojom),
-          sys.exc_info()[2])
+      raise ParseError('encountered exception {0} while parsing {1}'.format(
+          e, mojom))
+
+    # Files which are generated at compile time can't be checked by this script
+    # (at the moment) since they may not exist in the output directory.
+    generated_files_to_skip = {
+        ('third_party/blink/public/mojom/runtime_feature_state/'
+         'runtime_feature.mojom'),
+        ('third_party/blink/public/mojom/origin_trial_feature/'
+         'origin_trial_feature.mojom'),
+    }
+
+    ast.import_list.items = [
+        x for x in ast.import_list.items
+        if x.import_filename not in generated_files_to_skip
+    ]
+
     for imp in ast.import_list:
+      if (not file_overrides.get(imp.import_filename)
+          and not os.path.exists(os.path.join(root, imp.import_filename))):
+        # Speculatively construct a path prefix to locate the import_filename
+        mojom_path = os.path.dirname(os.path.normpath(mojom)).split(os.sep)
+        test_prefix = ''
+        for path_component in mojom_path:
+          test_prefix = os.path.join(test_prefix, path_component)
+          test_import_filename = os.path.join(test_prefix, imp.import_filename)
+          if os.path.exists(os.path.join(root, test_import_filename)):
+            imp.import_filename = test_import_filename
+            break
       parseMojom(imp.import_filename, file_overrides, override_modules)
 
     # Now that the transitive set of dependencies has been imported and parsed
@@ -89,10 +113,10 @@ def _ValidateDelta(root, delta):
     modules[mojom] = translate.OrderedModule(ast, mojom, all_modules)
 
   old_modules = {}
-  for mojom in old_files.keys():
+  for mojom in old_files:
     parseMojom(mojom, old_files, old_modules)
   new_modules = {}
-  for mojom in new_files.keys():
+  for mojom in new_files:
     parseMojom(mojom, new_files, new_modules)
 
   # At this point we have a complete set of translated Modules from both the
@@ -132,12 +156,21 @@ def _ValidateDelta(root, delta):
           'can be deleted by a subsequent change.' % qualified_name)
 
     checker = module.BackwardCompatibilityChecker()
-    if not checker.IsBackwardCompatible(new_types[new_name], kind):
-      raise Exception('Stable type %s appears to have changed in a way which '
-                      'breaks backward-compatibility. Please fix!\n\nIf you '
-                      'believe this assessment to be incorrect, please file a '
-                      'Chromium bug against the "Internals>Mojo>Bindings" '
-                      'component.' % qualified_name)
+    try:
+      if not checker.IsBackwardCompatible(new_types[new_name], kind):
+        raise Exception(
+            'Stable type %s appears to have changed in a way which '
+            'breaks backward-compatibility. Please fix!\n\nIf you '
+            'believe this assessment to be incorrect, please file a '
+            'Chromium bug against the "Internals>Mojo>Bindings" '
+            'component.' % qualified_name)
+    except Exception as e:
+      raise Exception(
+          'Stable type %s appears to have changed in a way which '
+          'breaks backward-compatibility: \n\n%s.\nPlease fix!\n\nIf you '
+          'believe this assessment to be incorrect, please file a '
+          'Chromium bug against the "Internals>Mojo>Bindings" '
+          'component.' % (qualified_name, e))
 
 
 def Run(command_line, delta=None):
diff --git a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility_unittest.py b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility_unittest.py
index 9f51ea77..06769c95 100755
--- a/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/check_stable_mojom_compatibility_unittest.py
@@ -1,5 +1,5 @@
-#!/usr/bin/env python
-# Copyright 2020 The Chromium Authors. All rights reserved.
+#!/usr/bin/env python3
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -15,7 +15,7 @@ import check_stable_mojom_compatibility
 from mojom.generate import module
 
 
-class Change(object):
+class Change:
   """Helper to clearly define a mojom file delta to be analyzed."""
 
   def __init__(self, filename, old=None, new=None):
@@ -28,7 +28,7 @@ class Change(object):
 
 class UnchangedFile(Change):
   def __init__(self, filename, contents):
-    super(UnchangedFile, self).__init__(filename, old=contents, new=contents)
+    super().__init__(filename, old=contents, new=contents)
 
 
 class CheckStableMojomCompatibilityTest(unittest.TestCase):
@@ -258,3 +258,82 @@ class CheckStableMojomCompatibilityTest(unittest.TestCase):
                [Stable] struct T { foo.S s; int32 x; };
                """)
     ])
+
+  def testWithPartialImport(self):
+    """The compatibility checking tool correctly parses imports with partial
+    paths."""
+    self.assertBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('foo/bar.mojom',
+               old="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('foo/bar.mojom',
+               old="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertNotBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('bar/bar.mojom',
+               old="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+    self.assertNotBackwardCompatible([
+        UnchangedFile('foo/foo.mojom', 'module foo; [Stable] struct S {};'),
+        Change('bar/bar.mojom',
+               old="""\
+               module bar;
+               import "foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """,
+               new="""\
+               module bar;
+               import "foo/foo.mojom";
+               [Stable] struct T { foo.S s; };
+               """)
+    ])
+
+  def testNewEnumDefault(self):
+    # Should be backwards compatible since it does not affect the wire format.
+    # This specific case also checks that the backwards compatibility checker
+    # does not throw an error due to the older version of the enum not
+    # specifying [Default].
+    self.assertBackwardCompatible([
+        Change('foo/foo.mojom',
+               old='[Extensible] enum E { One };',
+               new='[Extensible] enum E { [Default] One };')
+    ])
+    self.assertBackwardCompatible([
+        Change('foo/foo.mojom',
+               old='[Extensible] enum E { [Default] One, Two, };',
+               new='[Extensible] enum E { One, [Default] Two, };')
+    ])
diff --git a/utils/ipc/mojo/public/tools/mojom/const_unittest.py b/utils/ipc/mojo/public/tools/mojom/const_unittest.py
index cb42dfac..e8ed36a7 100644
--- a/utils/ipc/mojo/public/tools/mojom/const_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/const_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/enum_unittest.py b/utils/ipc/mojo/public/tools/mojom/enum_unittest.py
index d9005078..9269cde5 100644
--- a/utils/ipc/mojo/public/tools/mojom/enum_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/enum_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -90,3 +90,31 @@ class EnumTest(MojomParserTestCase):
     self.assertEqual('F', b.enums[0].mojom_name)
     self.assertEqual('kFoo', b.enums[0].fields[0].mojom_name)
     self.assertEqual(37, b.enums[0].fields[0].numeric_value)
+
+  def testEnumAttributesAreEnums(self):
+    """Verifies that enum values in attributes are really enum types."""
+    a_mojom = 'a.mojom'
+    self.WriteFile(a_mojom, 'module a; enum E { kFoo, kBar };')
+    b_mojom = 'b.mojom'
+    self.WriteFile(
+        b_mojom, 'module b;'
+        'import "a.mojom";'
+        '[MooCow=a.E.kFoo]'
+        'interface Foo { Foo(); };')
+    self.ParseMojoms([a_mojom, b_mojom])
+    b = self.LoadModule(b_mojom)
+    self.assertEqual(b.interfaces[0].attributes['MooCow'].mojom_name, 'kFoo')
+
+  def testConstantAttributes(self):
+    """Verifies that constants as attributes are translated to the constant."""
+    a_mojom = 'a.mojom'
+    self.WriteFile(
+        a_mojom, 'module a;'
+        'enum E { kFoo, kBar };'
+        'const E kB = E.kFoo;'
+        '[Attr=kB] interface Hello { Foo(); };')
+    self.ParseMojoms([a_mojom])
+    a = self.LoadModule(a_mojom)
+    self.assertEqual(a.interfaces[0].attributes['Attr'].mojom_name, 'kB')
+    self.assertEquals(a.interfaces[0].attributes['Attr'].value.mojom_name,
+                      'kFoo')
diff --git a/utils/ipc/mojo/public/tools/mojom/feature_unittest.py b/utils/ipc/mojo/public/tools/mojom/feature_unittest.py
new file mode 100644
index 00000000..5f014e1c
--- /dev/null
+++ b/utils/ipc/mojo/public/tools/mojom/feature_unittest.py
@@ -0,0 +1,84 @@
+# Copyright 2023 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from mojom_parser_test_case import MojomParserTestCase
+
+
+class FeatureTest(MojomParserTestCase):
+  """Tests feature parsing behavior."""
+  def testFeatureOff(self):
+    """Verifies basic parsing of feature types."""
+    types = self.ExtractTypes("""
+      // e.g. BASE_DECLARE_FEATURE(kFeature);
+      [AttributeOne=ValueOne]
+      feature kFeature {
+        // BASE_FEATURE(kFeature,"MyFeature",
+        //     base::FEATURE_DISABLED_BY_DEFAULT);
+        const string name = "MyFeature";
+        const bool default_state = false;
+      };
+    """)
+    self.assertEqual('name', types['kFeature'].constants[0].mojom_name)
+    self.assertEqual('"MyFeature"', types['kFeature'].constants[0].value)
+    self.assertEqual('default_state', types['kFeature'].constants[1].mojom_name)
+    self.assertEqual('false', types['kFeature'].constants[1].value)
+
+  def testFeatureOn(self):
+    """Verifies basic parsing of feature types."""
+    types = self.ExtractTypes("""
+      // e.g. BASE_DECLARE_FEATURE(kFeature);
+      feature kFeature {
+        // BASE_FEATURE(kFeature,"MyFeature",
+        //     base::FEATURE_ENABLED_BY_DEFAULT);
+        const string name = "MyFeature";
+        const bool default_state = true;
+      };
+    """)
+    self.assertEqual('name', types['kFeature'].constants[0].mojom_name)
+    self.assertEqual('"MyFeature"', types['kFeature'].constants[0].value)
+    self.assertEqual('default_state', types['kFeature'].constants[1].mojom_name)
+    self.assertEqual('true', types['kFeature'].constants[1].value)
+
+  def testFeatureWeakKeyword(self):
+    """Verifies that `feature` is a weak keyword."""
+    types = self.ExtractTypes("""
+      // e.g. BASE_DECLARE_FEATURE(kFeature);
+      [AttributeOne=ValueOne]
+      feature kFeature {
+        // BASE_FEATURE(kFeature,"MyFeature",
+        //     base::FEATURE_DISABLED_BY_DEFAULT);
+        const string name = "MyFeature";
+        const bool default_state = false;
+      };
+      struct MyStruct {
+         bool feature = true;
+      };
+      interface InterfaceName {
+         Method(string feature) => (int32 feature);
+      };
+    """)
+    self.assertEqual('name', types['kFeature'].constants[0].mojom_name)
+    self.assertEqual('"MyFeature"', types['kFeature'].constants[0].value)
+    self.assertEqual('default_state', types['kFeature'].constants[1].mojom_name)
+    self.assertEqual('false', types['kFeature'].constants[1].value)
+
+  def testFeatureAttributesAreFeatures(self):
+    """Verifies that feature values in attributes are really feature types."""
+    a_mojom = 'a.mojom'
+    self.WriteFile(
+        a_mojom, 'module a;'
+        'feature F { const string name = "f";'
+        'const bool default_state = false; };')
+    b_mojom = 'b.mojom'
+    self.WriteFile(
+        b_mojom, 'module b;'
+        'import "a.mojom";'
+        'feature G'
+        '{const string name = "g"; const bool default_state = false;};'
+        '[Attri=a.F] interface Foo { Foo(); };'
+        '[Boink=G] interface Bar {};')
+    self.ParseMojoms([a_mojom, b_mojom])
+    b = self.LoadModule(b_mojom)
+    self.assertEqual(b.interfaces[0].attributes['Attri'].mojom_name, 'F')
+    self.assertEqual(b.interfaces[1].attributes['Boink'].mojom_name, 'G')
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn b/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
index 51facc0c..a0edf0eb 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/BUILD.gn
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -8,6 +8,7 @@ group("mojom") {
     "error.py",
     "fileutil.py",
     "generate/__init__.py",
+    "generate/check.py",
     "generate/generator.py",
     "generate/module.py",
     "generate/pack.py",
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/error.py b/utils/ipc/mojo/public/tools/mojom/mojom/error.py
index 8a1e03da..dd53b835 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/error.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/error.py
@@ -1,4 +1,4 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/fileutil.py b/utils/ipc/mojo/public/tools/mojom/mojom/fileutil.py
index bf626f54..124f12c1 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/fileutil.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/fileutil.py
@@ -1,9 +1,8 @@
-# Copyright 2015 The Chromium Authors. All rights reserved.
+# Copyright 2015 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import errno
-import imp
 import os.path
 import sys
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/fileutil_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/fileutil_unittest.py
index ff5753a2..c93d2289 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/fileutil_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/fileutil_unittest.py
@@ -1,20 +1,17 @@
-# Copyright 2015 The Chromium Authors. All rights reserved.
+# Copyright 2015 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
 import os.path
 import shutil
-import sys
 import tempfile
 import unittest
 
 from mojom import fileutil
 
-
 class FileUtilTest(unittest.TestCase):
   def testEnsureDirectoryExists(self):
-    """Test that EnsureDirectoryExists fuctions correctly."""
+    """Test that EnsureDirectoryExists functions correctly."""
 
     temp_dir = tempfile.mkdtemp()
     try:
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/check.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/check.py
new file mode 100644
index 00000000..1efe2022
--- /dev/null
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/check.py
@@ -0,0 +1,26 @@
+# Copyright 2022 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+"""Code shared by the various pre-generation mojom checkers."""
+
+
+class CheckException(Exception):
+  def __init__(self, module, message):
+    self.module = module
+    self.message = message
+    super().__init__(self.message)
+
+  def __str__(self):
+    return "Failed mojo pre-generation check for {}:\n{}".format(
+        self.module.path, self.message)
+
+
+class Check:
+  def __init__(self, module):
+    self.module = module
+
+  def CheckModule(self):
+    """ Subclass should return True if its Checks pass, and throw an
+    exception otherwise. CheckModule will be called immediately before
+    mojom.generate.Generator.GenerateFiles()"""
+    raise NotImplementedError("Subclasses must override/implement this method")
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/constant_resolver.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/constant_resolver.py
deleted file mode 100644
index 0dfd996e..00000000
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/constant_resolver.py
+++ /dev/null
@@ -1,93 +0,0 @@
-# Copyright 2015 The Chromium Authors. All rights reserved.
-# Use of this source code is governed by a BSD-style license that can be
-# found in the LICENSE file.
-"""Resolves the values used for constants and enums."""
-
-from itertools import ifilter
-
-from mojom.generate import module as mojom
-
-
-def ResolveConstants(module, expression_to_text):
-  in_progress = set()
-  computed = set()
-
-  def GetResolvedValue(named_value):
-    assert isinstance(named_value, (mojom.EnumValue, mojom.ConstantValue))
-    if isinstance(named_value, mojom.EnumValue):
-      field = next(
-          ifilter(lambda field: field.name == named_value.name,
-                  named_value.enum.fields), None)
-      if not field:
-        raise RuntimeError(
-            'Unable to get computed value for field %s of enum %s' %
-            (named_value.name, named_value.enum.name))
-      if field not in computed:
-        ResolveEnum(named_value.enum)
-      return field.resolved_value
-    else:
-      ResolveConstant(named_value.constant)
-      named_value.resolved_value = named_value.constant.resolved_value
-      return named_value.resolved_value
-
-  def ResolveConstant(constant):
-    if constant in computed:
-      return
-    if constant in in_progress:
-      raise RuntimeError('Circular dependency for constant: %s' % constant.name)
-    in_progress.add(constant)
-    if isinstance(constant.value, (mojom.EnumValue, mojom.ConstantValue)):
-      resolved_value = GetResolvedValue(constant.value)
-    else:
-      resolved_value = expression_to_text(constant.value)
-    constant.resolved_value = resolved_value
-    in_progress.remove(constant)
-    computed.add(constant)
-
-  def ResolveEnum(enum):
-    def ResolveEnumField(enum, field, default_value):
-      if field in computed:
-        return
-      if field in in_progress:
-        raise RuntimeError('Circular dependency for enum: %s' % enum.name)
-      in_progress.add(field)
-      if field.value:
-        if isinstance(field.value, mojom.EnumValue):
-          resolved_value = GetResolvedValue(field.value)
-        elif isinstance(field.value, str):
-          resolved_value = int(field.value, 0)
-        else:
-          raise RuntimeError('Unexpected value: %s' % field.value)
-      else:
-        resolved_value = default_value
-      field.resolved_value = resolved_value
-      in_progress.remove(field)
-      computed.add(field)
-
-    current_value = 0
-    for field in enum.fields:
-      ResolveEnumField(enum, field, current_value)
-      current_value = field.resolved_value + 1
-
-  for constant in module.constants:
-    ResolveConstant(constant)
-
-  for enum in module.enums:
-    ResolveEnum(enum)
-
-  for struct in module.structs:
-    for constant in struct.constants:
-      ResolveConstant(constant)
-    for enum in struct.enums:
-      ResolveEnum(enum)
-    for field in struct.fields:
-      if isinstance(field.default, (mojom.ConstantValue, mojom.EnumValue)):
-        field.default.resolved_value = GetResolvedValue(field.default)
-
-  for interface in module.interfaces:
-    for constant in interface.constants:
-      ResolveConstant(constant)
-    for enum in interface.enums:
-      ResolveEnum(enum)
-
-  return module
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
index 4a1c73fc..96fe3a2d 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator.py
@@ -1,4 +1,4 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Code shared by the various language-specific code generators."""
@@ -97,7 +97,7 @@ def ToLowerSnakeCase(identifier):
   return _ToSnakeCase(identifier, upper=False)
 
 
-class Stylizer(object):
+class Stylizer:
   """Stylizers specify naming rules to map mojom names to names in generated
   code. For example, if you would like method_name in mojom to be mapped to
   MethodName in the generated code, you need to define a subclass of Stylizer
@@ -130,6 +130,9 @@ class Stylizer(object):
   def StylizeEnum(self, mojom_name):
     return mojom_name
 
+  def StylizeFeature(self, mojom_name):
+    return mojom_name
+
   def StylizeModule(self, mojom_namespace):
     return mojom_namespace
 
@@ -233,7 +236,7 @@ def AddComputedData(module):
     _AddInterfaceComputedData(interface)
 
 
-class Generator(object):
+class Generator:
   # Pass |output_dir| to emit files to disk. Omit |output_dir| to echo all
   # files to stdout.
   def __init__(self,
@@ -243,7 +246,6 @@ class Generator(object):
                variant=None,
                bytecode_path=None,
                for_blink=False,
-               js_bindings_mode="new",
                js_generate_struct_deserializers=False,
                export_attribute=None,
                export_header=None,
@@ -262,7 +264,6 @@ class Generator(object):
     self.variant = variant
     self.bytecode_path = bytecode_path
     self.for_blink = for_blink
-    self.js_bindings_mode = js_bindings_mode
     self.js_generate_struct_deserializers = js_generate_struct_deserializers
     self.export_attribute = export_attribute
     self.export_header = export_header
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator_unittest.py
index 32c884a8..7143e07c 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/generator_unittest.py
@@ -1,13 +1,12 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
+import importlib.util
 import os.path
 import sys
 import unittest
 
-
 def _GetDirAbove(dirname):
   """Returns the directory "above" this file containing |dirname| (which must
   also be "above" this file)."""
@@ -20,12 +19,11 @@ def _GetDirAbove(dirname):
 
 
 try:
-  imp.find_module("mojom")
+  importlib.util.find_spec("mojom")
 except ImportError:
   sys.path.append(os.path.join(_GetDirAbove("pylib"), "pylib"))
 from mojom.generate import generator
 
-
 class StringManipulationTest(unittest.TestCase):
   """generator contains some string utilities, this tests only those."""
 
@@ -69,6 +67,5 @@ class StringManipulationTest(unittest.TestCase):
     self.assertEquals("SNAKE_D3D11_CASE",
                       generator.ToUpperSnakeCase("snakeD3d11Case"))
 
-
 if __name__ == "__main__":
   unittest.main()
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
index 9bdb28e0..ca71059d 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module.py
@@ -1,4 +1,4 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -12,15 +12,14 @@
 # method = interface.AddMethod('Tat', 0)
 # method.AddParameter('baz', 0, mojom.INT32)
 
-import sys
-if sys.version_info.major == 2:
-  import cPickle as pickle
-else:
-  import pickle
+import pickle
+from collections import OrderedDict
 from uuid import UUID
 
+# pylint: disable=raise-missing-from
 
-class BackwardCompatibilityChecker(object):
+
+class BackwardCompatibilityChecker:
   """Used for memoization while recursively checking two type definitions for
   backward-compatibility."""
 
@@ -64,23 +63,20 @@ def Repr(obj, as_ref=True):
     return obj.Repr(as_ref=as_ref)
   # Since we cannot implement Repr for existing container types, we
   # handle them here.
-  elif isinstance(obj, list):
+  if isinstance(obj, list):
     if not obj:
       return '[]'
-    else:
-      return ('[\n%s\n]' % (',\n'.join(
-          '    %s' % Repr(elem, as_ref).replace('\n', '\n    ')
-          for elem in obj)))
-  elif isinstance(obj, dict):
+    return ('[\n%s\n]' %
+            (',\n'.join('    %s' % Repr(elem, as_ref).replace('\n', '\n    ')
+                        for elem in obj)))
+  if isinstance(obj, dict):
     if not obj:
       return '{}'
-    else:
-      return ('{\n%s\n}' % (',\n'.join(
-          '    %s: %s' % (Repr(key, as_ref).replace('\n', '\n    '),
-                          Repr(val, as_ref).replace('\n', '\n    '))
-          for key, val in obj.items())))
-  else:
-    return repr(obj)
+    return ('{\n%s\n}' % (',\n'.join('    %s: %s' %
+                                     (Repr(key, as_ref).replace('\n', '\n    '),
+                                      Repr(val, as_ref).replace('\n', '\n    '))
+                                     for key, val in obj.items())))
+  return repr(obj)
 
 
 def GenericRepr(obj, names):
@@ -104,7 +100,7 @@ def GenericRepr(obj, names):
       ReprIndent(name, as_ref) for (name, as_ref) in names.items()))
 
 
-class Kind(object):
+class Kind:
   """Kind represents a type (e.g. int8, string).
 
   Attributes:
@@ -112,16 +108,43 @@ class Kind(object):
     module: {Module} The defining module. Set to None for built-in types.
     parent_kind: The enclosing type. For example, an enum defined
         inside an interface has that interface as its parent. May be None.
+    is_nullable: True if the type is nullable.
   """
 
-  def __init__(self, spec=None, module=None):
+  def __init__(self, spec=None, is_nullable=False, module=None):
     self.spec = spec
     self.module = module
     self.parent_kind = None
+    self.is_nullable = is_nullable
+    self.shared_definition = {}
+
+  @classmethod
+  def AddSharedProperty(cls, name):
+    """Adds a property |name| to |cls|, which accesses the corresponding item in
+       |shared_definition|.
+
+       The reason of adding such indirection is to enable sharing definition
+       between a reference kind and its nullable variation. For example:
+         a = Struct('test_struct_1')
+         b = a.MakeNullableKind()
+         a.name = 'test_struct_2'
+         print(b.name)  # Outputs 'test_struct_2'.
+    """
+    def Get(self):
+      try:
+        return self.shared_definition[name]
+      except KeyError:  # Must raise AttributeError if property doesn't exist.
+        raise AttributeError
+
+    def Set(self, value):
+      self.shared_definition[name] = value
+
+    setattr(cls, name, property(Get, Set))
 
   def Repr(self, as_ref=True):
     # pylint: disable=unused-argument
-    return '<%s spec=%r>' % (self.__class__.__name__, self.spec)
+    return '<%s spec=%r is_nullable=%r>' % (self.__class__.__name__, self.spec,
+                                            self.is_nullable)
 
   def __repr__(self):
     # Gives us a decent __repr__ for all kinds.
@@ -130,7 +153,8 @@ class Kind(object):
   def __eq__(self, rhs):
     # pylint: disable=unidiomatic-typecheck
     return (type(self) == type(rhs)
-            and (self.spec, self.parent_kind) == (rhs.spec, rhs.parent_kind))
+            and (self.spec, self.parent_kind, self.is_nullable)
+            == (rhs.spec, rhs.parent_kind, rhs.is_nullable))
 
   def __hash__(self):
     # TODO(crbug.com/1060471): Remove this and other __hash__ methods on Kind
@@ -138,32 +162,113 @@ class Kind(object):
     # some primitive Kinds as dict keys. The default hash (object identity)
     # breaks these dicts when a pickled Module instance is unpickled and used
     # during a subsequent run of the parser.
-    return hash((self.spec, self.parent_kind))
+    return hash((self.spec, self.parent_kind, self.is_nullable))
 
   # pylint: disable=unused-argument
   def IsBackwardCompatible(self, rhs, checker):
     return self == rhs
 
 
+class ValueKind(Kind):
+  """ValueKind represents values that aren't reference kinds.
+
+  The primary difference is the wire representation for nullable value kinds
+  still reserves space for the value type itself, even if that value itself
+  is logically null.
+  """
+  def __init__(self, spec=None, is_nullable=False, module=None):
+    assert spec is None or is_nullable == spec.startswith('?')
+    Kind.__init__(self, spec, is_nullable, module)
+
+  def MakeNullableKind(self):
+    assert not self.is_nullable
+
+    if self == BOOL:
+      return NULLABLE_BOOL
+    if self == INT8:
+      return NULLABLE_INT8
+    if self == INT16:
+      return NULLABLE_INT16
+    if self == INT32:
+      return NULLABLE_INT32
+    if self == INT64:
+      return NULLABLE_INT64
+    if self == UINT8:
+      return NULLABLE_UINT8
+    if self == UINT16:
+      return NULLABLE_UINT16
+    if self == UINT32:
+      return NULLABLE_UINT32
+    if self == UINT64:
+      return NULLABLE_UINT64
+    if self == FLOAT:
+      return NULLABLE_FLOAT
+    if self == DOUBLE:
+      return NULLABLE_DOUBLE
+
+    nullable_kind = type(self)()
+    nullable_kind.shared_definition = self.shared_definition
+    if self.spec is not None:
+      nullable_kind.spec = '?' + self.spec
+    nullable_kind.is_nullable = True
+    nullable_kind.parent_kind = self.parent_kind
+    nullable_kind.module = self.module
+
+    return nullable_kind
+
+  def MakeUnnullableKind(self):
+    assert self.is_nullable
+
+    if self == NULLABLE_BOOL:
+      return BOOL
+    if self == NULLABLE_INT8:
+      return INT8
+    if self == NULLABLE_INT16:
+      return INT16
+    if self == NULLABLE_INT32:
+      return INT32
+    if self == NULLABLE_INT64:
+      return INT64
+    if self == NULLABLE_UINT8:
+      return UINT8
+    if self == NULLABLE_UINT16:
+      return UINT16
+    if self == NULLABLE_UINT32:
+      return UINT32
+    if self == NULLABLE_UINT64:
+      return UINT64
+    if self == NULLABLE_FLOAT:
+      return FLOAT
+    if self == NULLABLE_DOUBLE:
+      return DOUBLE
+
+    nullable_kind = type(self)()
+    nullable_kind.shared_definition = self.shared_definition
+    if self.spec is not None:
+      nullable_kind.spec = self.spec[1:]
+    nullable_kind.is_nullable = False
+    nullable_kind.parent_kind = self.parent_kind
+    nullable_kind.module = self.module
+
+    return nullable_kind
+
+  def __eq__(self, rhs):
+    return (isinstance(rhs, ValueKind) and super().__eq__(rhs))
+
+  def __hash__(self):  # pylint: disable=useless-super-delegation
+    return super().__hash__()
+
+
 class ReferenceKind(Kind):
   """ReferenceKind represents pointer and handle types.
 
   A type is nullable if null (for pointer types) or invalid handle (for handle
   types) is a legal value for the type.
-
-  Attributes:
-    is_nullable: True if the type is nullable.
   """
 
   def __init__(self, spec=None, is_nullable=False, module=None):
     assert spec is None or is_nullable == spec.startswith('?')
-    Kind.__init__(self, spec, module)
-    self.is_nullable = is_nullable
-    self.shared_definition = {}
-
-  def Repr(self, as_ref=True):
-    return '<%s spec=%r is_nullable=%r>' % (self.__class__.__name__, self.spec,
-                                            self.is_nullable)
+    Kind.__init__(self, spec, is_nullable, module)
 
   def MakeNullableKind(self):
     assert not self.is_nullable
@@ -193,55 +298,65 @@ class ReferenceKind(Kind):
 
     return nullable_kind
 
-  @classmethod
-  def AddSharedProperty(cls, name):
-    """Adds a property |name| to |cls|, which accesses the corresponding item in
-       |shared_definition|.
-
-       The reason of adding such indirection is to enable sharing definition
-       between a reference kind and its nullable variation. For example:
-         a = Struct('test_struct_1')
-         b = a.MakeNullableKind()
-         a.name = 'test_struct_2'
-         print(b.name)  # Outputs 'test_struct_2'.
-    """
-
-    def Get(self):
-      try:
-        return self.shared_definition[name]
-      except KeyError:  # Must raise AttributeError if property doesn't exist.
-        raise AttributeError
-
-    def Set(self, value):
-      self.shared_definition[name] = value
+  def MakeUnnullableKind(self):
+    assert self.is_nullable
+
+    if self == NULLABLE_STRING:
+      return STRING
+    if self == NULLABLE_HANDLE:
+      return HANDLE
+    if self == NULLABLE_DCPIPE:
+      return DCPIPE
+    if self == NULLABLE_DPPIPE:
+      return DPPIPE
+    if self == NULLABLE_MSGPIPE:
+      return MSGPIPE
+    if self == NULLABLE_SHAREDBUFFER:
+      return SHAREDBUFFER
+    if self == NULLABLE_PLATFORMHANDLE:
+      return PLATFORMHANDLE
+
+    unnullable_kind = type(self)()
+    unnullable_kind.shared_definition = self.shared_definition
+    if self.spec is not None:
+      assert self.spec[0] == '?'
+      unnullable_kind.spec = self.spec[1:]
+    unnullable_kind.is_nullable = False
+    unnullable_kind.parent_kind = self.parent_kind
+    unnullable_kind.module = self.module
 
-    setattr(cls, name, property(Get, Set))
+    return unnullable_kind
 
   def __eq__(self, rhs):
-    return (isinstance(rhs, ReferenceKind)
-            and super(ReferenceKind, self).__eq__(rhs)
-            and self.is_nullable == rhs.is_nullable)
+    return (isinstance(rhs, ReferenceKind) and super().__eq__(rhs))
 
-  def __hash__(self):
-    return hash((super(ReferenceKind, self).__hash__(), self.is_nullable))
-
-  def IsBackwardCompatible(self, rhs, checker):
-    return (super(ReferenceKind, self).IsBackwardCompatible(rhs, checker)
-            and self.is_nullable == rhs.is_nullable)
+  def __hash__(self):  # pylint: disable=useless-super-delegation
+    return super().__hash__()
 
 
 # Initialize the set of primitive types. These can be accessed by clients.
-BOOL = Kind('b')
-INT8 = Kind('i8')
-INT16 = Kind('i16')
-INT32 = Kind('i32')
-INT64 = Kind('i64')
-UINT8 = Kind('u8')
-UINT16 = Kind('u16')
-UINT32 = Kind('u32')
-UINT64 = Kind('u64')
-FLOAT = Kind('f')
-DOUBLE = Kind('d')
+BOOL = ValueKind('b')
+INT8 = ValueKind('i8')
+INT16 = ValueKind('i16')
+INT32 = ValueKind('i32')
+INT64 = ValueKind('i64')
+UINT8 = ValueKind('u8')
+UINT16 = ValueKind('u16')
+UINT32 = ValueKind('u32')
+UINT64 = ValueKind('u64')
+FLOAT = ValueKind('f')
+DOUBLE = ValueKind('d')
+NULLABLE_BOOL = ValueKind('?b', True)
+NULLABLE_INT8 = ValueKind('?i8', True)
+NULLABLE_INT16 = ValueKind('?i16', True)
+NULLABLE_INT32 = ValueKind('?i32', True)
+NULLABLE_INT64 = ValueKind('?i64', True)
+NULLABLE_UINT8 = ValueKind('?u8', True)
+NULLABLE_UINT16 = ValueKind('?u16', True)
+NULLABLE_UINT32 = ValueKind('?u32', True)
+NULLABLE_UINT64 = ValueKind('?u64', True)
+NULLABLE_FLOAT = ValueKind('?f', True)
+NULLABLE_DOUBLE = ValueKind('?d', True)
 STRING = ReferenceKind('s')
 HANDLE = ReferenceKind('h')
 DCPIPE = ReferenceKind('h:d:c')
@@ -270,6 +385,17 @@ PRIMITIVES = (
     UINT64,
     FLOAT,
     DOUBLE,
+    NULLABLE_BOOL,
+    NULLABLE_INT8,
+    NULLABLE_INT16,
+    NULLABLE_INT32,
+    NULLABLE_INT64,
+    NULLABLE_UINT8,
+    NULLABLE_UINT16,
+    NULLABLE_UINT32,
+    NULLABLE_UINT64,
+    NULLABLE_FLOAT,
+    NULLABLE_DOUBLE,
     STRING,
     HANDLE,
     DCPIPE,
@@ -291,12 +417,17 @@ ATTRIBUTE_DEFAULT = 'Default'
 ATTRIBUTE_EXTENSIBLE = 'Extensible'
 ATTRIBUTE_NO_INTERRUPT = 'NoInterrupt'
 ATTRIBUTE_STABLE = 'Stable'
+ATTRIBUTE_SUPPORTS_URGENT = 'SupportsUrgent'
 ATTRIBUTE_SYNC = 'Sync'
 ATTRIBUTE_UNLIMITED_SIZE = 'UnlimitedSize'
 ATTRIBUTE_UUID = 'Uuid'
+ATTRIBUTE_SERVICE_SANDBOX = 'ServiceSandbox'
+ATTRIBUTE_REQUIRE_CONTEXT = 'RequireContext'
+ATTRIBUTE_ALLOWED_CONTEXT = 'AllowedContext'
+ATTRIBUTE_RUNTIME_FEATURE = 'RuntimeFeature'
 
 
-class NamedValue(object):
+class NamedValue:
   def __init__(self, module, parent_kind, mojom_name):
     self.module = module
     self.parent_kind = parent_kind
@@ -316,7 +447,7 @@ class NamedValue(object):
     return hash((self.parent_kind, self.mojom_name))
 
 
-class BuiltinValue(object):
+class BuiltinValue:
   def __init__(self, value):
     self.value = value
 
@@ -350,7 +481,7 @@ class EnumValue(NamedValue):
     return self.field.name
 
 
-class Constant(object):
+class Constant:
   def __init__(self, mojom_name=None, kind=None, value=None, parent_kind=None):
     self.mojom_name = mojom_name
     self.name = None
@@ -368,7 +499,7 @@ class Constant(object):
                                        rhs.parent_kind))
 
 
-class Field(object):
+class Field:
   def __init__(self,
                mojom_name=None,
                kind=None,
@@ -414,7 +545,18 @@ class StructField(Field):
 
 
 class UnionField(Field):
-  pass
+  def __init__(self,
+               mojom_name=None,
+               kind=None,
+               ordinal=None,
+               default=None,
+               attributes=None):
+    Field.__init__(self, mojom_name, kind, ordinal, default, attributes)
+
+  @property
+  def is_default(self):
+    return self.attributes.get(ATTRIBUTE_DEFAULT, False) \
+        if self.attributes else False
 
 
 def _IsFieldBackwardCompatible(new_field, old_field, checker):
@@ -424,6 +566,38 @@ def _IsFieldBackwardCompatible(new_field, old_field, checker):
   return checker.IsBackwardCompatible(new_field.kind, old_field.kind)
 
 
+class Feature(ReferenceKind):
+  """A runtime enabled feature defined from mojom.
+
+  Attributes:
+    mojom_name: {str} The name of the feature type as defined in mojom.
+    name: {str} The stylized name. (Note: not the "name" used by FeatureList.)
+    constants: {List[Constant]} The constants defined in the feature scope.
+    attributes: {dict} Additional information about the feature.
+  """
+
+  Kind.AddSharedProperty('mojom_name')
+  Kind.AddSharedProperty('name')
+  Kind.AddSharedProperty('constants')
+  Kind.AddSharedProperty('attributes')
+
+  def __init__(self, mojom_name=None, module=None, attributes=None):
+    if mojom_name is not None:
+      spec = 'x:' + mojom_name
+    else:
+      spec = None
+    ReferenceKind.__init__(self, spec, False, module)
+    self.mojom_name = mojom_name
+    self.name = None
+    self.constants = []
+    self.attributes = attributes
+
+  def Stylize(self, stylizer):
+    self.name = stylizer.StylizeFeature(self.mojom_name)
+    for constant in self.constants:
+      constant.Stylize(stylizer)
+
+
 class Struct(ReferenceKind):
   """A struct with typed fields.
 
@@ -441,14 +615,14 @@ class Struct(ReferenceKind):
         if it's a native struct.
   """
 
-  ReferenceKind.AddSharedProperty('mojom_name')
-  ReferenceKind.AddSharedProperty('name')
-  ReferenceKind.AddSharedProperty('native_only')
-  ReferenceKind.AddSharedProperty('custom_serializer')
-  ReferenceKind.AddSharedProperty('fields')
-  ReferenceKind.AddSharedProperty('enums')
-  ReferenceKind.AddSharedProperty('constants')
-  ReferenceKind.AddSharedProperty('attributes')
+  Kind.AddSharedProperty('mojom_name')
+  Kind.AddSharedProperty('name')
+  Kind.AddSharedProperty('native_only')
+  Kind.AddSharedProperty('custom_serializer')
+  Kind.AddSharedProperty('fields')
+  Kind.AddSharedProperty('enums')
+  Kind.AddSharedProperty('constants')
+  Kind.AddSharedProperty('attributes')
 
   def __init__(self, mojom_name=None, module=None, attributes=None):
     if mojom_name is not None:
@@ -470,12 +644,11 @@ class Struct(ReferenceKind):
       return '<%s mojom_name=%r module=%s>' % (self.__class__.__name__,
                                                self.mojom_name,
                                                Repr(self.module, as_ref=True))
-    else:
-      return GenericRepr(self, {
-          'mojom_name': False,
-          'fields': False,
-          'module': True
-      })
+    return GenericRepr(self, {
+        'mojom_name': False,
+        'fields': False,
+        'module': True
+    })
 
   def AddField(self,
                mojom_name,
@@ -496,13 +669,13 @@ class Struct(ReferenceKind):
     for constant in self.constants:
       constant.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_struct, checker):
-    """This struct is backward-compatible with older_struct if and only if all
-    of the following conditions hold:
+  def IsBackwardCompatible(self, rhs, checker):
+    """This struct is backward-compatible with rhs (older_struct) if and only if
+    all of the following conditions hold:
       - Any newly added field is tagged with a [MinVersion] attribute specifying
         a version number greater than all previously used [MinVersion]
         attributes within the struct.
-      - All fields present in older_struct remain present in the new struct,
+      - All fields present in rhs remain present in the new struct,
         with the same ordinal position, same optional or non-optional status,
         same (or backward-compatible) type and where applicable, the same
         [MinVersion] attribute value.
@@ -521,7 +694,7 @@ class Struct(ReferenceKind):
       return fields_by_ordinal
 
     new_fields = buildOrdinalFieldMap(self)
-    old_fields = buildOrdinalFieldMap(older_struct)
+    old_fields = buildOrdinalFieldMap(rhs)
     if len(new_fields) < len(old_fields):
       # At least one field was removed, which is not OK.
       return False
@@ -574,11 +747,18 @@ class Struct(ReferenceKind):
       prefix = self.module.GetNamespacePrefix()
     return '%s%s' % (prefix, self.mojom_name)
 
+  def _tuple(self):
+    return (self.mojom_name, self.native_only, self.fields, self.constants,
+            self.attributes)
+
   def __eq__(self, rhs):
-    return (isinstance(rhs, Struct) and
-            (self.mojom_name, self.native_only, self.fields, self.constants,
-             self.attributes) == (rhs.mojom_name, rhs.native_only, rhs.fields,
-                                  rhs.constants, rhs.attributes))
+    return isinstance(rhs, Struct) and self._tuple() == rhs._tuple()
+
+  def __lt__(self, rhs):
+    if not isinstance(self, type(rhs)):
+      return str(type(self)) < str(type(rhs))
+
+    return self._tuple() < rhs._tuple()
 
   def __hash__(self):
     return id(self)
@@ -595,10 +775,11 @@ class Union(ReferenceKind):
         which Java class name to use to represent it in the generated
         bindings.
   """
-  ReferenceKind.AddSharedProperty('mojom_name')
-  ReferenceKind.AddSharedProperty('name')
-  ReferenceKind.AddSharedProperty('fields')
-  ReferenceKind.AddSharedProperty('attributes')
+  Kind.AddSharedProperty('mojom_name')
+  Kind.AddSharedProperty('name')
+  Kind.AddSharedProperty('fields')
+  Kind.AddSharedProperty('attributes')
+  Kind.AddSharedProperty('default_field')
 
   def __init__(self, mojom_name=None, module=None, attributes=None):
     if mojom_name is not None:
@@ -610,14 +791,14 @@ class Union(ReferenceKind):
     self.name = None
     self.fields = []
     self.attributes = attributes
+    self.default_field = None
 
   def Repr(self, as_ref=True):
     if as_ref:
       return '<%s spec=%r is_nullable=%r fields=%s>' % (
           self.__class__.__name__, self.spec, self.is_nullable, Repr(
               self.fields))
-    else:
-      return GenericRepr(self, {'fields': True, 'is_nullable': False})
+    return GenericRepr(self, {'fields': True, 'is_nullable': False})
 
   def AddField(self, mojom_name, kind, ordinal=None, attributes=None):
     field = UnionField(mojom_name, kind, ordinal, None, attributes)
@@ -629,13 +810,13 @@ class Union(ReferenceKind):
     for field in self.fields:
       field.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_union, checker):
-    """This union is backward-compatible with older_union if and only if all
-    of the following conditions hold:
+  def IsBackwardCompatible(self, rhs, checker):
+    """This union is backward-compatible with rhs (older_union) if and only if
+    all of the following conditions hold:
       - Any newly added field is tagged with a [MinVersion] attribute specifying
         a version number greater than all previously used [MinVersion]
         attributes within the union.
-      - All fields present in older_union remain present in the new union,
+      - All fields present in rhs remain present in the new union,
         with the same ordinal value, same optional or non-optional status,
         same (or backward-compatible) type, and where applicable, the same
         [MinVersion] attribute value.
@@ -651,7 +832,7 @@ class Union(ReferenceKind):
       return fields_by_ordinal
 
     new_fields = buildOrdinalFieldMap(self)
-    old_fields = buildOrdinalFieldMap(older_union)
+    old_fields = buildOrdinalFieldMap(rhs)
     if len(new_fields) < len(old_fields):
       # At least one field was removed, which is not OK.
       return False
@@ -678,6 +859,11 @@ class Union(ReferenceKind):
     return True
 
   @property
+  def extensible(self):
+    return self.attributes.get(ATTRIBUTE_EXTENSIBLE, False) \
+        if self.attributes else False
+
+  @property
   def stable(self):
     return self.attributes.get(ATTRIBUTE_STABLE, False) \
         if self.attributes else False
@@ -690,10 +876,17 @@ class Union(ReferenceKind):
       prefix = self.module.GetNamespacePrefix()
     return '%s%s' % (prefix, self.mojom_name)
 
+  def _tuple(self):
+    return (self.mojom_name, self.fields, self.attributes)
+
   def __eq__(self, rhs):
-    return (isinstance(rhs, Union) and
-            (self.mojom_name, self.fields,
-             self.attributes) == (rhs.mojom_name, rhs.fields, rhs.attributes))
+    return isinstance(rhs, Union) and self._tuple() == rhs._tuple()
+
+  def __lt__(self, rhs):
+    if not isinstance(self, type(rhs)):
+      return str(type(self)) < str(type(rhs))
+
+    return self._tuple() < rhs._tuple()
 
   def __hash__(self):
     return id(self)
@@ -707,8 +900,8 @@ class Array(ReferenceKind):
     length: The number of elements. None if unknown.
   """
 
-  ReferenceKind.AddSharedProperty('kind')
-  ReferenceKind.AddSharedProperty('length')
+  Kind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('length')
 
   def __init__(self, kind=None, length=None):
     if kind is not None:
@@ -728,12 +921,11 @@ class Array(ReferenceKind):
       return '<%s spec=%r is_nullable=%r kind=%s length=%r>' % (
           self.__class__.__name__, self.spec, self.is_nullable, Repr(
               self.kind), self.length)
-    else:
-      return GenericRepr(self, {
-          'kind': True,
-          'length': False,
-          'is_nullable': False
-      })
+    return GenericRepr(self, {
+        'kind': True,
+        'length': False,
+        'is_nullable': False
+    })
 
   def __eq__(self, rhs):
     return (isinstance(rhs, Array)
@@ -754,8 +946,8 @@ class Map(ReferenceKind):
     key_kind: {Kind} The type of the keys. May be None.
     value_kind: {Kind} The type of the elements. May be None.
   """
-  ReferenceKind.AddSharedProperty('key_kind')
-  ReferenceKind.AddSharedProperty('value_kind')
+  Kind.AddSharedProperty('key_kind')
+  Kind.AddSharedProperty('value_kind')
 
   def __init__(self, key_kind=None, value_kind=None):
     if (key_kind is not None and value_kind is not None):
@@ -780,8 +972,7 @@ class Map(ReferenceKind):
       return '<%s spec=%r is_nullable=%r key_kind=%s value_kind=%s>' % (
           self.__class__.__name__, self.spec, self.is_nullable,
           Repr(self.key_kind), Repr(self.value_kind))
-    else:
-      return GenericRepr(self, {'key_kind': True, 'value_kind': True})
+    return GenericRepr(self, {'key_kind': True, 'value_kind': True})
 
   def __eq__(self, rhs):
     return (isinstance(rhs, Map) and
@@ -797,7 +988,7 @@ class Map(ReferenceKind):
 
 
 class PendingRemote(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -822,7 +1013,7 @@ class PendingRemote(ReferenceKind):
 
 
 class PendingReceiver(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -847,7 +1038,7 @@ class PendingReceiver(ReferenceKind):
 
 
 class PendingAssociatedRemote(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -873,7 +1064,7 @@ class PendingAssociatedRemote(ReferenceKind):
 
 
 class PendingAssociatedReceiver(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -899,7 +1090,7 @@ class PendingAssociatedReceiver(ReferenceKind):
 
 
 class InterfaceRequest(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -923,7 +1114,7 @@ class InterfaceRequest(ReferenceKind):
 
 
 class AssociatedInterfaceRequest(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -949,7 +1140,7 @@ class AssociatedInterfaceRequest(ReferenceKind):
             self.kind, rhs.kind)
 
 
-class Parameter(object):
+class Parameter:
   def __init__(self,
                mojom_name=None,
                kind=None,
@@ -983,7 +1174,7 @@ class Parameter(object):
                                       rhs.default, rhs.attributes))
 
 
-class Method(object):
+class Method:
   def __init__(self, interface, mojom_name, ordinal=None, attributes=None):
     self.interface = interface
     self.mojom_name = mojom_name
@@ -999,12 +1190,11 @@ class Method(object):
   def Repr(self, as_ref=True):
     if as_ref:
       return '<%s mojom_name=%r>' % (self.__class__.__name__, self.mojom_name)
-    else:
-      return GenericRepr(self, {
-          'mojom_name': False,
-          'parameters': True,
-          'response_parameters': True
-      })
+    return GenericRepr(self, {
+        'mojom_name': False,
+        'parameters': True,
+        'response_parameters': True
+    })
 
   def AddParameter(self,
                    mojom_name,
@@ -1061,21 +1251,49 @@ class Method(object):
     return self.attributes.get(ATTRIBUTE_UNLIMITED_SIZE) \
         if self.attributes else False
 
+  @property
+  def allowed_context(self):
+    return self.attributes.get(ATTRIBUTE_ALLOWED_CONTEXT) \
+        if self.attributes else None
+
+  @property
+  def supports_urgent(self):
+    return self.attributes.get(ATTRIBUTE_SUPPORTS_URGENT) \
+        if self.attributes else None
+
+  @property
+  def runtime_feature(self):
+    if not self.attributes:
+      return None
+    runtime_feature = self.attributes.get(ATTRIBUTE_RUNTIME_FEATURE, None)
+    if runtime_feature is None:
+      return None
+    if not isinstance(runtime_feature, Feature):
+      raise Exception("RuntimeFeature attribute on %s must be a feature." %
+                      self.name)
+    return runtime_feature
+
+  def _tuple(self):
+    return (self.mojom_name, self.ordinal, self.parameters,
+            self.response_parameters, self.attributes)
+
   def __eq__(self, rhs):
-    return (isinstance(rhs, Method) and
-            (self.mojom_name, self.ordinal, self.parameters,
-             self.response_parameters,
-             self.attributes) == (rhs.mojom_name, rhs.ordinal, rhs.parameters,
-                                  rhs.response_parameters, rhs.attributes))
+    return isinstance(rhs, Method) and self._tuple() == rhs._tuple()
+
+  def __lt__(self, rhs):
+    if not isinstance(self, type(rhs)):
+      return str(type(self)) < str(type(rhs))
+
+    return self._tuple() < rhs._tuple()
 
 
 class Interface(ReferenceKind):
-  ReferenceKind.AddSharedProperty('mojom_name')
-  ReferenceKind.AddSharedProperty('name')
-  ReferenceKind.AddSharedProperty('methods')
-  ReferenceKind.AddSharedProperty('enums')
-  ReferenceKind.AddSharedProperty('constants')
-  ReferenceKind.AddSharedProperty('attributes')
+  Kind.AddSharedProperty('mojom_name')
+  Kind.AddSharedProperty('name')
+  Kind.AddSharedProperty('methods')
+  Kind.AddSharedProperty('enums')
+  Kind.AddSharedProperty('constants')
+  Kind.AddSharedProperty('attributes')
 
   def __init__(self, mojom_name=None, module=None, attributes=None):
     if mojom_name is not None:
@@ -1093,12 +1311,11 @@ class Interface(ReferenceKind):
   def Repr(self, as_ref=True):
     if as_ref:
       return '<%s mojom_name=%r>' % (self.__class__.__name__, self.mojom_name)
-    else:
-      return GenericRepr(self, {
-          'mojom_name': False,
-          'attributes': False,
-          'methods': False
-      })
+    return GenericRepr(self, {
+        'mojom_name': False,
+        'attributes': False,
+        'methods': False
+    })
 
   def AddMethod(self, mojom_name, ordinal=None, attributes=None):
     method = Method(self, mojom_name, ordinal, attributes)
@@ -1114,10 +1331,10 @@ class Interface(ReferenceKind):
     for constant in self.constants:
       constant.Stylize(stylizer)
 
-  def IsBackwardCompatible(self, older_interface, checker):
-    """This interface is backward-compatible with older_interface if and only
-    if all of the following conditions hold:
-      - All defined methods in older_interface (when identified by ordinal) have
+  def IsBackwardCompatible(self, rhs, checker):
+    """This interface is backward-compatible with rhs (older_interface) if and
+    only if all of the following conditions hold:
+      - All defined methods in rhs (when identified by ordinal) have
         backward-compatible definitions in this interface. For each method this
         means:
           - The parameter list is backward-compatible, according to backward-
@@ -1131,7 +1348,7 @@ class Interface(ReferenceKind):
             rules for structs.
       - All newly introduced methods in this interface have a [MinVersion]
         attribute specifying a version greater than any method in
-        older_interface.
+        rhs.
     """
 
     def buildOrdinalMethodMap(interface):
@@ -1144,7 +1361,7 @@ class Interface(ReferenceKind):
       return methods_by_ordinal
 
     new_methods = buildOrdinalMethodMap(self)
-    old_methods = buildOrdinalMethodMap(older_interface)
+    old_methods = buildOrdinalMethodMap(rhs)
     max_old_min_version = 0
     for ordinal, old_method in old_methods.items():
       new_method = new_methods.get(ordinal)
@@ -1187,6 +1404,39 @@ class Interface(ReferenceKind):
     return True
 
   @property
+  def service_sandbox(self):
+    if not self.attributes:
+      return None
+    service_sandbox = self.attributes.get(ATTRIBUTE_SERVICE_SANDBOX, None)
+    if service_sandbox is None:
+      return None
+    # Constants are only allowed to refer to an enum here, so replace.
+    if isinstance(service_sandbox, Constant):
+      service_sandbox = service_sandbox.value
+    if not isinstance(service_sandbox, EnumValue):
+      raise Exception("ServiceSandbox attribute on %s must be an enum value." %
+                      self.module.name)
+    return service_sandbox
+
+  @property
+  def runtime_feature(self):
+    if not self.attributes:
+      return None
+    runtime_feature = self.attributes.get(ATTRIBUTE_RUNTIME_FEATURE, None)
+    if runtime_feature is None:
+      return None
+    if not isinstance(runtime_feature, Feature):
+      raise Exception("RuntimeFeature attribute on %s must be a feature." %
+                      self.name)
+    return runtime_feature
+
+  @property
+  def require_context(self):
+    if not self.attributes:
+      return None
+    return self.attributes.get(ATTRIBUTE_REQUIRE_CONTEXT, None)
+
+  @property
   def stable(self):
     return self.attributes.get(ATTRIBUTE_STABLE, False) \
         if self.attributes else False
@@ -1199,11 +1449,18 @@ class Interface(ReferenceKind):
       prefix = self.module.GetNamespacePrefix()
     return '%s%s' % (prefix, self.mojom_name)
 
+  def _tuple(self):
+    return (self.mojom_name, self.methods, self.enums, self.constants,
+            self.attributes)
+
   def __eq__(self, rhs):
-    return (isinstance(rhs, Interface)
-            and (self.mojom_name, self.methods, self.enums, self.constants,
-                 self.attributes) == (rhs.mojom_name, rhs.methods, rhs.enums,
-                                      rhs.constants, rhs.attributes))
+    return isinstance(rhs, Interface) and self._tuple() == rhs._tuple()
+
+  def __lt__(self, rhs):
+    if not isinstance(self, type(rhs)):
+      return str(type(self)) < str(type(rhs))
+
+    return self._tuple() < rhs._tuple()
 
   @property
   def uuid(self):
@@ -1224,7 +1481,7 @@ class Interface(ReferenceKind):
 
 
 class AssociatedInterface(ReferenceKind):
-  ReferenceKind.AddSharedProperty('kind')
+  Kind.AddSharedProperty('kind')
 
   def __init__(self, kind=None):
     if kind is not None:
@@ -1249,7 +1506,7 @@ class AssociatedInterface(ReferenceKind):
                           self.kind, rhs.kind)
 
 
-class EnumField(object):
+class EnumField:
   def __init__(self,
                mojom_name=None,
                value=None,
@@ -1281,16 +1538,25 @@ class EnumField(object):
                                          rhs.attributes, rhs.numeric_value))
 
 
-class Enum(Kind):
+class Enum(ValueKind):
+  Kind.AddSharedProperty('mojom_name')
+  Kind.AddSharedProperty('name')
+  Kind.AddSharedProperty('native_only')
+  Kind.AddSharedProperty('fields')
+  Kind.AddSharedProperty('attributes')
+  Kind.AddSharedProperty('min_value')
+  Kind.AddSharedProperty('max_value')
+  Kind.AddSharedProperty('default_field')
+
   def __init__(self, mojom_name=None, module=None, attributes=None):
-    self.mojom_name = mojom_name
-    self.name = None
-    self.native_only = False
     if mojom_name is not None:
       spec = 'x:' + mojom_name
     else:
       spec = None
-    Kind.__init__(self, spec, module)
+    ValueKind.__init__(self, spec, False, module)
+    self.mojom_name = mojom_name
+    self.name = None
+    self.native_only = False
     self.fields = []
     self.attributes = attributes
     self.min_value = None
@@ -1300,8 +1566,7 @@ class Enum(Kind):
   def Repr(self, as_ref=True):
     if as_ref:
       return '<%s mojom_name=%r>' % (self.__class__.__name__, self.mojom_name)
-    else:
-      return GenericRepr(self, {'mojom_name': False, 'fields': False})
+    return GenericRepr(self, {'mojom_name': False, 'fields': False})
 
   def Stylize(self, stylizer):
     self.name = stylizer.StylizeEnum(self.mojom_name)
@@ -1327,14 +1592,14 @@ class Enum(Kind):
     return '%s%s' % (prefix, self.mojom_name)
 
   # pylint: disable=unused-argument
-  def IsBackwardCompatible(self, older_enum, checker):
-    """This enum is backward-compatible with older_enum if and only if one of
-    the following conditions holds:
+  def IsBackwardCompatible(self, rhs, checker):
+    """This enum is backward-compatible with rhs (older_enum) if and only if one
+    of the following conditions holds:
         - Neither enum is [Extensible] and both have the exact same set of valid
           numeric values. Field names and aliases for the same numeric value do
           not affect compatibility.
-        - older_enum is [Extensible], and for every version defined by
-          older_enum, this enum has the exact same set of valid numeric values.
+        - rhs is [Extensible], and for every version defined by
+          rhs, this enum has the exact same set of valid numeric values.
     """
 
     def buildVersionFieldMap(enum):
@@ -1345,32 +1610,49 @@ class Enum(Kind):
         fields_by_min_version[field.min_version].add(field.numeric_value)
       return fields_by_min_version
 
-    old_fields = buildVersionFieldMap(older_enum)
+    old_fields = buildVersionFieldMap(rhs)
     new_fields = buildVersionFieldMap(self)
 
-    if new_fields.keys() != old_fields.keys() and not older_enum.extensible:
-      return False
+    if new_fields.keys() != old_fields.keys() and not rhs.extensible:
+      raise Exception("Non-extensible enum cannot be modified")
 
     for min_version, valid_values in old_fields.items():
-      if (min_version not in new_fields
-          or new_fields[min_version] != valid_values):
-        return False
+      if min_version not in new_fields:
+        raise Exception('New values added to an extensible enum '
+                        'do not specify MinVersion: %s' % new_fields)
+
+      if (new_fields[min_version] != valid_values):
+        if (len(new_fields[min_version]) < len(valid_values)):
+          raise Exception('Removing values for an existing MinVersion %s '
+                          'is not allowed' % min_version)
 
+        raise Exception(
+            'New values don\'t match old values'
+            'for an existing MinVersion %s,'
+            ' please specify MinVersion equal to "Next version" '
+            'in the enum description'
+            ' for the following values:\n%s' %
+            (min_version, new_fields[min_version].difference(valid_values)))
     return True
 
+  def _tuple(self):
+    return (self.mojom_name, self.native_only, self.fields, self.attributes,
+            self.min_value, self.max_value, self.default_field)
+
   def __eq__(self, rhs):
-    return (isinstance(rhs, Enum) and
-            (self.mojom_name, self.native_only, self.fields, self.attributes,
-             self.min_value, self.max_value,
-             self.default_field) == (rhs.mojom_name, rhs.native_only,
-                                     rhs.fields, rhs.attributes, rhs.min_value,
-                                     rhs.max_value, rhs.default_field))
+    return isinstance(rhs, Enum) and self._tuple() == rhs._tuple()
+
+  def __lt__(self, rhs):
+    if not isinstance(self, type(rhs)):
+      return str(type(self)) < str(type(rhs))
+
+    return self._tuple() < rhs._tuple()
 
   def __hash__(self):
     return id(self)
 
 
-class Module(object):
+class Module:
   def __init__(self, path=None, mojom_namespace=None, attributes=None):
     self.path = path
     self.mojom_namespace = mojom_namespace
@@ -1379,24 +1661,26 @@ class Module(object):
     self.unions = []
     self.interfaces = []
     self.enums = []
+    self.features = []
     self.constants = []
-    self.kinds = {}
+    self.kinds = OrderedDict()
     self.attributes = attributes
     self.imports = []
-    self.imported_kinds = {}
-    self.metadata = {}
+    self.imported_kinds = OrderedDict()
+    self.metadata = OrderedDict()
 
   def __repr__(self):
     # Gives us a decent __repr__ for modules.
     return self.Repr()
 
   def __eq__(self, rhs):
-    return (isinstance(rhs, Module) and
-            (self.path, self.attributes, self.mojom_namespace, self.imports,
-             self.constants, self.enums, self.structs, self.unions,
-             self.interfaces) == (rhs.path, rhs.attributes, rhs.mojom_namespace,
-                                  rhs.imports, rhs.constants, rhs.enums,
-                                  rhs.structs, rhs.unions, rhs.interfaces))
+    return (isinstance(rhs, Module)
+            and (self.path, self.attributes, self.mojom_namespace, self.imports,
+                 self.constants, self.enums, self.structs, self.unions,
+                 self.interfaces, self.features)
+            == (rhs.path, rhs.attributes, rhs.mojom_namespace, rhs.imports,
+                rhs.constants, rhs.enums, rhs.structs, rhs.unions,
+                rhs.interfaces, rhs.features))
 
   def __hash__(self):
     return id(self)
@@ -1405,16 +1689,16 @@ class Module(object):
     if as_ref:
       return '<%s path=%r mojom_namespace=%r>' % (
           self.__class__.__name__, self.path, self.mojom_namespace)
-    else:
-      return GenericRepr(
-          self, {
-              'path': False,
-              'mojom_namespace': False,
-              'attributes': False,
-              'structs': False,
-              'interfaces': False,
-              'unions': False
-          })
+    return GenericRepr(
+        self, {
+            'path': False,
+            'mojom_namespace': False,
+            'attributes': False,
+            'structs': False,
+            'interfaces': False,
+            'unions': False,
+            'features': False,
+        })
 
   def GetNamespacePrefix(self):
     return '%s.' % self.mojom_namespace if self.mojom_namespace else ''
@@ -1434,6 +1718,11 @@ class Module(object):
     self.unions.append(union)
     return union
 
+  def AddFeature(self, mojom_name, attributes=None):
+    feature = Feature(mojom_name, self, attributes)
+    self.features.append(feature)
+    return feature
+
   def Stylize(self, stylizer):
     self.namespace = stylizer.StylizeModule(self.mojom_namespace)
     for struct in self.structs:
@@ -1446,12 +1735,14 @@ class Module(object):
       enum.Stylize(stylizer)
     for constant in self.constants:
       constant.Stylize(stylizer)
+    for feature in self.features:
+      feature.Stylize(stylizer)
 
     for imported_module in self.imports:
       imported_module.Stylize(stylizer)
 
   def Dump(self, f):
-    pickle.dump(self, f, 2)
+    pickle.dump(self, f)
 
   @classmethod
   def Load(cls, f):
@@ -1461,15 +1752,15 @@ class Module(object):
 
 
 def IsBoolKind(kind):
-  return kind.spec == BOOL.spec
+  return kind.spec == BOOL.spec or kind.spec == NULLABLE_BOOL.spec
 
 
 def IsFloatKind(kind):
-  return kind.spec == FLOAT.spec
+  return kind.spec == FLOAT.spec or kind.spec == NULLABLE_FLOAT.spec
 
 
 def IsDoubleKind(kind):
-  return kind.spec == DOUBLE.spec
+  return kind.spec == DOUBLE.spec or kind.spec == NULLABLE_DOUBLE.spec
 
 
 def IsIntegralKind(kind):
@@ -1477,7 +1768,14 @@ def IsIntegralKind(kind):
           or kind.spec == INT16.spec or kind.spec == INT32.spec
           or kind.spec == INT64.spec or kind.spec == UINT8.spec
           or kind.spec == UINT16.spec or kind.spec == UINT32.spec
-          or kind.spec == UINT64.spec)
+          or kind.spec == UINT64.spec or kind.spec == NULLABLE_BOOL.spec
+          or kind.spec == NULLABLE_INT8.spec or kind.spec == NULLABLE_INT16.spec
+          or kind.spec == NULLABLE_INT32.spec
+          or kind.spec == NULLABLE_INT64.spec
+          or kind.spec == NULLABLE_UINT8.spec
+          or kind.spec == NULLABLE_UINT16.spec
+          or kind.spec == NULLABLE_UINT32.spec
+          or kind.spec == NULLABLE_UINT64.spec)
 
 
 def IsStringKind(kind):
@@ -1522,6 +1820,10 @@ def IsArrayKind(kind):
   return isinstance(kind, Array)
 
 
+def IsFeatureKind(kind):
+  return isinstance(kind, Feature)
+
+
 def IsInterfaceKind(kind):
   return isinstance(kind, Interface)
 
@@ -1558,12 +1860,16 @@ def IsEnumKind(kind):
   return isinstance(kind, Enum)
 
 
+def IsValueKind(kind):
+  return isinstance(kind, ValueKind)
+
+
 def IsReferenceKind(kind):
   return isinstance(kind, ReferenceKind)
 
 
 def IsNullableKind(kind):
-  return IsReferenceKind(kind) and kind.is_nullable
+  return kind.is_nullable
 
 
 def IsMapKind(kind):
@@ -1664,11 +1970,8 @@ def MethodPassesInterfaces(method):
   return _AnyMethodParameterRecursive(method, IsInterfaceKind)
 
 
-def HasSyncMethods(interface):
-  for method in interface.methods:
-    if method.sync:
-      return True
-  return False
+def GetSyncMethodOrdinals(interface):
+  return [method.ordinal for method in interface.methods if method.sync]
 
 
 def HasUninterruptableMethods(interface):
@@ -1700,18 +2003,17 @@ def ContainsHandlesOrInterfaces(kind):
     checked.add(kind.spec)
     if IsStructKind(kind):
       return any(Check(field.kind) for field in kind.fields)
-    elif IsUnionKind(kind):
+    if IsUnionKind(kind):
       return any(Check(field.kind) for field in kind.fields)
-    elif IsAnyHandleKind(kind):
+    if IsAnyHandleKind(kind):
       return True
-    elif IsAnyInterfaceKind(kind):
+    if IsAnyInterfaceKind(kind):
       return True
-    elif IsArrayKind(kind):
+    if IsArrayKind(kind):
       return Check(kind.kind)
-    elif IsMapKind(kind):
+    if IsMapKind(kind):
       return Check(kind.key_kind) or Check(kind.value_kind)
-    else:
-      return False
+    return False
 
   return Check(kind)
 
@@ -1738,21 +2040,20 @@ def ContainsNativeTypes(kind):
     checked.add(kind.spec)
     if IsEnumKind(kind):
       return kind.native_only
-    elif IsStructKind(kind):
+    if IsStructKind(kind):
       if kind.native_only:
         return True
       if any(enum.native_only for enum in kind.enums):
         return True
       return any(Check(field.kind) for field in kind.fields)
-    elif IsUnionKind(kind):
+    if IsUnionKind(kind):
       return any(Check(field.kind) for field in kind.fields)
-    elif IsInterfaceKind(kind):
+    if IsInterfaceKind(kind):
       return any(enum.native_only for enum in kind.enums)
-    elif IsArrayKind(kind):
+    if IsArrayKind(kind):
       return Check(kind.kind)
-    elif IsMapKind(kind):
+    if IsMapKind(kind):
       return Check(kind.key_kind) or Check(kind.value_kind)
-    else:
-      return False
+    return False
 
   return Check(kind)
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module_unittest.py
index e8fd4936..2a4e852c 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/module_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/module_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack.py
index 88b77c98..61240426 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack.py
@@ -1,7 +1,8 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import copy
 from mojom.generate import module as mojom
 
 # This module provides a mechanism for determining the packed order and offsets
@@ -15,7 +16,7 @@ from mojom.generate import module as mojom
 HEADER_SIZE = 8
 
 
-class PackedField(object):
+class PackedField:
   kind_to_size = {
       mojom.BOOL: 1,
       mojom.INT8: 1,
@@ -75,18 +76,55 @@ class PackedField(object):
       return 8
     return cls.GetSizeForKind(kind)
 
-  def __init__(self, field, index, ordinal):
+  def __init__(self,
+               field,
+               index,
+               ordinal,
+               original_field=None,
+               sub_ordinal=None,
+               linked_value_packed_field=None):
     """
     Args:
       field: the original field.
       index: the position of the original field in the struct.
       ordinal: the ordinal of the field for serialization.
+      original_field: See below.
+      sub_ordinal: See below.
+      linked_value_packed_field: See below.
+
+    original_field, sub_ordinal, and linked_value_packed_field are used to
+    support nullable ValueKind fields. For legacy reasons, nullable ValueKind
+    fields actually generate two PackedFields. This allows:
+
+    - backwards compatibility prior to Mojo support for nullable ValueKinds.
+    - correct packing of fields for the aforementioned backwards compatibility.
+
+    When translating Fields to PackedFields, the original field is turned into
+    two PackedFields: the first PackedField always has type mojom.BOOL, while
+    the second PackedField has the non-nullable version of the field's kind.
+
+    When constructing these PackedFields, original_field references the field
+    as defined in the mojom; the name as defined in the mojom will be used for
+    all layers above the wire/data layer.
+
+    sub_ordinal is used to sort the two PackedFields correctly with respect to
+    each other: the first mojom.BOOL field always has sub_ordinal 0, while the
+    second field always has sub_ordinal 1.
+
+    Finally, linked_value_packed_field is used by the serialization and
+    deserialization helpers, which generally just iterate over a PackedStruct's
+    PackedField's in ordinal order. This allows the helpers to easily reference
+    any related PackedFields rather than having to lookup related PackedFields
+    by index while iterating.
     """
     self.field = field
     self.index = index
     self.ordinal = ordinal
-    self.size = self.GetSizeForKind(field.kind)
-    self.alignment = self.GetAlignmentForKind(field.kind)
+    self.original_field = original_field
+    self.sub_ordinal = sub_ordinal
+    self.linked_value_packed_field = linked_value_packed_field
+    self.size = self.GetSizeForKind(self.field.kind)
+    self.alignment = self.GetAlignmentForKind(self.field.kind)
     self.offset = None
     self.bit = None
     self.min_version = None
@@ -120,7 +158,33 @@ def GetPayloadSizeUpToField(field):
   return offset + pad
 
 
-class PackedStruct(object):
+def IsNullableValueKindPackedField(field):
+  """Returns true if `field` is derived from a nullable ValueKind field.
+
+  Nullable ValueKind fields often require special handling in the bindings due
+  to the way the implementation is constrained for wire compatibility.
+  """
+  assert isinstance(field, PackedField)
+  return field.sub_ordinal is not None
+
+
+def IsPrimaryNullableValueKindPackedField(field):
+  """Returns true if `field` is derived from a nullable ValueKind mojom field
+  and is the "primary" field.
+
+  The primary field is a bool PackedField that controls if the field should be
+  considered as present or not; it will have a reference to the PackedField that
+  holds the actual value representation if considered present.
+
+  Bindings code that translates between the wire protocol and the higher layers
+  can use this to simplify mapping multiple PackedFields to the single field
+  that is logically exposed to bindings consumers.
+  """
+  assert isinstance(field, PackedField)
+  return field.linked_value_packed_field is not None
+
+
+class PackedStruct:
   def __init__(self, struct):
     self.struct = struct
     # |packed_fields| contains all the fields, in increasing offset order.
@@ -139,9 +203,41 @@ class PackedStruct(object):
     for index, field in enumerate(struct.fields):
       if field.ordinal is not None:
         ordinal = field.ordinal
-      src_fields.append(PackedField(field, index, ordinal))
+      # Nullable value types are a bit weird: they generate two PackedFields
+      # despite being a single ValueKind. This is for wire compatibility to
+      # ease the transition from legacy mojom syntax where nullable value types
+      # were not supported.
+      if isinstance(field.kind, mojom.ValueKind) and field.kind.is_nullable:
+        # The suffixes intentionally use Unicode codepoints which are considered
+        # valid C++/Java/JavaScript identifiers, yet are unlikely to be used in
+        # actual user code.
+        has_value_field = copy.copy(field)
+        has_value_field.name = f'{field.mojom_name}_$flag'
+        has_value_field.kind = mojom.BOOL
+
+        value_field = copy.copy(field)
+        value_field.name = f'{field.mojom_name}_$value'
+        value_field.kind = field.kind.MakeUnnullableKind()
+
+        value_packed_field = PackedField(value_field,
+                                         index,
+                                         ordinal,
+                                         original_field=field,
+                                         sub_ordinal=1,
+                                         linked_value_packed_field=None)
+        has_value_packed_field = PackedField(
+            has_value_field,
+            index,
+            ordinal,
+            original_field=field,
+            sub_ordinal=0,
+            linked_value_packed_field=value_packed_field)
+        src_fields.append(has_value_packed_field)
+        src_fields.append(value_packed_field)
+      else:
+        src_fields.append(PackedField(field, index, ordinal))
       ordinal += 1
-    src_fields.sort(key=lambda field: field.ordinal)
+    src_fields.sort(key=lambda field: (field.ordinal, field.sub_ordinal))
 
     # Set |min_version| for each field.
     next_min_version = 0
@@ -156,10 +252,11 @@ class PackedStruct(object):
       if (packed_field.min_version != 0
           and mojom.IsReferenceKind(packed_field.field.kind)
           and not packed_field.field.kind.is_nullable):
-        raise Exception("Non-nullable fields are only allowed in version 0 of "
-                        "a struct. %s.%s is defined with [MinVersion=%d]." %
-                        (self.struct.name, packed_field.field.name,
-                         packed_field.min_version))
+        raise Exception(
+            "Non-nullable reference fields are only allowed in version 0 of a "
+            "struct. %s.%s is defined with [MinVersion=%d]." %
+            (self.struct.name, packed_field.field.name,
+             packed_field.min_version))
 
     src_field = src_fields[0]
     src_field.offset = 0
@@ -186,7 +283,7 @@ class PackedStruct(object):
         dst_fields.append(src_field)
 
 
-class ByteInfo(object):
+class ByteInfo:
   def __init__(self):
     self.is_padding = False
     self.packed_fields = []
@@ -214,10 +311,11 @@ def GetByteLayout(packed_struct):
   return byte_info
 
 
-class VersionInfo(object):
-  def __init__(self, version, num_fields, num_bytes):
+class VersionInfo:
+  def __init__(self, version, num_fields, num_packed_fields, num_bytes):
     self.version = version
     self.num_fields = num_fields
+    self.num_packed_fields = num_packed_fields
     self.num_bytes = num_bytes
 
 
@@ -235,24 +333,35 @@ def GetVersionInfo(packed_struct):
   versions = []
   last_version = 0
   last_num_fields = 0
+  last_num_packed_fields = 0
   last_payload_size = 0
 
   for packed_field in packed_struct.packed_fields_in_ordinal_order:
     if packed_field.min_version != last_version:
       versions.append(
-          VersionInfo(last_version, last_num_fields,
+          VersionInfo(last_version, last_num_fields, last_num_packed_fields,
                       last_payload_size + HEADER_SIZE))
       last_version = packed_field.min_version
 
-    last_num_fields += 1
+    # Nullable numeric fields (e.g. `int32?`) expand to two packed fields, so to
+    # avoid double-counting, only increment if the field is:
+    # - not used for representing a nullable value kind field, or
+    # - the primary field representing the nullable value kind field.
+    last_num_fields += 1 if (
+        not IsNullableValueKindPackedField(packed_field)
+        or IsPrimaryNullableValueKindPackedField(packed_field)) else 0
+
+    last_num_packed_fields += 1
+
     # The fields are iterated in ordinal order here. However, the size of a
     # version is determined by the last field of that version in pack order,
     # instead of ordinal order. Therefore, we need to calculate the max value.
-    last_payload_size = max(
-        GetPayloadSizeUpToField(packed_field), last_payload_size)
+    last_payload_size = max(GetPayloadSizeUpToField(packed_field),
+                            last_payload_size)
 
-  assert len(versions) == 0 or last_num_fields != versions[-1].num_fields
+  assert len(
+      versions) == 0 or last_num_packed_fields != versions[-1].num_packed_fields
   versions.append(
-      VersionInfo(last_version, last_num_fields,
+      VersionInfo(last_version, last_num_fields, last_num_packed_fields,
                   last_payload_size + HEADER_SIZE))
   return versions
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack_unittest.py
index 98c705ad..7d8e4e01 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/pack_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -205,6 +205,34 @@ class PackTest(unittest.TestCase):
     self.assertEqual(4, versions[2].num_fields)
     self.assertEqual(32, versions[2].num_bytes)
 
+  def testGetVersionInfoPackedStruct(self):
+    """Tests that pack.GetVersionInfo() correctly sets version, num_fields,
+    and num_packed_fields for a packed struct.
+    """
+    struct = mojom.Struct('test')
+    struct.AddField('field_0', mojom.BOOL, ordinal=0)
+    struct.AddField('field_1',
+                    mojom.NULLABLE_BOOL,
+                    ordinal=1,
+                    attributes={'MinVersion': 1})
+    struct.AddField('field_2',
+                    mojom.NULLABLE_BOOL,
+                    ordinal=2,
+                    attributes={'MinVersion': 2})
+    ps = pack.PackedStruct(struct)
+    versions = pack.GetVersionInfo(ps)
+
+    self.assertEqual(3, len(versions))
+    self.assertEqual(0, versions[0].version)
+    self.assertEqual(1, versions[1].version)
+    self.assertEqual(2, versions[2].version)
+    self.assertEqual(1, versions[0].num_fields)
+    self.assertEqual(2, versions[1].num_fields)
+    self.assertEqual(3, versions[2].num_fields)
+    self.assertEqual(1, versions[0].num_packed_fields)
+    self.assertEqual(3, versions[1].num_packed_fields)
+    self.assertEqual(5, versions[2].num_packed_fields)
+
   def testInterfaceAlignment(self):
     """Tests that interfaces are aligned on 4-byte boundaries, although the size
     of an interface is 8 bytes.
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
index 0da90058..807e2a4f 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/template_expander.py
@@ -1,4 +1,4 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
index 7580b780..83bb297f 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate.py
@@ -1,4 +1,4 @@
-# Copyright 2013 The Chromium Authors. All rights reserved.
+# Copyright 2013 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Convert parse tree to AST.
@@ -12,17 +12,294 @@ already been parsed and converted to ASTs before.
 import itertools
 import os
 import re
-import sys
 
+from collections import OrderedDict
 from mojom.generate import generator
 from mojom.generate import module as mojom
 from mojom.parse import ast
 
 
-def _IsStrOrUnicode(x):
-  if sys.version_info[0] < 3:
-    return isinstance(x, (unicode, str))
-  return isinstance(x, str)
+is_running_backwards_compatibility_check_hack = False
+
+### DO NOT ADD ENTRIES TO THIS LIST. ###
+_EXTENSIBLE_ENUMS_MISSING_DEFAULT = (
+    'x:arc.keymaster.mojom.Algorithm',
+    'x:arc.keymaster.mojom.Digest',
+    'x:arc.keymaster.mojom.SignatureResult',
+    'x:arc.mojom.AccessibilityActionType',
+    'x:arc.mojom.AccessibilityBooleanProperty',
+    'x:arc.mojom.AccessibilityEventIntListProperty',
+    'x:arc.mojom.AccessibilityEventIntProperty',
+    'x:arc.mojom.AccessibilityEventStringProperty',
+    'x:arc.mojom.AccessibilityEventType',
+    'x:arc.mojom.AccessibilityFilterType',
+    'x:arc.mojom.AccessibilityIntListProperty',
+    'x:arc.mojom.AccessibilityIntProperty',
+    'x:arc.mojom.AccessibilityLiveRegionType',
+    'x:arc.mojom.AccessibilityNotificationStateType',
+    'x:arc.mojom.AccessibilityRangeType',
+    'x:arc.mojom.AccessibilitySelectionMode',
+    'x:arc.mojom.AccessibilityStringListProperty',
+    'x:arc.mojom.AccessibilityStringProperty',
+    'x:arc.mojom.AccessibilityWindowBooleanProperty',
+    'x:arc.mojom.AccessibilityWindowIntListProperty',
+    'x:arc.mojom.AccessibilityWindowIntProperty',
+    'x:arc.mojom.AccessibilityWindowStringProperty',
+    'x:arc.mojom.AccessibilityWindowType',
+    'x:arc.mojom.AccountCheckStatus',
+    'x:arc.mojom.AccountUpdateType',
+    'x:arc.mojom.ActionType',
+    'x:arc.mojom.Algorithm',
+    'x:arc.mojom.AndroidIdSource',
+    'x:arc.mojom.AnrSource',
+    'x:arc.mojom.AnrType',
+    'x:arc.mojom.AppDiscoveryRequestState',
+    'x:arc.mojom.AppKillType',
+    'x:arc.mojom.AppPermission',
+    'x:arc.mojom.AppPermissionGroup',
+    'x:arc.mojom.AppReinstallState',
+    'x:arc.mojom.AppShortcutItemType',
+    'x:arc.mojom.ArcAuthCodeStatus',
+    'x:arc.mojom.ArcClipboardDragDropEvent',
+    'x:arc.mojom.ArcCorePriAbiMigEvent',
+    'x:arc.mojom.ArcDnsQuery',
+    'x:arc.mojom.ArcImageCopyPasteCompatAction',
+    'x:arc.mojom.ArcNetworkError',
+    'x:arc.mojom.ArcNetworkEvent',
+    'x:arc.mojom.ArcNotificationEvent',
+    'x:arc.mojom.ArcNotificationExpandState',
+    'x:arc.mojom.ArcNotificationPriority',
+    'x:arc.mojom.ArcNotificationRemoteInputState',
+    'x:arc.mojom.ArcNotificationShownContents',
+    'x:arc.mojom.ArcNotificationStyle',
+    'x:arc.mojom.ArcNotificationType',
+    'x:arc.mojom.ArcPipEvent',
+    'x:arc.mojom.ArcResizeLockState',
+    'x:arc.mojom.ArcSignInSuccess',
+    'x:arc.mojom.ArcTimerResult',
+    'x:arc.mojom.AudioSwitch',
+    'x:arc.mojom.BluetoothAclState',
+    'x:arc.mojom.BluetoothAdapterState',
+    'x:arc.mojom.BluetoothAdvertisingDataType',
+    'x:arc.mojom.BluetoothBondState',
+    'x:arc.mojom.BluetoothDeviceType',
+    'x:arc.mojom.BluetoothDiscoveryState',
+    'x:arc.mojom.BluetoothGattDBAttributeType',
+    'x:arc.mojom.BluetoothGattStatus',
+    'x:arc.mojom.BluetoothPropertyType',
+    'x:arc.mojom.BluetoothScanMode',
+    'x:arc.mojom.BluetoothSdpAttributeType',
+    'x:arc.mojom.BluetoothSocketType',
+    'x:arc.mojom.BluetoothStatus',
+    'x:arc.mojom.BootType',
+    'x:arc.mojom.CaptionTextShadowType',
+    'x:arc.mojom.ChangeType',
+    'x:arc.mojom.ChromeAccountType',
+    'x:arc.mojom.ChromeApp',
+    'x:arc.mojom.ChromePage',
+    'x:arc.mojom.ClockId',
+    'x:arc.mojom.CloudProvisionFlowError',
+    'x:arc.mojom.CommandResultType',
+    'x:arc.mojom.CompanionLibApiId',
+    'x:arc.mojom.ConnectionStateType',
+    'x:arc.mojom.ContentChangeType',
+    'x:arc.mojom.CpuRestrictionState',
+    'x:arc.mojom.CursorCoordinateSpace',
+    'x:arc.mojom.DataRestoreStatus',
+    'x:arc.mojom.DecoderStatus',
+    'x:arc.mojom.DeviceType',
+    'x:arc.mojom.Digest',
+    'x:arc.mojom.DisplayWakeLockType',
+    'x:arc.mojom.EapMethod',
+    'x:arc.mojom.EapPhase2Method',
+    'x:arc.mojom.FileSelectorEventType',
+    'x:arc.mojom.GMSCheckInError',
+    'x:arc.mojom.GMSSignInError',
+    'x:arc.mojom.GeneralSignInError',
+    'x:arc.mojom.GetNetworksRequestType',
+    'x:arc.mojom.HalPixelFormat',
+    'x:arc.mojom.IPAddressType',
+    'x:arc.mojom.InstallErrorReason',
+    'x:arc.mojom.KeyFormat',
+    'x:arc.mojom.KeyManagement',
+    'x:arc.mojom.KeyPurpose',
+    'x:arc.mojom.KeymasterError',
+    'x:arc.mojom.MainAccountHashMigrationStatus',
+    'x:arc.mojom.MainAccountResolutionStatus',
+    'x:arc.mojom.ManagementChangeStatus',
+    'x:arc.mojom.ManagementState',
+    'x:arc.mojom.MessageCenterVisibility',
+    'x:arc.mojom.MetricsType',
+    'x:arc.mojom.MountEvent',
+    'x:arc.mojom.NativeBridgeType',
+    'x:arc.mojom.NetworkResult',
+    'x:arc.mojom.NetworkType',
+    'x:arc.mojom.OemCryptoAlgorithm',
+    'x:arc.mojom.OemCryptoCipherMode',
+    'x:arc.mojom.OemCryptoHdcpCapability',
+    'x:arc.mojom.OemCryptoLicenseType',
+    'x:arc.mojom.OemCryptoPrivateKey',
+    'x:arc.mojom.OemCryptoProvisioningMethod',
+    'x:arc.mojom.OemCryptoResult',
+    'x:arc.mojom.OemCryptoRsaPaddingScheme',
+    'x:arc.mojom.OemCryptoUsageEntryStatus',
+    'x:arc.mojom.Padding',
+    'x:arc.mojom.PaiFlowState',
+    'x:arc.mojom.PatternType',
+    'x:arc.mojom.PressureLevel',
+    'x:arc.mojom.PrintColorMode',
+    'x:arc.mojom.PrintContentType',
+    'x:arc.mojom.PrintDuplexMode',
+    'x:arc.mojom.PrinterStatus',
+    'x:arc.mojom.ProcessState',
+    'x:arc.mojom.PurchaseState',
+    'x:arc.mojom.ReauthReason',
+    'x:arc.mojom.ScaleFactor',
+    'x:arc.mojom.SecurityType',
+    'x:arc.mojom.SegmentStyle',
+    'x:arc.mojom.SelectFilesActionType',
+    'x:arc.mojom.SetNativeChromeVoxResponse',
+    'x:arc.mojom.ShowPackageInfoPage',
+    'x:arc.mojom.SpanType',
+    'x:arc.mojom.SupportedLinkChangeSource',
+    'x:arc.mojom.TetheringClientState',
+    'x:arc.mojom.TextInputType',
+    'x:arc.mojom.TtsEventType',
+    'x:arc.mojom.VideoCodecProfile',
+    'x:arc.mojom.VideoDecodeAccelerator.Result',
+    'x:arc.mojom.VideoEncodeAccelerator.Error',
+    'x:arc.mojom.VideoFrameStorageType',
+    'x:arc.mojom.VideoPixelFormat',
+    'x:arc.mojom.WakefulnessMode',
+    'x:arc.mojom.WebApkInstallResult',
+    'x:ash.ime.mojom.InputFieldType',
+    'x:ash.ime.mojom.PersonalizationMode',
+    'x:ash.language.mojom.FeatureId',
+    'x:blink.mojom.ScrollRestorationType',
+    'x:chromeos.cdm.mojom.CdmKeyStatus',
+    'x:chromeos.cdm.mojom.CdmMessageType',
+    'x:chromeos.cdm.mojom.CdmSessionType',
+    'x:chromeos.cdm.mojom.DecryptStatus',
+    'x:chromeos.cdm.mojom.EmeInitDataType',
+    'x:chromeos.cdm.mojom.EncryptionScheme',
+    'x:chromeos.cdm.mojom.HdcpVersion',
+    'x:chromeos.cdm.mojom.OutputProtection.LinkType',
+    'x:chromeos.cdm.mojom.OutputProtection.ProtectionType',
+    'x:chromeos.cdm.mojom.PromiseException',
+    'x:chromeos.cfm.mojom.EnqueuePriority',
+    'x:chromeos.cfm.mojom.LoggerErrorCode',
+    'x:chromeos.cfm.mojom.LoggerState',
+    'x:chromeos.cros_healthd.mojom.CryptoAlgorithm',
+    'x:chromeos.cros_healthd.mojom.EncryptionState',
+    'x:chromeos.machine_learning.mojom.AnnotationUsecase',
+    'x:chromeos.machine_learning.mojom.BuiltinModelId',
+    'x:chromeos.machine_learning.mojom.CreateGraphExecutorResult',
+    'x:chromeos.machine_learning.mojom.DocumentScannerResultStatus',
+    'x:chromeos.machine_learning.mojom.EndpointReason',
+    'x:chromeos.machine_learning.mojom.EndpointerType',
+    'x:chromeos.machine_learning.mojom.ExecuteResult',
+    'x:chromeos.machine_learning.mojom.GrammarCheckerResult.Status',
+    'x:chromeos.machine_learning.mojom.HandwritingRecognizerResult.Status',
+    'x:chromeos.machine_learning.mojom.LoadHandwritingModelResult',
+    'x:chromeos.machine_learning.mojom.LoadModelResult',
+    'x:chromeos.machine_learning.mojom.Rotation',
+    'x:chromeos.network_config.mojom.ConnectionStateType',
+    'x:chromeos.network_config.mojom.DeviceStateType',
+    'x:chromeos.network_config.mojom.IPConfigType',
+    'x:chromeos.network_config.mojom.NetworkType',
+    'x:chromeos.network_config.mojom.OncSource',
+    'x:chromeos.network_config.mojom.PolicySource',
+    'x:chromeos.network_config.mojom.PortalState',
+    'x:chromeos.wilco_dtc_supportd.mojom.WilcoDtcSupportdEvent',
+    'x:chromeos.wilco_dtc_supportd.mojom.WilcoDtcSupportdWebRequestHttpMethod',
+    'x:chromeos.wilco_dtc_supportd.mojom.WilcoDtcSupportdWebRequestStatus',
+    'x:cros.mojom.CameraClientType',
+    'x:cros.mojom.CameraMetadataSectionStart',
+    'x:cros.mojom.CameraMetadataTag',
+    'x:cros.mojom.HalPixelFormat',
+    'x:crosapi.mojom.AllowedPaths',
+    'x:crosapi.mojom.BrowserAppInstanceType',
+    'x:crosapi.mojom.CreationResult',
+    'x:crosapi.mojom.DeviceAccessResultCode',
+    'x:crosapi.mojom.DeviceMode',
+    'x:crosapi.mojom.DlpRestrictionLevel',
+    'x:crosapi.mojom.ExoImeSupport',
+    'x:crosapi.mojom.FullscreenVisibility',
+    'x:crosapi.mojom.GoogleServiceAuthError.State',
+    'x:crosapi.mojom.IsInstallableResult',
+    'x:crosapi.mojom.KeyTag',
+    'x:crosapi.mojom.KeystoreSigningAlgorithmName',
+    'x:crosapi.mojom.KeystoreType',
+    'x:crosapi.mojom.LacrosFeedbackSource',
+    'x:crosapi.mojom.MemoryPressureLevel',
+    'x:crosapi.mojom.MetricsReportingManaged',
+    'x:crosapi.mojom.NotificationType',
+    'x:crosapi.mojom.OndeviceHandwritingSupport',
+    'x:crosapi.mojom.OpenResult',
+    'x:crosapi.mojom.PolicyDomain',
+    'x:crosapi.mojom.RegistrationCodeType',
+    'x:crosapi.mojom.ScaleFactor',
+    'x:crosapi.mojom.SearchResult.OptionalBool',
+    'x:crosapi.mojom.SelectFileDialogType',
+    'x:crosapi.mojom.SelectFileResult',
+    'x:crosapi.mojom.SharesheetResult',
+    'x:crosapi.mojom.TouchEventType',
+    'x:crosapi.mojom.VideoRotation',
+    'x:crosapi.mojom.WallpaperLayout',
+    'x:crosapi.mojom.WebAppInstallResultCode',
+    'x:crosapi.mojom.WebAppUninstallResultCode',
+    'x:device.mojom.HidBusType',
+    'x:device.mojom.WakeLockReason',
+    'x:device.mojom.WakeLockType',
+    'x:drivefs.mojom.DialogReason.Type',
+    'x:drivefs.mojom.DriveError.Type',
+    'x:drivefs.mojom.DriveFsDelegate.ExtensionConnectionStatus',
+    'x:drivefs.mojom.FileMetadata.CanPinStatus',
+    'x:drivefs.mojom.FileMetadata.Type',
+    'x:drivefs.mojom.ItemEventReason',
+    'x:drivefs.mojom.MirrorPathStatus',
+    'x:drivefs.mojom.MirrorSyncStatus',
+    'x:drivefs.mojom.QueryParameters.SortField',
+    'x:fuzz.mojom.FuzzEnum',
+    'x:media.mojom.FillLightMode',
+    'x:media.mojom.MeteringMode',
+    'x:media.mojom.PowerLineFrequency',
+    'x:media.mojom.RedEyeReduction',
+    'x:media.mojom.ResolutionChangePolicy',
+    'x:media.mojom.VideoCaptureApi',
+    'x:media.mojom.VideoCaptureBufferType',
+    'x:media.mojom.VideoCaptureError',
+    'x:media.mojom.VideoCaptureFrameDropReason',
+    'x:media.mojom.VideoCapturePixelFormat',
+    'x:media.mojom.VideoCaptureTransportType',
+    'x:media.mojom.VideoFacingMode',
+    'x:media_session.mojom.AudioFocusType',
+    'x:media_session.mojom.CameraState',
+    'x:media_session.mojom.EnforcementMode',
+    'x:media_session.mojom.MediaAudioVideoState',
+    'x:media_session.mojom.MediaImageBitmapColorType',
+    'x:media_session.mojom.MediaPictureInPictureState',
+    'x:media_session.mojom.MediaPlaybackState',
+    'x:media_session.mojom.MediaSession.SuspendType',
+    'x:media_session.mojom.MediaSessionAction',
+    'x:media_session.mojom.MediaSessionImageType',
+    'x:media_session.mojom.MediaSessionInfo.SessionState',
+    'x:media_session.mojom.MicrophoneState',
+    'x:ml.model_loader.mojom.ComputeResult',
+    'x:ml.model_loader.mojom.CreateModelLoaderResult',
+    'x:ml.model_loader.mojom.LoadModelResult',
+    'x:mojo.test.AnExtensibleEnum',
+    'x:mojo.test.EnumB',
+    'x:mojo.test.ExtensibleEmptyEnum',
+    'x:mojo.test.enum_default_unittest.mojom.ExtensibleEnumWithoutDefault',
+    'x:network.mojom.WebSandboxFlags',
+    'x:payments.mojom.BillingResponseCode',
+    'x:payments.mojom.CreateDigitalGoodsResponseCode',
+    'x:payments.mojom.ItemType',
+    'x:printing.mojom.PrinterType',
+    'x:ui.mojom.KeyboardCode',
+)
+### DO NOT ADD ENTRIES TO THIS LIST. ###
 
 
 def _DuplicateName(values):
@@ -98,12 +375,6 @@ def _MapKind(kind):
   }
   if kind.endswith('?'):
     base_kind = _MapKind(kind[0:-1])
-    # NOTE: This doesn't rule out enum types. Those will be detected later, when
-    # cross-reference is established.
-    reference_kinds = ('m', 's', 'h', 'a', 'r', 'x', 'asso', 'rmt', 'rcv',
-                       'rma', 'rca')
-    if re.split('[^a-z]', base_kind, 1)[0] not in reference_kinds:
-      raise Exception('A type (spec "%s") cannot be made nullable' % base_kind)
     return '?' + base_kind
   if kind.endswith('}'):
     lbracket = kind.rfind('{')
@@ -113,8 +384,6 @@ def _MapKind(kind):
     lbracket = kind.rfind('[')
     typename = kind[0:lbracket]
     return 'a' + kind[lbracket + 1:-1] + ':' + _MapKind(typename)
-  if kind.endswith('&'):
-    return 'r:' + _MapKind(kind[0:-1])
   if kind.startswith('asso<'):
     assert kind.endswith('>')
     return 'asso:' + _MapKind(kind[5:-1])
@@ -135,13 +404,45 @@ def _MapKind(kind):
   return 'x:' + kind
 
 
-def _AttributeListToDict(attribute_list):
+def _MapAttributeValue(module, kind, value):
+  # True/False/None
+  if value is None:
+    return value
+  if not isinstance(value, str):
+    return value
+  # Is the attribute value the name of a feature?
+  try:
+    # Features cannot be nested in other types, so lookup in the global scope.
+    trial = _LookupKind(module.kinds, 'x:' + value,
+                        _GetScopeForKind(module, kind))
+    if isinstance(trial, mojom.Feature):
+      return trial
+  except ValueError:
+    pass
+  # Is the attribute value a constant or enum value?
+  try:
+    trial = _LookupValue(module, None, None, ('IDENTIFIER', value))
+    if isinstance(trial, mojom.ConstantValue):
+      return trial.constant
+    if isinstance(trial, mojom.EnumValue):
+      return trial
+  except ValueError:
+    pass
+  # If not a referenceable mojo type - return as a string.
+  return value
+
+
+def _AttributeListToDict(module, kind, attribute_list):
   if attribute_list is None:
     return None
   assert isinstance(attribute_list, ast.AttributeList)
-  # TODO(vtl): Check for duplicate keys here.
-  return dict(
-      [(attribute.key, attribute.value) for attribute in attribute_list])
+  attributes = dict()
+  for attribute in attribute_list:
+    if attribute.key in attributes:
+      raise Exception("Duplicate key (%s) in attribute list" % attribute.key)
+    attributes[attribute.key] = _MapAttributeValue(module, kind,
+                                                   attribute.value)
+  return attributes
 
 
 builtin_values = frozenset([
@@ -257,7 +558,8 @@ def _Kind(kinds, spec, scope):
     return kind
 
   if spec.startswith('?'):
-    kind = _Kind(kinds, spec[1:], scope).MakeNullableKind()
+    kind = _Kind(kinds, spec[1:], scope)
+    kind = kind.MakeNullableKind()
   elif spec.startswith('a:'):
     kind = mojom.Array(_Kind(kinds, spec[2:], scope))
   elif spec.startswith('asso:'):
@@ -303,7 +605,8 @@ def _Kind(kinds, spec, scope):
 
 def _Import(module, import_module):
   # Copy the struct kinds from our imports into the current module.
-  importable_kinds = (mojom.Struct, mojom.Union, mojom.Enum, mojom.Interface)
+  importable_kinds = (mojom.Struct, mojom.Union, mojom.Enum, mojom.Interface,
+                      mojom.Feature)
   for kind in import_module.kinds.values():
     if (isinstance(kind, importable_kinds)
         and kind.module.path == import_module.path):
@@ -316,6 +619,32 @@ def _Import(module, import_module):
   return import_module
 
 
+def _Feature(module, parsed_feature):
+  """
+  Args:
+    module: {mojom.Module} Module currently being constructed.
+    parsed_feature: {ast.Feature} Parsed feature.
+
+  Returns:
+    {mojom.Feature} AST feature.
+  """
+  feature = mojom.Feature(module=module)
+  feature.mojom_name = parsed_feature.mojom_name
+  feature.spec = 'x:' + module.GetNamespacePrefix() + feature.mojom_name
+  module.kinds[feature.spec] = feature
+  feature.constants = []
+  _ProcessElements(
+      parsed_feature.mojom_name, parsed_feature.body, {
+          ast.Const:
+          lambda const: feature.constants.append(
+              _Constant(module, const, feature)),
+      })
+
+  feature.attributes = _AttributeListToDict(module, feature,
+                                            parsed_feature.attribute_list)
+  return feature
+
+
 def _Struct(module, parsed_struct):
   """
   Args:
@@ -345,7 +674,8 @@ def _Struct(module, parsed_struct):
             struct.fields_data.append,
         })
 
-  struct.attributes = _AttributeListToDict(parsed_struct.attribute_list)
+  struct.attributes = _AttributeListToDict(module, struct,
+                                           parsed_struct.attribute_list)
 
   # Enforce that a [Native] attribute is set to make native-only struct
   # declarations more explicit.
@@ -377,7 +707,8 @@ def _Union(module, parsed_union):
   union.fields_data = []
   _ProcessElements(parsed_union.mojom_name, parsed_union.body,
                    {ast.UnionField: union.fields_data.append})
-  union.attributes = _AttributeListToDict(parsed_union.attribute_list)
+  union.attributes = _AttributeListToDict(module, union,
+                                          parsed_union.attribute_list)
   return union
 
 
@@ -398,7 +729,8 @@ def _StructField(module, parsed_field, struct):
   field.ordinal = parsed_field.ordinal.value if parsed_field.ordinal else None
   field.default = _LookupValue(module, struct, field.kind,
                                parsed_field.default_value)
-  field.attributes = _AttributeListToDict(parsed_field.attribute_list)
+  field.attributes = _AttributeListToDict(module, field,
+                                          parsed_field.attribute_list)
   return field
 
 
@@ -414,11 +746,22 @@ def _UnionField(module, parsed_field, union):
   """
   field = mojom.UnionField()
   field.mojom_name = parsed_field.mojom_name
+  # Disallow unions from being self-recursive.
+  parsed_typename = parsed_field.typename
+  if parsed_typename.endswith('?'):
+    parsed_typename = parsed_typename[:-1]
+  assert parsed_typename != union.mojom_name
   field.kind = _Kind(module.kinds, _MapKind(parsed_field.typename),
                      (module.mojom_namespace, union.mojom_name))
   field.ordinal = parsed_field.ordinal.value if parsed_field.ordinal else None
   field.default = None
-  field.attributes = _AttributeListToDict(parsed_field.attribute_list)
+  field.attributes = _AttributeListToDict(module, field,
+                                          parsed_field.attribute_list)
+  if field.is_default and not mojom.IsNullableKind(field.kind) and \
+     not mojom.IsIntegralKind(field.kind):
+    raise Exception(
+        '[Default] field for union %s must be nullable or integral type.' %
+        union.mojom_name)
   return field
 
 
@@ -439,7 +782,8 @@ def _Parameter(module, parsed_param, interface):
   parameter.ordinal = (parsed_param.ordinal.value
                        if parsed_param.ordinal else None)
   parameter.default = None  # TODO(tibell): We never have these. Remove field?
-  parameter.attributes = _AttributeListToDict(parsed_param.attribute_list)
+  parameter.attributes = _AttributeListToDict(module, parameter,
+                                              parsed_param.attribute_list)
   return parameter
 
 
@@ -464,7 +808,8 @@ def _Method(module, parsed_method, interface):
     method.response_parameters = list(
         map(lambda parameter: _Parameter(module, parameter, interface),
             parsed_method.response_parameter_list))
-  method.attributes = _AttributeListToDict(parsed_method.attribute_list)
+  method.attributes = _AttributeListToDict(module, method,
+                                           parsed_method.attribute_list)
 
   # Enforce that only methods with response can have a [Sync] attribute.
   if method.sync and method.response_parameters is None:
@@ -492,7 +837,8 @@ def _Interface(module, parsed_iface):
   interface.mojom_name = parsed_iface.mojom_name
   interface.spec = 'x:' + module.GetNamespacePrefix() + interface.mojom_name
   module.kinds[interface.spec] = interface
-  interface.attributes = _AttributeListToDict(parsed_iface.attribute_list)
+  interface.attributes = _AttributeListToDict(module, interface,
+                                              parsed_iface.attribute_list)
   interface.enums = []
   interface.constants = []
   interface.methods_data = []
@@ -522,7 +868,8 @@ def _EnumField(module, enum, parsed_field):
   field = mojom.EnumField()
   field.mojom_name = parsed_field.mojom_name
   field.value = _LookupValue(module, enum, None, parsed_field.value)
-  field.attributes = _AttributeListToDict(parsed_field.attribute_list)
+  field.attributes = _AttributeListToDict(module, field,
+                                          parsed_field.attribute_list)
   value = mojom.EnumValue(module, enum, field)
   module.values[value.GetSpec()] = value
   return field
@@ -544,7 +891,7 @@ def _ResolveNumericEnumValues(enum):
       prev_value += 1
 
     # Integral value (e.g: BEGIN = -0x1).
-    elif _IsStrOrUnicode(field.value):
+    elif isinstance(field.value, str):
       prev_value = int(field.value, 0)
 
     # Reference to a previous enum value (e.g: INIT = BEGIN).
@@ -560,7 +907,10 @@ def _ResolveNumericEnumValues(enum):
     else:
       raise Exception('Unresolved enum value for %s' % field.value.GetSpec())
 
-    #resolved_enum_values[field.mojom_name] = prev_value
+    if prev_value in (-128, -127):
+      raise Exception(f'{field.mojom_name} in {enum.spec} has the value '
+                      f'{prev_value}, which is reserved for WTF::HashTrait\'s '
+                      'default enum specialization and may not be used.')
     field.numeric_value = prev_value
     if min_value is None or prev_value < min_value:
       min_value = prev_value
@@ -588,7 +938,8 @@ def _Enum(module, parsed_enum, parent_kind):
     mojom_name = parent_kind.mojom_name + '.' + mojom_name
   enum.spec = 'x:%s.%s' % (module.mojom_namespace, mojom_name)
   enum.parent_kind = parent_kind
-  enum.attributes = _AttributeListToDict(parsed_enum.attribute_list)
+  enum.attributes = _AttributeListToDict(module, enum,
+                                         parsed_enum.attribute_list)
 
   if not enum.native_only:
     enum.fields = list(
@@ -600,11 +951,18 @@ def _Enum(module, parsed_enum, parent_kind):
     for field in enum.fields:
       if field.default:
         if not enum.extensible:
-          raise Exception('Non-extensible enums may not specify a default')
-        if enum.default_field is not None:
           raise Exception(
-              'Only one enumerator value may be specified as the default')
+              f'Non-extensible enum {enum.spec} may not specify a default')
+        if enum.default_field is not None:
+          raise Exception(f'Multiple [Default] enumerators in enum {enum.spec}')
         enum.default_field = field
+    # While running the backwards compatibility check, ignore errors because the
+    # old version of the enum might not specify [Default].
+    if (enum.extensible and enum.default_field is None
+        and enum.spec not in _EXTENSIBLE_ENUMS_MISSING_DEFAULT
+        and not is_running_backwards_compatibility_check_hack):
+      raise Exception(
+          f'Extensible enum {enum.spec} must specify a [Default] enumerator')
 
   module.kinds[enum.spec] = enum
 
@@ -696,6 +1054,11 @@ def _CollectReferencedKinds(module, all_defined_kinds):
         for referenced_kind in extract_referenced_user_kinds(param.kind):
           sanitized_kind = sanitize_kind(referenced_kind)
           referenced_user_kinds[sanitized_kind.spec] = sanitized_kind
+  # Consts can reference imported enums.
+  for const in module.constants:
+    if not const.kind in mojom.PRIMITIVES:
+      sanitized_kind = sanitize_kind(const.kind)
+      referenced_user_kinds[sanitized_kind.spec] = sanitized_kind
 
   return referenced_user_kinds
 
@@ -741,6 +1104,16 @@ def _AssertTypeIsStable(kind):
           assertDependencyIsStable(response_param.kind)
 
 
+def _AssertStructIsValid(kind):
+  expected_ordinals = set(range(0, len(kind.fields)))
+  ordinals = set(map(lambda field: field.ordinal, kind.fields))
+  if ordinals != expected_ordinals:
+    raise Exception(
+        'Structs must use contiguous ordinals starting from 0. ' +
+        '{} is missing the following ordinals: {}.'.format(
+            kind.mojom_name, ', '.join(map(str, expected_ordinals - ordinals))))
+
+
 def _Module(tree, path, imports):
   """
   Args:
@@ -778,6 +1151,8 @@ def _Module(tree, path, imports):
   module.structs = []
   module.unions = []
   module.interfaces = []
+  module.features = []
+
   _ProcessElements(
       filename, tree.definition_list, {
           ast.Const:
@@ -791,6 +1166,8 @@ def _Module(tree, path, imports):
           ast.Interface:
           lambda interface: module.interfaces.append(
               _Interface(module, interface)),
+          ast.Feature:
+          lambda feature: module.features.append(_Feature(module, feature)),
       })
 
   # Second pass expands fields and methods. This allows fields and parameters
@@ -806,12 +1183,24 @@ def _Module(tree, path, imports):
     for enum in struct.enums:
       all_defined_kinds[enum.spec] = enum
 
+  for feature in module.features:
+    all_defined_kinds[feature.spec] = feature
+
   for union in module.unions:
     union.fields = list(
         map(lambda field: _UnionField(module, field, union), union.fields_data))
     _AssignDefaultOrdinals(union.fields)
+    for field in union.fields:
+      if field.is_default:
+        if union.default_field is not None:
+          raise Exception('Multiple [Default] fields in union %s.' %
+                          union.mojom_name)
+        union.default_field = field
     del union.fields_data
     all_defined_kinds[union.spec] = union
+    if union.extensible and union.default_field is None:
+      raise Exception('Extensible union %s must specify a [Default] field' %
+                      union.mojom_name)
 
   for interface in module.interfaces:
     interface.methods = list(
@@ -829,8 +1218,8 @@ def _Module(tree, path, imports):
                                                  all_defined_kinds.values())
   imported_kind_specs = set(all_referenced_kinds.keys()).difference(
       set(all_defined_kinds.keys()))
-  module.imported_kinds = dict(
-      (spec, all_referenced_kinds[spec]) for spec in imported_kind_specs)
+  module.imported_kinds = OrderedDict((spec, all_referenced_kinds[spec])
+                                      for spec in sorted(imported_kind_specs))
 
   generator.AddComputedData(module)
   for iface in module.interfaces:
@@ -847,6 +1236,9 @@ def _Module(tree, path, imports):
       if kind.stable:
         _AssertTypeIsStable(kind)
 
+  for kind in module.structs:
+    _AssertStructIsValid(kind)
+
   return module
 
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate_unittest.py
index 19905c8a..b4fea924 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/generate/translate_unittest.py
@@ -1,17 +1,13 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
-import os.path
-import sys
 import unittest
 
 from mojom.generate import module as mojom
 from mojom.generate import translate
 from mojom.parse import ast
 
-
 class TranslateTest(unittest.TestCase):
   """Tests |parser.Parse()|."""
 
@@ -69,5 +65,77 @@ class TranslateTest(unittest.TestCase):
     # pylint: disable=W0212
     self.assertEquals(
         translate._MapKind("asso<SomeInterface>?"), "?asso:x:SomeInterface")
-    self.assertEquals(
-        translate._MapKind("asso<SomeInterface&>?"), "?asso:r:x:SomeInterface")
+    self.assertEquals(translate._MapKind("rca<SomeInterface>?"),
+                      "?rca:x:SomeInterface")
+
+  def testSelfRecursiveUnions(self):
+    """Verifies _UnionField() raises when a union is self-recursive."""
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Union("SomeUnion", None,
+                  ast.UnionBody([ast.UnionField("a", None, None, "SomeUnion")]))
+    ])
+    with self.assertRaises(Exception):
+      translate.OrderedModule(tree, "mojom_tree", [])
+
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Union(
+            "SomeUnion", None,
+            ast.UnionBody([ast.UnionField("a", None, None, "SomeUnion?")]))
+    ])
+    with self.assertRaises(Exception):
+      translate.OrderedModule(tree, "mojom_tree", [])
+
+  def testDuplicateAttributesException(self):
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Union(
+            "FakeUnion",
+            ast.AttributeList([
+                ast.Attribute("key1", "value"),
+                ast.Attribute("key1", "value")
+            ]),
+            ast.UnionBody([
+                ast.UnionField("a", None, None, "int32"),
+                ast.UnionField("b", None, None, "string")
+            ]))
+    ])
+    with self.assertRaises(Exception):
+      translate.OrderedModule(tree, "mojom_tree", [])
+
+  def testEnumWithReservedValues(self):
+    """Verifies that assigning reserved values to enumerators fails."""
+    # -128 is reserved for the empty representation in WTF::HashTraits.
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Enum(
+            "MyEnum", None,
+            ast.EnumValueList([
+                ast.EnumValue('kReserved', None, '-128'),
+            ]))
+    ])
+    with self.assertRaises(Exception) as context:
+      translate.OrderedModule(tree, "mojom_tree", [])
+    self.assertIn("reserved for WTF::HashTrait", str(context.exception))
+
+    # -127 is reserved for the deleted representation in WTF::HashTraits.
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Enum(
+            "MyEnum", None,
+            ast.EnumValueList([
+                ast.EnumValue('kReserved', None, '-127'),
+            ]))
+    ])
+    with self.assertRaises(Exception) as context:
+      translate.OrderedModule(tree, "mojom_tree", [])
+    self.assertIn("reserved for WTF::HashTrait", str(context.exception))
+
+    # Implicitly assigning a reserved value should also fail.
+    tree = ast.Mojom(None, ast.ImportList(), [
+        ast.Enum(
+            "MyEnum", None,
+            ast.EnumValueList([
+                ast.EnumValue('kNotReserved', None, '-129'),
+                ast.EnumValue('kImplicitlyReserved', None, None),
+            ]))
+    ])
+    with self.assertRaises(Exception) as context:
+      translate.OrderedModule(tree, "mojom_tree", [])
+    self.assertIn("reserved for WTF::HashTrait", str(context.exception))
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast.py
index 1f0db200..aae9cdb6 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast.py
@@ -1,4 +1,4 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Node classes for the AST for a Mojo IDL file."""
@@ -8,17 +8,14 @@
 # and lineno). You may also define __repr__() to help with analyzing test
 # failures, especially for more complex types.
 
+import os.path
 
-import sys
 
+# Instance of 'NodeListBase' has no '_list_item_type' member (no-member)
+# pylint: disable=no-member
 
-def _IsStrOrUnicode(x):
-  if sys.version_info[0] < 3:
-    return isinstance(x, (unicode, str))
-  return isinstance(x, str)
 
-
-class NodeBase(object):
+class NodeBase:
   """Base class for nodes in the AST."""
 
   def __init__(self, filename=None, lineno=None):
@@ -43,7 +40,7 @@ class NodeListBase(NodeBase):
   classes, in a tuple) of the members of the list.)"""
 
   def __init__(self, item_or_items=None, **kwargs):
-    super(NodeListBase, self).__init__(**kwargs)
+    super().__init__(**kwargs)
     self.items = []
     if item_or_items is None:
       pass
@@ -62,7 +59,7 @@ class NodeListBase(NodeBase):
     return self.items.__iter__()
 
   def __eq__(self, other):
-    return super(NodeListBase, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.items == other.items
 
   # Implement this so that on failure, we get slightly more sensible output.
@@ -96,7 +93,7 @@ class Definition(NodeBase):
   include parameter definitions.) This class is meant to be subclassed."""
 
   def __init__(self, mojom_name, **kwargs):
-    assert _IsStrOrUnicode(mojom_name)
+    assert isinstance(mojom_name, str)
     NodeBase.__init__(self, **kwargs)
     self.mojom_name = mojom_name
 
@@ -108,13 +105,13 @@ class Attribute(NodeBase):
   """Represents an attribute."""
 
   def __init__(self, key, value, **kwargs):
-    assert _IsStrOrUnicode(key)
-    super(Attribute, self).__init__(**kwargs)
+    assert isinstance(key, str)
+    super().__init__(**kwargs)
     self.key = key
     self.value = value
 
   def __eq__(self, other):
-    return super(Attribute, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.key == other.key and \
            self.value == other.value
 
@@ -131,17 +128,17 @@ class Const(Definition):
   def __init__(self, mojom_name, attribute_list, typename, value, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     # The typename is currently passed through as a string.
-    assert _IsStrOrUnicode(typename)
+    assert isinstance(typename, str)
     # The value is either a literal (currently passed through as a string) or a
     # "wrapped identifier".
-    assert _IsStrOrUnicode or isinstance(value, tuple)
-    super(Const, self).__init__(mojom_name, **kwargs)
+    assert isinstance(value, (tuple, str))
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.typename = typename
     self.value = value
 
   def __eq__(self, other):
-    return super(Const, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.typename == other.typename and \
            self.value == other.value
@@ -153,12 +150,12 @@ class Enum(Definition):
   def __init__(self, mojom_name, attribute_list, enum_value_list, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert enum_value_list is None or isinstance(enum_value_list, EnumValueList)
-    super(Enum, self).__init__(mojom_name, **kwargs)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.enum_value_list = enum_value_list
 
   def __eq__(self, other):
-    return super(Enum, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.enum_value_list == other.enum_value_list
 
@@ -170,13 +167,13 @@ class EnumValue(Definition):
     # The optional value is either an int (which is current a string) or a
     # "wrapped identifier".
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
-    assert value is None or _IsStrOrUnicode(value) or isinstance(value, tuple)
-    super(EnumValue, self).__init__(mojom_name, **kwargs)
+    assert value is None or isinstance(value, (tuple, str))
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.value = value
 
   def __eq__(self, other):
-    return super(EnumValue, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.value == other.value
 
@@ -188,18 +185,47 @@ class EnumValueList(NodeListBase):
   _list_item_type = EnumValue
 
 
+class Feature(Definition):
+  """Represents a runtime feature definition."""
+  def __init__(self, mojom_name, attribute_list, body, **kwargs):
+    assert attribute_list is None or isinstance(attribute_list, AttributeList)
+    assert isinstance(body, FeatureBody) or body is None
+    super().__init__(mojom_name, **kwargs)
+    self.attribute_list = attribute_list
+    self.body = body
+
+  def __eq__(self, other):
+    return super().__eq__(other) and \
+           self.attribute_list == other.attribute_list and \
+           self.body == other.body
+
+  def __repr__(self):
+    return "Feature(mojom_name = %s, attribute_list = %s, body = %s)" % (
+        self.mojom_name, self.attribute_list, self.body)
+
+
+# This needs to be declared after `FeatureConst` and `FeatureField`.
+class FeatureBody(NodeListBase):
+  """Represents the body of (i.e., list of definitions inside) a feature."""
+
+  # Features are compile time helpers so all fields are initializers/consts
+  # for the underlying platform feature type.
+  _list_item_type = (Const)
+
+
 class Import(NodeBase):
   """Represents an import statement."""
 
   def __init__(self, attribute_list, import_filename, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
-    assert _IsStrOrUnicode(import_filename)
-    super(Import, self).__init__(**kwargs)
+    assert isinstance(import_filename, str)
+    super().__init__(**kwargs)
     self.attribute_list = attribute_list
-    self.import_filename = import_filename
+    # TODO(crbug.com/953884): Use pathlib once we're migrated fully to Python 3.
+    self.import_filename = os.path.normpath(import_filename).replace('\\', '/')
 
   def __eq__(self, other):
-    return super(Import, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.import_filename == other.import_filename
 
@@ -216,12 +242,12 @@ class Interface(Definition):
   def __init__(self, mojom_name, attribute_list, body, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert isinstance(body, InterfaceBody)
-    super(Interface, self).__init__(mojom_name, **kwargs)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.body = body
 
   def __eq__(self, other):
-    return super(Interface, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.body == other.body
 
@@ -236,14 +262,14 @@ class Method(Definition):
     assert isinstance(parameter_list, ParameterList)
     assert response_parameter_list is None or \
            isinstance(response_parameter_list, ParameterList)
-    super(Method, self).__init__(mojom_name, **kwargs)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.ordinal = ordinal
     self.parameter_list = parameter_list
     self.response_parameter_list = response_parameter_list
 
   def __eq__(self, other):
-    return super(Method, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.ordinal == other.ordinal and \
            self.parameter_list == other.parameter_list and \
@@ -264,12 +290,12 @@ class Module(NodeBase):
     # |mojom_namespace| is either none or a "wrapped identifier".
     assert mojom_namespace is None or isinstance(mojom_namespace, tuple)
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
-    super(Module, self).__init__(**kwargs)
+    super().__init__(**kwargs)
     self.mojom_namespace = mojom_namespace
     self.attribute_list = attribute_list
 
   def __eq__(self, other):
-    return super(Module, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.mojom_namespace == other.mojom_namespace and \
            self.attribute_list == other.attribute_list
 
@@ -281,13 +307,13 @@ class Mojom(NodeBase):
     assert module is None or isinstance(module, Module)
     assert isinstance(import_list, ImportList)
     assert isinstance(definition_list, list)
-    super(Mojom, self).__init__(**kwargs)
+    super().__init__(**kwargs)
     self.module = module
     self.import_list = import_list
     self.definition_list = definition_list
 
   def __eq__(self, other):
-    return super(Mojom, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.module == other.module and \
            self.import_list == other.import_list and \
            self.definition_list == other.definition_list
@@ -302,11 +328,11 @@ class Ordinal(NodeBase):
 
   def __init__(self, value, **kwargs):
     assert isinstance(value, int)
-    super(Ordinal, self).__init__(**kwargs)
+    super().__init__(**kwargs)
     self.value = value
 
   def __eq__(self, other):
-    return super(Ordinal, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.value == other.value
 
 
@@ -314,18 +340,18 @@ class Parameter(NodeBase):
   """Represents a method request or response parameter."""
 
   def __init__(self, mojom_name, attribute_list, ordinal, typename, **kwargs):
-    assert _IsStrOrUnicode(mojom_name)
+    assert isinstance(mojom_name, str)
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert ordinal is None or isinstance(ordinal, Ordinal)
-    assert _IsStrOrUnicode(typename)
-    super(Parameter, self).__init__(**kwargs)
+    assert isinstance(typename, str)
+    super().__init__(**kwargs)
     self.mojom_name = mojom_name
     self.attribute_list = attribute_list
     self.ordinal = ordinal
     self.typename = typename
 
   def __eq__(self, other):
-    return super(Parameter, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.mojom_name == other.mojom_name and \
            self.attribute_list == other.attribute_list and \
            self.ordinal == other.ordinal and \
@@ -344,42 +370,51 @@ class Struct(Definition):
   def __init__(self, mojom_name, attribute_list, body, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert isinstance(body, StructBody) or body is None
-    super(Struct, self).__init__(mojom_name, **kwargs)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.body = body
 
   def __eq__(self, other):
-    return super(Struct, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.body == other.body
 
+  def __repr__(self):
+    return "Struct(mojom_name = %s, attribute_list = %s, body = %s)" % (
+        self.mojom_name, self.attribute_list, self.body)
+
 
 class StructField(Definition):
   """Represents a struct field definition."""
 
   def __init__(self, mojom_name, attribute_list, ordinal, typename,
                default_value, **kwargs):
-    assert _IsStrOrUnicode(mojom_name)
+    assert isinstance(mojom_name, str)
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert ordinal is None or isinstance(ordinal, Ordinal)
-    assert _IsStrOrUnicode(typename)
+    assert isinstance(typename, str)
     # The optional default value is currently either a value as a string or a
     # "wrapped identifier".
-    assert default_value is None or _IsStrOrUnicode(default_value) or \
-        isinstance(default_value, tuple)
-    super(StructField, self).__init__(mojom_name, **kwargs)
+    assert default_value is None or isinstance(default_value, (str, tuple))
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.ordinal = ordinal
     self.typename = typename
     self.default_value = default_value
 
   def __eq__(self, other):
-    return super(StructField, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.ordinal == other.ordinal and \
            self.typename == other.typename and \
            self.default_value == other.default_value
 
+  def __repr__(self):
+    return ("StructField(mojom_name = %s, attribute_list = %s, ordinal = %s, "
+            "typename = %s, default_value = %s") % (
+                self.mojom_name, self.attribute_list, self.ordinal,
+                self.typename, self.default_value)
+
 
 # This needs to be declared after |StructField|.
 class StructBody(NodeListBase):
@@ -394,29 +429,29 @@ class Union(Definition):
   def __init__(self, mojom_name, attribute_list, body, **kwargs):
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert isinstance(body, UnionBody)
-    super(Union, self).__init__(mojom_name, **kwargs)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.body = body
 
   def __eq__(self, other):
-    return super(Union, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.body == other.body
 
 
 class UnionField(Definition):
   def __init__(self, mojom_name, attribute_list, ordinal, typename, **kwargs):
-    assert _IsStrOrUnicode(mojom_name)
+    assert isinstance(mojom_name, str)
     assert attribute_list is None or isinstance(attribute_list, AttributeList)
     assert ordinal is None or isinstance(ordinal, Ordinal)
-    assert _IsStrOrUnicode(typename)
-    super(UnionField, self).__init__(mojom_name, **kwargs)
+    assert isinstance(typename, str)
+    super().__init__(mojom_name, **kwargs)
     self.attribute_list = attribute_list
     self.ordinal = ordinal
     self.typename = typename
 
   def __eq__(self, other):
-    return super(UnionField, self).__eq__(other) and \
+    return super().__eq__(other) and \
            self.attribute_list == other.attribute_list and \
            self.ordinal == other.ordinal and \
            self.typename == other.typename
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast_unittest.py
index 62798631..b289f7b1 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/ast_unittest.py
@@ -1,32 +1,26 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
-import os.path
-import sys
 import unittest
 
 from mojom.parse import ast
 
-
 class _TestNode(ast.NodeBase):
   """Node type for tests."""
 
   def __init__(self, value, **kwargs):
-    super(_TestNode, self).__init__(**kwargs)
+    super().__init__(**kwargs)
     self.value = value
 
   def __eq__(self, other):
-    return super(_TestNode, self).__eq__(other) and self.value == other.value
-
+    return super().__eq__(other) and self.value == other.value
 
 class _TestNodeList(ast.NodeListBase):
   """Node list type for tests."""
 
   _list_item_type = _TestNode
 
-
 class ASTTest(unittest.TestCase):
   """Tests various AST classes."""
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features.py
index 3cb73c5d..9687edbf 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features.py
@@ -1,4 +1,4 @@
-# Copyright 2018 The Chromium Authors. All rights reserved.
+# Copyright 2018 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Helpers for processing conditionally enabled features in a mojom."""
@@ -17,8 +17,10 @@ class EnableIfError(Error):
 def _IsEnabled(definition, enabled_features):
   """Returns true if a definition is enabled.
 
-  A definition is enabled if it has no EnableIf attribute, or if the value of
-  the EnableIf attribute is in enabled_features.
+  A definition is enabled if it has no EnableIf/EnableIfNot attribute.
+  It is retained if it has an EnableIf attribute and the attribute is in
+  enabled_features. It is retained if it has an EnableIfNot attribute and the
+  attribute is not in enabled features.
   """
   if not hasattr(definition, "attribute_list"):
     return True
@@ -27,17 +29,19 @@ def _IsEnabled(definition, enabled_features):
 
   already_defined = False
   for a in definition.attribute_list:
-    if a.key == 'EnableIf':
+    if a.key == 'EnableIf' or a.key == 'EnableIfNot':
       if already_defined:
         raise EnableIfError(
             definition.filename,
-            "EnableIf attribute may only be defined once per field.",
+            "EnableIf/EnableIfNot attribute may only be set once per field.",
             definition.lineno)
       already_defined = True
 
   for attribute in definition.attribute_list:
     if attribute.key == 'EnableIf' and attribute.value not in enabled_features:
       return False
+    if attribute.key == 'EnableIfNot' and attribute.value in enabled_features:
+      return False
   return True
 
 
@@ -56,15 +60,12 @@ def _FilterDefinition(definition, enabled_features):
   """Filters definitions with a body."""
   if isinstance(definition, ast.Enum):
     _FilterDisabledFromNodeList(definition.enum_value_list, enabled_features)
-  elif isinstance(definition, ast.Interface):
-    _FilterDisabledFromNodeList(definition.body, enabled_features)
   elif isinstance(definition, ast.Method):
     _FilterDisabledFromNodeList(definition.parameter_list, enabled_features)
     _FilterDisabledFromNodeList(definition.response_parameter_list,
                                 enabled_features)
-  elif isinstance(definition, ast.Struct):
-    _FilterDisabledFromNodeList(definition.body, enabled_features)
-  elif isinstance(definition, ast.Union):
+  elif isinstance(definition,
+                  (ast.Interface, ast.Struct, ast.Union, ast.Feature)):
     _FilterDisabledFromNodeList(definition.body, enabled_features)
 
 
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features_unittest.py
index aa609be7..cca1764b 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/conditional_features_unittest.py
@@ -1,13 +1,12 @@
-# Copyright 2018 The Chromium Authors. All rights reserved.
+# Copyright 2018 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
+import importlib.util
 import os
 import sys
 import unittest
 
-
 def _GetDirAbove(dirname):
   """Returns the directory "above" this file containing |dirname| (which must
   also be "above" this file)."""
@@ -18,9 +17,8 @@ def _GetDirAbove(dirname):
     if tail == dirname:
       return path
 
-
 try:
-  imp.find_module('mojom')
+  importlib.util.find_spec("mojom")
 except ImportError:
   sys.path.append(os.path.join(_GetDirAbove('pylib'), 'pylib'))
 import mojom.parse.ast as ast
@@ -29,7 +27,6 @@ import mojom.parse.parser as parser
 
 ENABLED_FEATURES = frozenset({'red', 'green', 'blue'})
 
-
 class ConditionalFeaturesTest(unittest.TestCase):
   """Tests |mojom.parse.conditional_features|."""
 
@@ -55,6 +52,48 @@ class ConditionalFeaturesTest(unittest.TestCase):
     """
     self.parseAndAssertEqual(const_source, expected_source)
 
+  def testFilterIfNotConst(self):
+    """Test that Consts are correctly filtered."""
+    const_source = """
+      [EnableIfNot=blue]
+      const int kMyConst1 = 1;
+      [EnableIfNot=orange]
+      const double kMyConst2 = 2;
+      [EnableIf=blue]
+      const int kMyConst3 = 3;
+      [EnableIfNot=blue]
+      const int kMyConst4 = 4;
+      [EnableIfNot=purple]
+      const int kMyConst5 = 5;
+    """
+    expected_source = """
+      [EnableIfNot=orange]
+      const double kMyConst2 = 2;
+      [EnableIf=blue]
+      const int kMyConst3 = 3;
+      [EnableIfNot=purple]
+      const int kMyConst5 = 5;
+    """
+    self.parseAndAssertEqual(const_source, expected_source)
+
+  def testFilterIfNotMultipleConst(self):
+    """Test that Consts are correctly filtered."""
+    const_source = """
+      [EnableIfNot=blue]
+      const int kMyConst1 = 1;
+      [EnableIfNot=orange]
+      const double kMyConst2 = 2;
+      [EnableIfNot=orange]
+      const int kMyConst3 = 3;
+    """
+    expected_source = """
+      [EnableIfNot=orange]
+      const double kMyConst2 = 2;
+      [EnableIfNot=orange]
+      const int kMyConst3 = 3;
+    """
+    self.parseAndAssertEqual(const_source, expected_source)
+
   def testFilterEnum(self):
     """Test that EnumValues are correctly filtered from an Enum."""
     enum_source = """
@@ -91,6 +130,24 @@ class ConditionalFeaturesTest(unittest.TestCase):
     """
     self.parseAndAssertEqual(import_source, expected_source)
 
+  def testFilterIfNotImport(self):
+    """Test that imports are correctly filtered from a Mojom."""
+    import_source = """
+      [EnableIf=blue]
+      import "foo.mojom";
+      [EnableIfNot=purple]
+      import "bar.mojom";
+      [EnableIfNot=green]
+      import "baz.mojom";
+    """
+    expected_source = """
+      [EnableIf=blue]
+      import "foo.mojom";
+      [EnableIfNot=purple]
+      import "bar.mojom";
+    """
+    self.parseAndAssertEqual(import_source, expected_source)
+
   def testFilterInterface(self):
     """Test that definitions are correctly filtered from an Interface."""
     interface_source = """
@@ -175,6 +232,50 @@ class ConditionalFeaturesTest(unittest.TestCase):
     """
     self.parseAndAssertEqual(struct_source, expected_source)
 
+  def testFilterIfNotStruct(self):
+    """Test that definitions are correctly filtered from a Struct."""
+    struct_source = """
+      struct MyStruct {
+        [EnableIf=blue]
+        enum MyEnum {
+          VALUE1,
+          [EnableIfNot=red]
+          VALUE2,
+        };
+        [EnableIfNot=yellow]
+        const double kMyConst = 1.23;
+        [EnableIf=green]
+        int32 a;
+        double b;
+        [EnableIfNot=purple]
+        int32 c;
+        [EnableIf=blue]
+        double d;
+        int32 e;
+        [EnableIfNot=red]
+        double f;
+      };
+    """
+    expected_source = """
+      struct MyStruct {
+        [EnableIf=blue]
+        enum MyEnum {
+          VALUE1,
+        };
+        [EnableIfNot=yellow]
+        const double kMyConst = 1.23;
+        [EnableIf=green]
+        int32 a;
+        double b;
+        [EnableIfNot=purple]
+        int32 c;
+        [EnableIf=blue]
+        double d;
+        int32 e;
+      };
+    """
+    self.parseAndAssertEqual(struct_source, expected_source)
+
   def testFilterUnion(self):
     """Test that UnionFields are correctly filtered from a Union."""
     union_source = """
@@ -216,6 +317,25 @@ class ConditionalFeaturesTest(unittest.TestCase):
     """
     self.parseAndAssertEqual(mojom_source, expected_source)
 
+  def testFeaturesWithEnableIf(self):
+    mojom_source = """
+      feature Foo {
+        const string name = "FooFeature";
+        [EnableIf=red]
+        const bool default_state = false;
+        [EnableIf=yellow]
+        const bool default_state = true;
+      };
+    """
+    expected_source = """
+      feature Foo {
+        const string name = "FooFeature";
+        [EnableIf=red]
+        const bool default_state = false;
+      };
+    """
+    self.parseAndAssertEqual(mojom_source, expected_source)
+
   def testMultipleEnableIfs(self):
     source = """
       enum Foo {
@@ -228,6 +348,29 @@ class ConditionalFeaturesTest(unittest.TestCase):
                       conditional_features.RemoveDisabledDefinitions,
                       definition, ENABLED_FEATURES)
 
+  def testMultipleEnableIfs(self):
+    source = """
+      enum Foo {
+        [EnableIf=red,EnableIfNot=yellow]
+        kBarValue = 5,
+      };
+    """
+    definition = parser.Parse(source, "my_file.mojom")
+    self.assertRaises(conditional_features.EnableIfError,
+                      conditional_features.RemoveDisabledDefinitions,
+                      definition, ENABLED_FEATURES)
+
+  def testMultipleEnableIfs(self):
+    source = """
+      enum Foo {
+        [EnableIfNot=red,EnableIfNot=yellow]
+        kBarValue = 5,
+      };
+    """
+    definition = parser.Parse(source, "my_file.mojom")
+    self.assertRaises(conditional_features.EnableIfError,
+                      conditional_features.RemoveDisabledDefinitions,
+                      definition, ENABLED_FEATURES)
 
 if __name__ == '__main__':
   unittest.main()
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer.py
index 3e084bbf..00136a8b 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer.py
@@ -1,8 +1,7 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
 import os.path
 import sys
 
@@ -22,7 +21,7 @@ class LexError(Error):
 
 # We have methods which look like they could be functions:
 # pylint: disable=R0201
-class Lexer(object):
+class Lexer:
   def __init__(self, filename):
     self.filename = filename
 
@@ -56,6 +55,7 @@ class Lexer(object):
       'PENDING_RECEIVER',
       'PENDING_ASSOCIATED_REMOTE',
       'PENDING_ASSOCIATED_RECEIVER',
+      'FEATURE',
   )
 
   keyword_map = {}
@@ -81,7 +81,6 @@ class Lexer(object):
       # Operators
       'MINUS',
       'PLUS',
-      'AMP',
       'QSTN',
 
       # Assignment
@@ -168,7 +167,6 @@ class Lexer(object):
   # Operators
   t_MINUS = r'-'
   t_PLUS = r'\+'
-  t_AMP = r'&'
   t_QSTN = r'\?'
 
   # =
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer_unittest.py
index eadc6587..bc9f8354 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/lexer_unittest.py
@@ -1,13 +1,12 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
+import importlib.util
 import os.path
 import sys
 import unittest
 
-
 def _GetDirAbove(dirname):
   """Returns the directory "above" this file containing |dirname| (which must
   also be "above" this file)."""
@@ -18,17 +17,15 @@ def _GetDirAbove(dirname):
     if tail == dirname:
       return path
 
-
 sys.path.insert(1, os.path.join(_GetDirAbove("mojo"), "third_party"))
 from ply import lex
 
 try:
-  imp.find_module("mojom")
+  importlib.util.find_spec("mojom")
 except ImportError:
   sys.path.append(os.path.join(_GetDirAbove("pylib"), "pylib"))
 import mojom.parse.lexer
 
-
 # This (monkey-patching LexToken to make comparison value-based) is evil, but
 # we'll do it anyway. (I'm pretty sure ply's lexer never cares about comparing
 # for object identity.)
@@ -146,7 +143,6 @@ class LexerTest(unittest.TestCase):
         self._SingleTokenForInput("+"), _MakeLexToken("PLUS", "+"))
     self.assertEquals(
         self._SingleTokenForInput("-"), _MakeLexToken("MINUS", "-"))
-    self.assertEquals(self._SingleTokenForInput("&"), _MakeLexToken("AMP", "&"))
     self.assertEquals(
         self._SingleTokenForInput("?"), _MakeLexToken("QSTN", "?"))
     self.assertEquals(
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser.py
index b3b803d6..1dffd98b 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser.py
@@ -1,8 +1,11 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Generates a syntax tree from a Mojo IDL file."""
 
+# Breaking parser stanzas is unhelpful so allow longer lines.
+# pylint: disable=line-too-long
+
 import os.path
 import sys
 
@@ -33,7 +36,7 @@ class ParseError(Error):
 
 # We have methods which look like they could be functions:
 # pylint: disable=R0201
-class Parser(object):
+class Parser:
   def __init__(self, lexer, source, filename):
     self.tokens = lexer.tokens
     self.source = source
@@ -111,7 +114,8 @@ class Parser(object):
                   | union
                   | interface
                   | enum
-                  | const"""
+                  | const
+                  | feature"""
     p[0] = p[1]
 
   def p_attribute_section_1(self, p):
@@ -140,12 +144,19 @@ class Parser(object):
     p[0].Append(p[3])
 
   def p_attribute_1(self, p):
-    """attribute : NAME EQUALS evaled_literal
-                 | NAME EQUALS NAME"""
-    p[0] = ast.Attribute(p[1], p[3], filename=self.filename, lineno=p.lineno(1))
+    """attribute : name_wrapped EQUALS identifier_wrapped"""
+    p[0] = ast.Attribute(p[1],
+                         p[3][1],
+                         filename=self.filename,
+                         lineno=p.lineno(1))
 
   def p_attribute_2(self, p):
-    """attribute : NAME"""
+    """attribute : name_wrapped EQUALS evaled_literal
+                 | name_wrapped EQUALS name_wrapped"""
+    p[0] = ast.Attribute(p[1], p[3], filename=self.filename, lineno=p.lineno(1))
+
+  def p_attribute_3(self, p):
+    """attribute : name_wrapped"""
     p[0] = ast.Attribute(p[1], True, filename=self.filename, lineno=p.lineno(1))
 
   def p_evaled_literal(self, p):
@@ -161,11 +172,11 @@ class Parser(object):
       p[0] = eval(p[1])
 
   def p_struct_1(self, p):
-    """struct : attribute_section STRUCT NAME LBRACE struct_body RBRACE SEMI"""
+    """struct : attribute_section STRUCT name_wrapped LBRACE struct_body RBRACE SEMI"""
     p[0] = ast.Struct(p[3], p[1], p[5])
 
   def p_struct_2(self, p):
-    """struct : attribute_section STRUCT NAME SEMI"""
+    """struct : attribute_section STRUCT name_wrapped SEMI"""
     p[0] = ast.Struct(p[3], p[1], None)
 
   def p_struct_body_1(self, p):
@@ -180,11 +191,24 @@ class Parser(object):
     p[0].Append(p[2])
 
   def p_struct_field(self, p):
-    """struct_field : attribute_section typename NAME ordinal default SEMI"""
+    """struct_field : attribute_section typename name_wrapped ordinal default SEMI"""
     p[0] = ast.StructField(p[3], p[1], p[4], p[2], p[5])
 
+  def p_feature(self, p):
+    """feature : attribute_section FEATURE NAME LBRACE feature_body RBRACE SEMI"""
+    p[0] = ast.Feature(p[3], p[1], p[5])
+
+  def p_feature_body_1(self, p):
+    """feature_body : """
+    p[0] = ast.FeatureBody()
+
+  def p_feature_body_2(self, p):
+    """feature_body : feature_body const"""
+    p[0] = p[1]
+    p[0].Append(p[2])
+
   def p_union(self, p):
-    """union : attribute_section UNION NAME LBRACE union_body RBRACE SEMI"""
+    """union : attribute_section UNION name_wrapped LBRACE union_body RBRACE SEMI"""
     p[0] = ast.Union(p[3], p[1], p[5])
 
   def p_union_body_1(self, p):
@@ -197,7 +221,7 @@ class Parser(object):
     p[1].Append(p[2])
 
   def p_union_field(self, p):
-    """union_field : attribute_section typename NAME ordinal SEMI"""
+    """union_field : attribute_section typename name_wrapped ordinal SEMI"""
     p[0] = ast.UnionField(p[3], p[1], p[4], p[2])
 
   def p_default_1(self, p):
@@ -209,8 +233,7 @@ class Parser(object):
     p[0] = p[2]
 
   def p_interface(self, p):
-    """interface : attribute_section INTERFACE NAME LBRACE interface_body \
-                       RBRACE SEMI"""
+    """interface : attribute_section INTERFACE name_wrapped LBRACE interface_body RBRACE SEMI"""
     p[0] = ast.Interface(p[3], p[1], p[5])
 
   def p_interface_body_1(self, p):
@@ -233,8 +256,7 @@ class Parser(object):
     p[0] = p[3]
 
   def p_method(self, p):
-    """method : attribute_section NAME ordinal LPAREN parameter_list RPAREN \
-                    response SEMI"""
+    """method : attribute_section name_wrapped ordinal LPAREN parameter_list RPAREN response SEMI"""
     p[0] = ast.Method(p[2], p[1], p[3], p[5], p[7])
 
   def p_parameter_list_1(self, p):
@@ -255,7 +277,7 @@ class Parser(object):
     p[0].Append(p[3])
 
   def p_parameter(self, p):
-    """parameter : attribute_section typename NAME ordinal"""
+    """parameter : attribute_section typename name_wrapped ordinal"""
     p[0] = ast.Parameter(
         p[3], p[1], p[4], p[2], filename=self.filename, lineno=p.lineno(3))
 
@@ -271,8 +293,7 @@ class Parser(object):
     """nonnullable_typename : basictypename
                             | array
                             | fixed_array
-                            | associative_array
-                            | interfacerequest"""
+                            | associative_array"""
     p[0] = p[1]
 
   def p_basictypename(self, p):
@@ -297,18 +318,16 @@ class Parser(object):
     p[0] = "rcv<%s>" % p[3]
 
   def p_associatedremotetype(self, p):
-    """associatedremotetype : PENDING_ASSOCIATED_REMOTE LANGLE identifier \
-                                  RANGLE"""
+    """associatedremotetype : PENDING_ASSOCIATED_REMOTE LANGLE identifier RANGLE"""
     p[0] = "rma<%s>" % p[3]
 
   def p_associatedreceivertype(self, p):
-    """associatedreceivertype : PENDING_ASSOCIATED_RECEIVER LANGLE identifier \
-                                    RANGLE"""
+    """associatedreceivertype : PENDING_ASSOCIATED_RECEIVER LANGLE identifier RANGLE"""
     p[0] = "rca<%s>" % p[3]
 
   def p_handletype(self, p):
     """handletype : HANDLE
-                  | HANDLE LANGLE NAME RANGLE"""
+                  | HANDLE LANGLE name_wrapped RANGLE"""
     if len(p) == 2:
       p[0] = p[1]
     else:
@@ -342,14 +361,6 @@ class Parser(object):
     """associative_array : MAP LANGLE identifier COMMA typename RANGLE"""
     p[0] = p[5] + "{" + p[3] + "}"
 
-  def p_interfacerequest(self, p):
-    """interfacerequest : identifier AMP
-                        | ASSOCIATED identifier AMP"""
-    if len(p) == 3:
-      p[0] = p[1] + "&"
-    else:
-      p[0] = "asso<" + p[2] + "&>"
-
   def p_ordinal_1(self, p):
     """ordinal : """
     p[0] = None
@@ -366,15 +377,14 @@ class Parser(object):
     p[0] = ast.Ordinal(value, filename=self.filename, lineno=p.lineno(1))
 
   def p_enum_1(self, p):
-    """enum : attribute_section ENUM NAME LBRACE enum_value_list \
-                  RBRACE SEMI
-            | attribute_section ENUM NAME LBRACE nonempty_enum_value_list \
-                  COMMA RBRACE SEMI"""
+    """enum : attribute_section ENUM name_wrapped LBRACE enum_value_list RBRACE SEMI
+            | attribute_section ENUM name_wrapped LBRACE \
+                    nonempty_enum_value_list COMMA RBRACE SEMI"""
     p[0] = ast.Enum(
         p[3], p[1], p[5], filename=self.filename, lineno=p.lineno(2))
 
   def p_enum_2(self, p):
-    """enum : attribute_section ENUM NAME SEMI"""
+    """enum : attribute_section ENUM name_wrapped SEMI"""
     p[0] = ast.Enum(
         p[3], p[1], None, filename=self.filename, lineno=p.lineno(2))
 
@@ -396,9 +406,9 @@ class Parser(object):
     p[0].Append(p[3])
 
   def p_enum_value(self, p):
-    """enum_value : attribute_section NAME
-                  | attribute_section NAME EQUALS int
-                  | attribute_section NAME EQUALS identifier_wrapped"""
+    """enum_value : attribute_section name_wrapped
+                  | attribute_section name_wrapped EQUALS int
+                  | attribute_section name_wrapped EQUALS identifier_wrapped"""
     p[0] = ast.EnumValue(
         p[2],
         p[1],
@@ -407,7 +417,7 @@ class Parser(object):
         lineno=p.lineno(2))
 
   def p_const(self, p):
-    """const : attribute_section CONST typename NAME EQUALS constant SEMI"""
+    """const : attribute_section CONST typename name_wrapped EQUALS constant SEMI"""
     p[0] = ast.Const(p[4], p[1], p[3], p[6])
 
   def p_constant(self, p):
@@ -422,10 +432,16 @@ class Parser(object):
   # TODO(vtl): Make this produce a "wrapped" identifier (probably as an
   # |ast.Identifier|, to be added) and get rid of identifier_wrapped.
   def p_identifier(self, p):
-    """identifier : NAME
-                  | NAME DOT identifier"""
+    """identifier : name_wrapped
+                  | name_wrapped DOT identifier"""
     p[0] = ''.join(p[1:])
 
+  # Allow 'feature' to be a name literal not just a keyword.
+  def p_name_wrapped(self, p):
+    """name_wrapped : NAME
+                    | FEATURE"""
+    p[0] = p[1]
+
   def p_literal(self, p):
     """literal : int
                | float
@@ -458,6 +474,12 @@ class Parser(object):
       # TODO(vtl): Can we figure out what's missing?
       raise ParseError(self.filename, "Unexpected end of file")
 
+    if e.value == 'feature':
+      raise ParseError(self.filename,
+                       "`feature` is reserved for a future mojom keyword",
+                       lineno=e.lineno,
+                       snippet=self._GetSnippet(e.lineno))
+
     raise ParseError(
         self.filename,
         "Unexpected %r:" % e.value,
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser_unittest.py
index 6d6b7153..0a26307b 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom/parse/parser_unittest.py
@@ -1,17 +1,13 @@
-# Copyright 2014 The Chromium Authors. All rights reserved.
+# Copyright 2014 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import imp
-import os.path
-import sys
 import unittest
 
 from mojom.parse import ast
 from mojom.parse import lexer
 from mojom.parse import parser
 
-
 class ParserTest(unittest.TestCase):
   """Tests |parser.Parse()|."""
 
@@ -1086,7 +1082,7 @@ class ParserTest(unittest.TestCase):
           handle<data_pipe_producer>? k;
           handle<message_pipe>? l;
           handle<shared_buffer>? m;
-          some_interface&? n;
+          pending_receiver<some_interface>? n;
           handle<platform>? o;
         };
         """
@@ -1110,7 +1106,7 @@ class ParserTest(unittest.TestCase):
                 ast.StructField('l', None, None, 'handle<message_pipe>?', None),
                 ast.StructField('m', None, None, 'handle<shared_buffer>?',
                                 None),
-                ast.StructField('n', None, None, 'some_interface&?', None),
+                ast.StructField('n', None, None, 'rcv<some_interface>?', None),
                 ast.StructField('o', None, None, 'handle<platform>?', None)
             ]))
     ])
@@ -1138,16 +1134,6 @@ class ParserTest(unittest.TestCase):
         r" *handle\?<data_pipe_consumer> a;$"):
       parser.Parse(source2, "my_file.mojom")
 
-    source3 = """\
-        struct MyStruct {
-          some_interface?& a;
-        };
-        """
-    with self.assertRaisesRegexp(
-        parser.ParseError, r"^my_file\.mojom:2: Error: Unexpected '&':\n"
-        r" *some_interface\?& a;$"):
-      parser.Parse(source3, "my_file.mojom")
-
   def testSimpleUnion(self):
     """Tests a simple .mojom source that just defines a union."""
     source = """\
@@ -1317,9 +1303,9 @@ class ParserTest(unittest.TestCase):
     source1 = """\
         struct MyStruct {
           associated MyInterface a;
-          associated MyInterface& b;
+          pending_associated_receiver<MyInterface> b;
           associated MyInterface? c;
-          associated MyInterface&? d;
+          pending_associated_receiver<MyInterface>? d;
         };
         """
     expected1 = ast.Mojom(None, ast.ImportList(), [
@@ -1327,16 +1313,16 @@ class ParserTest(unittest.TestCase):
             'MyStruct', None,
             ast.StructBody([
                 ast.StructField('a', None, None, 'asso<MyInterface>', None),
-                ast.StructField('b', None, None, 'asso<MyInterface&>', None),
+                ast.StructField('b', None, None, 'rca<MyInterface>', None),
                 ast.StructField('c', None, None, 'asso<MyInterface>?', None),
-                ast.StructField('d', None, None, 'asso<MyInterface&>?', None)
+                ast.StructField('d', None, None, 'rca<MyInterface>?', None)
             ]))
     ])
     self.assertEquals(parser.Parse(source1, "my_file.mojom"), expected1)
 
     source2 = """\
         interface MyInterface {
-          MyMethod(associated A a) =>(associated B& b);
+          MyMethod(associated A a) =>(pending_associated_receiver<B> b);
         };"""
     expected2 = ast.Mojom(None, ast.ImportList(), [
         ast.Interface(
@@ -1344,10 +1330,10 @@ class ParserTest(unittest.TestCase):
             ast.InterfaceBody(
                 ast.Method(
                     'MyMethod', None, None,
-                    ast.ParameterList(
-                        ast.Parameter('a', None, None, 'asso<A>')),
-                    ast.ParameterList(
-                        ast.Parameter('b', None, None, 'asso<B&>')))))
+                    ast.ParameterList(ast.Parameter('a', None, None,
+                                                    'asso<A>')),
+                    ast.ParameterList(ast.Parameter('b', None, None,
+                                                    'rca<B>')))))
     ])
     self.assertEquals(parser.Parse(source2, "my_file.mojom"), expected2)
 
@@ -1385,6 +1371,5 @@ class ParserTest(unittest.TestCase):
         r" *associated\? MyInterface& a;$"):
       parser.Parse(source3, "my_file.mojom")
 
-
 if __name__ == "__main__":
   unittest.main()
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom_parser.py b/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
index eb90c825..9693090e 100755
--- a/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom_parser.py
@@ -1,5 +1,5 @@
-#!/usr/bin/env python
-# Copyright 2020 The Chromium Authors. All rights reserved.
+#!/usr/bin/env python3
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 """Parses mojom IDL files.
@@ -11,6 +11,7 @@ generate usable language bindings.
 """
 
 import argparse
+import builtins
 import codecs
 import errno
 import json
@@ -19,6 +20,7 @@ import multiprocessing
 import os
 import os.path
 import sys
+import traceback
 from collections import defaultdict
 
 from mojom.generate import module
@@ -28,16 +30,12 @@ from mojom.parse import conditional_features
 
 
 # Disable this for easier debugging.
-# In Python 2, subprocesses just hang when exceptions are thrown :(.
-_ENABLE_MULTIPROCESSING = sys.version_info[0] > 2
+_ENABLE_MULTIPROCESSING = True
 
-if sys.version_info < (3, 4):
-  _MULTIPROCESSING_USES_FORK = sys.platform.startswith('linux')
-else:
-  # https://docs.python.org/3/library/multiprocessing.html#:~:text=bpo-33725
-  if __name__ == '__main__' and sys.platform == 'darwin':
-    multiprocessing.set_start_method('fork')
-  _MULTIPROCESSING_USES_FORK = multiprocessing.get_start_method() == 'fork'
+# https://docs.python.org/3/library/multiprocessing.html#:~:text=bpo-33725
+if __name__ == '__main__' and sys.platform == 'darwin':
+  multiprocessing.set_start_method('fork')
+_MULTIPROCESSING_USES_FORK = multiprocessing.get_start_method() == 'fork'
 
 
 def _ResolveRelativeImportPath(path, roots):
@@ -63,7 +61,7 @@ def _ResolveRelativeImportPath(path, roots):
   raise ValueError('"%s" does not exist in any of %s' % (path, roots))
 
 
-def _RebaseAbsolutePath(path, roots):
+def RebaseAbsolutePath(path, roots):
   """Rewrites an absolute file path as relative to an absolute directory path in
   roots.
 
@@ -139,7 +137,7 @@ def _EnsureInputLoaded(mojom_abspath, module_path, abs_paths, asts,
     # Already done.
     return
 
-  for dep_abspath, dep_path in dependencies[mojom_abspath]:
+  for dep_abspath, dep_path in sorted(dependencies[mojom_abspath]):
     if dep_abspath not in loaded_modules:
       _EnsureInputLoaded(dep_abspath, dep_path, abs_paths, asts, dependencies,
                          loaded_modules, module_metadata)
@@ -159,11 +157,19 @@ def _CollectAllowedImportsFromBuildMetadata(build_metadata_filename):
 
   def collect(metadata_filename):
     processed_deps.add(metadata_filename)
+
+    # Paths in the metadata file are relative to the metadata file's dir.
+    metadata_dir = os.path.abspath(os.path.dirname(metadata_filename))
+
+    def to_abs(s):
+      return os.path.normpath(os.path.join(metadata_dir, s))
+
     with open(metadata_filename) as f:
       metadata = json.load(f)
       allowed_imports.update(
-          map(os.path.normcase, map(os.path.normpath, metadata['sources'])))
+          [os.path.normcase(to_abs(s)) for s in metadata['sources']])
       for dep_metadata in metadata['deps']:
+        dep_metadata = to_abs(dep_metadata)
         if dep_metadata not in processed_deps:
           collect(dep_metadata)
 
@@ -172,8 +178,7 @@ def _CollectAllowedImportsFromBuildMetadata(build_metadata_filename):
 
 
 # multiprocessing helper.
-def _ParseAstHelper(args):
-  mojom_abspath, enabled_features = args
+def _ParseAstHelper(mojom_abspath, enabled_features):
   with codecs.open(mojom_abspath, encoding='utf-8') as f:
     ast = parser.Parse(f.read(), mojom_abspath)
     conditional_features.RemoveDisabledDefinitions(ast, enabled_features)
@@ -181,8 +186,7 @@ def _ParseAstHelper(args):
 
 
 # multiprocessing helper.
-def _SerializeHelper(args):
-  mojom_abspath, mojom_path = args
+def _SerializeHelper(mojom_abspath, mojom_path):
   module_path = os.path.join(_SerializeHelper.output_root_path,
                              _GetModuleFilename(mojom_path))
   module_dir = os.path.dirname(module_path)
@@ -199,12 +203,33 @@ def _SerializeHelper(args):
     _SerializeHelper.loaded_modules[mojom_abspath].Dump(f)
 
 
-def _Shard(target_func, args, processes=None):
-  args = list(args)
+class _ExceptionWrapper:
+  def __init__(self):
+    # Do not capture exception object to ensure pickling works.
+    self.formatted_trace = traceback.format_exc()
+
+
+class _FuncWrapper:
+  """Marshals exceptions and spreads args."""
+
+  def __init__(self, func):
+    self._func = func
+
+  def __call__(self, args):
+    # multiprocessing does not gracefully handle excptions.
+    # https://crbug.com/1219044
+    try:
+      return self._func(*args)
+    except:  # pylint: disable=bare-except
+      return _ExceptionWrapper()
+
+
+def _Shard(target_func, arg_list, processes=None):
+  arg_list = list(arg_list)
   if processes is None:
     processes = multiprocessing.cpu_count()
   # Seems optimal to have each process perform at least 2 tasks.
-  processes = min(processes, len(args) // 2)
+  processes = min(processes, len(arg_list) // 2)
 
   if sys.platform == 'win32':
     # TODO(crbug.com/1190269) - we can't use more than 56
@@ -213,13 +238,17 @@ def _Shard(target_func, args, processes=None):
 
   # Don't spin up processes unless there is enough work to merit doing so.
   if not _ENABLE_MULTIPROCESSING or processes < 2:
-    for result in map(target_func, args):
-      yield result
+    for arg_tuple in arg_list:
+      yield target_func(*arg_tuple)
     return
 
   pool = multiprocessing.Pool(processes=processes)
   try:
-    for result in pool.imap_unordered(target_func, args):
+    wrapped_func = _FuncWrapper(target_func)
+    for result in pool.imap_unordered(wrapped_func, arg_list):
+      if isinstance(result, _ExceptionWrapper):
+        sys.stderr.write(result.formatted_trace)
+        sys.exit(1)
       yield result
   finally:
     pool.close()
@@ -230,6 +259,7 @@ def _Shard(target_func, args, processes=None):
 def _ParseMojoms(mojom_files,
                  input_root_paths,
                  output_root_path,
+                 module_root_paths,
                  enabled_features,
                  module_metadata,
                  allowed_imports=None):
@@ -245,8 +275,10 @@ def _ParseMojoms(mojom_files,
         are based on the mojom's relative path, rebased onto this path.
         Additionally, the script expects this root to contain already-generated
         modules for any transitive dependencies not listed in mojom_files.
+    module_root_paths: A list of absolute filesystem paths which contain
+        already-generated modules for any non-transitive dependencies.
     enabled_features: A list of enabled feature names, controlling which AST
-        nodes are filtered by [EnableIf] attributes.
+        nodes are filtered by [EnableIf] or [EnableIfNot] attributes.
     module_metadata: A list of 2-tuples representing metadata key-value pairs to
         attach to each compiled module output.
 
@@ -262,7 +294,7 @@ def _ParseMojoms(mojom_files,
   loaded_modules = {}
   input_dependencies = defaultdict(set)
   mojom_files_to_parse = dict((os.path.normcase(abs_path),
-                               _RebaseAbsolutePath(abs_path, input_root_paths))
+                               RebaseAbsolutePath(abs_path, input_root_paths))
                               for abs_path in mojom_files)
   abs_paths = dict(
       (path, abs_path) for abs_path, path in mojom_files_to_parse.items())
@@ -274,7 +306,7 @@ def _ParseMojoms(mojom_files,
     loaded_mojom_asts[mojom_abspath] = ast
 
   logging.info('Processing dependencies')
-  for mojom_abspath, ast in loaded_mojom_asts.items():
+  for mojom_abspath, ast in sorted(loaded_mojom_asts.items()):
     invalid_imports = []
     for imp in ast.import_list:
       import_abspath = _ResolveRelativeImportPath(imp.import_filename,
@@ -295,8 +327,8 @@ def _ParseMojoms(mojom_files,
         # be parsed and have a module file sitting in a corresponding output
         # location.
         module_path = _GetModuleFilename(imp.import_filename)
-        module_abspath = _ResolveRelativeImportPath(module_path,
-                                                    [output_root_path])
+        module_abspath = _ResolveRelativeImportPath(
+            module_path, module_root_paths + [output_root_path])
         with open(module_abspath, 'rb') as module_file:
           loaded_modules[import_abspath] = module.Module.Load(module_file)
 
@@ -371,6 +403,15 @@ already present in the provided output root.""")
       'ROOT is also searched for existing modules of any transitive imports '
       'which were not included in the set of inputs.')
   arg_parser.add_argument(
+      '--module-root',
+      default=[],
+      action='append',
+      metavar='ROOT',
+      dest='module_root_paths',
+      help='Adds ROOT to the set of root paths to search for existing modules '
+      'of non-transitive imports. Provided root paths are always searched in '
+      'order from longest absolute path to shortest.')
+  arg_parser.add_argument(
       '--mojoms',
       nargs='+',
       dest='mojom_files',
@@ -396,9 +437,9 @@ already present in the provided output root.""")
       help='Enables a named feature when parsing the given mojoms. Features '
       'are identified by arbitrary string values. Specifying this flag with a '
       'given FEATURE name will cause the parser to process any syntax elements '
-      'tagged with an [EnableIf=FEATURE] attribute. If this flag is not '
-      'provided for a given FEATURE, such tagged elements are discarded by the '
-      'parser and will not be present in the compiled output.')
+      'tagged with an [EnableIf=FEATURE] or [EnableIfNot] attribute. If this '
+      'flag is not provided for a given FEATURE, such tagged elements are '
+      'discarded by the parser and will not be present in the compiled output.')
   arg_parser.add_argument(
       '--check-imports',
       dest='build_metadata_filename',
@@ -436,6 +477,7 @@ already present in the provided output root.""")
   mojom_files = list(map(os.path.abspath, args.mojom_files))
   input_roots = list(map(os.path.abspath, args.input_root_paths))
   output_root = os.path.abspath(args.output_root_path)
+  module_roots = list(map(os.path.abspath, args.module_root_paths))
 
   if args.build_metadata_filename:
     allowed_imports = _CollectAllowedImportsFromBuildMetadata(
@@ -445,13 +487,16 @@ already present in the provided output root.""")
 
   module_metadata = list(
       map(lambda kvp: tuple(kvp.split('=')), args.module_metadata))
-  _ParseMojoms(mojom_files, input_roots, output_root, args.enabled_features,
-               module_metadata, allowed_imports)
+  _ParseMojoms(mojom_files, input_roots, output_root, module_roots,
+               args.enabled_features, module_metadata, allowed_imports)
   logging.info('Finished')
-  # Exit without running GC, which can save multiple seconds due the large
-  # number of object created.
-  os._exit(0)
 
 
 if __name__ == '__main__':
   Run(sys.argv[1:])
+  # Exit without running GC, which can save multiple seconds due to the large
+  # number of object created. But flush is necessary as os._exit doesn't do
+  # that.
+  sys.stdout.flush()
+  sys.stderr.flush()
+  os._exit(0)
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom_parser_test_case.py b/utils/ipc/mojo/public/tools/mojom/mojom_parser_test_case.py
index e213fbfa..f0ee6966 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom_parser_test_case.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom_parser_test_case.py
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -20,7 +20,7 @@ class MojomParserTestCase(unittest.TestCase):
   resolution, and module serialization and deserialization."""
 
   def __init__(self, method_name):
-    super(MojomParserTestCase, self).__init__(method_name)
+    super().__init__(method_name)
     self._temp_dir = None
 
   def setUp(self):
@@ -67,7 +67,7 @@ class MojomParserTestCase(unittest.TestCase):
     self.ParseMojoms([filename])
     m = self.LoadModule(filename)
     definitions = {}
-    for kinds in (m.enums, m.structs, m.unions, m.interfaces):
+    for kinds in (m.enums, m.structs, m.unions, m.interfaces, m.features):
       for kind in kinds:
         definitions[kind.mojom_name] = kind
     return definitions
diff --git a/utils/ipc/mojo/public/tools/mojom/mojom_parser_unittest.py b/utils/ipc/mojo/public/tools/mojom/mojom_parser_unittest.py
index a93f34ba..353a2b6e 100644
--- a/utils/ipc/mojo/public/tools/mojom/mojom_parser_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/mojom_parser_unittest.py
@@ -1,7 +1,9 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import json
+
 from mojom_parser_test_case import MojomParserTestCase
 
 
@@ -119,15 +121,22 @@ class MojomParserTest(MojomParserTestCase):
     c = 'c.mojom'
     c_metadata = 'out/c.build_metadata'
     self.WriteFile(a_metadata,
-                   '{"sources": ["%s"], "deps": []}\n' % self.GetPath(a))
+                   json.dumps({
+                       "sources": [self.GetPath(a)],
+                       "deps": []
+                   }))
     self.WriteFile(
         b_metadata,
-        '{"sources": ["%s"], "deps": ["%s"]}\n' % (self.GetPath(b),
-                                                   self.GetPath(a_metadata)))
+        json.dumps({
+            "sources": [self.GetPath(b)],
+            "deps": [self.GetPath(a_metadata)]
+        }))
     self.WriteFile(
         c_metadata,
-        '{"sources": ["%s"], "deps": ["%s"]}\n' % (self.GetPath(c),
-                                                   self.GetPath(b_metadata)))
+        json.dumps({
+            "sources": [self.GetPath(c)],
+            "deps": [self.GetPath(b_metadata)]
+        }))
     self.WriteFile(a, """\
         module a;
         struct Bar {};""")
@@ -154,9 +163,15 @@ class MojomParserTest(MojomParserTestCase):
     b = 'b.mojom'
     b_metadata = 'out/b.build_metadata'
     self.WriteFile(a_metadata,
-                   '{"sources": ["%s"], "deps": []}\n' % self.GetPath(a))
+                   json.dumps({
+                       "sources": [self.GetPath(a)],
+                       "deps": []
+                   }))
     self.WriteFile(b_metadata,
-                   '{"sources": ["%s"], "deps": []}\n' % self.GetPath(b))
+                   json.dumps({
+                       "sources": [self.GetPath(b)],
+                       "deps": []
+                   }))
     self.WriteFile(a, """\
         module a;
         struct Bar {};""")
diff --git a/utils/ipc/mojo/public/tools/mojom/stable_attribute_unittest.py b/utils/ipc/mojo/public/tools/mojom/stable_attribute_unittest.py
index d45ec586..d10d69c6 100644
--- a/utils/ipc/mojo/public/tools/mojom/stable_attribute_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/stable_attribute_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
diff --git a/utils/ipc/mojo/public/tools/mojom/union_unittest.py b/utils/ipc/mojo/public/tools/mojom/union_unittest.py
new file mode 100644
index 00000000..6b2525e5
--- /dev/null
+++ b/utils/ipc/mojo/public/tools/mojom/union_unittest.py
@@ -0,0 +1,44 @@
+# Copyright 2022 The Chromium Authors
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+from mojom_parser_test_case import MojomParserTestCase
+
+
+class UnionTest(MojomParserTestCase):
+  """Tests union parsing behavior."""
+
+  def testExtensibleMustHaveDefault(self):
+    """Verifies that extensible unions must have a default field."""
+    mojom = 'foo.mojom'
+    self.WriteFile(mojom, 'module foo; [Extensible] union U { bool x; };')
+    with self.assertRaisesRegexp(Exception, 'must specify a \[Default\]'):
+      self.ParseMojoms([mojom])
+
+  def testExtensibleSingleDefault(self):
+    """Verifies that extensible unions must not have multiple default fields."""
+    mojom = 'foo.mojom'
+    self.WriteFile(
+        mojom, """\
+               module foo;
+               [Extensible] union U {
+                 [Default] bool x;
+                 [Default] bool y;
+               };
+               """)
+    with self.assertRaisesRegexp(Exception, 'Multiple \[Default\] fields'):
+      self.ParseMojoms([mojom])
+
+  def testExtensibleDefaultTypeValid(self):
+    """Verifies that an extensible union's default field must be nullable or
+    integral type."""
+    mojom = 'foo.mojom'
+    self.WriteFile(
+        mojom, """\
+               module foo;
+               [Extensible] union U {
+                 [Default] handle<message_pipe> p;
+               };
+               """)
+    with self.assertRaisesRegexp(Exception, 'must be nullable or integral'):
+      self.ParseMojoms([mojom])
diff --git a/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py b/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
index 65db4dc9..45e45ec5 100644
--- a/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
+++ b/utils/ipc/mojo/public/tools/mojom/version_compatibility_unittest.py
@@ -1,4 +1,4 @@
-# Copyright 2020 The Chromium Authors. All rights reserved.
+# Copyright 2020 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
@@ -23,9 +23,12 @@ class VersionCompatibilityTest(MojomParserTestCase):
 
     checker = module.BackwardCompatibilityChecker()
     compatibility_map = {}
-    for name in old.keys():
-      compatibility_map[name] = checker.IsBackwardCompatible(
-          new[name], old[name])
+    for name in old:
+      try:
+        compatibility_map[name] = checker.IsBackwardCompatible(
+            new[name], old[name])
+      except Exception:
+        compatibility_map[name] = False
     return compatibility_map
 
   def assertBackwardCompatible(self, old_mojom, new_mojom):
@@ -60,40 +63,48 @@ class VersionCompatibilityTest(MojomParserTestCase):
     """Adding a value to an existing version is not allowed, even if the old
     enum was marked [Extensible]. Note that it is irrelevant whether or not the
     new enum is marked [Extensible]."""
-    self.assertNotBackwardCompatible('[Extensible] enum E { kFoo, kBar };',
-                                     'enum E { kFoo, kBar, kBaz };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kFoo, kBar };',
-        '[Extensible] enum E { kFoo, kBar, kBaz };')
+        '[Extensible] enum E { [Default] kFoo, kBar };',
+        'enum E { kFoo, kBar, kBaz };')
+    self.assertNotBackwardCompatible(
+        '[Extensible] enum E { [Default] kFoo, kBar };',
+        '[Extensible] enum E { [Default] kFoo, kBar, kBaz };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kFoo, [MinVersion=1] kBar };',
+        '[Extensible] enum E { [Default] kFoo, [MinVersion=1] kBar };',
         'enum E { kFoo, [MinVersion=1] kBar, [MinVersion=1] kBaz };')
 
   def testEnumValueRemoval(self):
     """Removal of an enum value is never valid even for [Extensible] enums."""
     self.assertNotBackwardCompatible('enum E { kFoo, kBar };',
                                      'enum E { kFoo };')
-    self.assertNotBackwardCompatible('[Extensible] enum E { kFoo, kBar };',
-                                     '[Extensible] enum E { kFoo };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kA, [MinVersion=1] kB };',
-        '[Extensible] enum E { kA, };')
+        '[Extensible] enum E { [Default] kFoo, kBar };',
+        '[Extensible] enum E { [Default] kFoo };')
+    self.assertNotBackwardCompatible(
+        '[Extensible] enum E { [Default] kA, [MinVersion=1] kB };',
+        '[Extensible] enum E { [Default] kA, };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kA, [MinVersion=1] kB, [MinVersion=1] kZ };',
-        '[Extensible] enum E { kA, [MinVersion=1] kB };')
+        """[Extensible] enum E {
+          [Default] kA,
+          [MinVersion=1] kB,
+          [MinVersion=1] kZ };""",
+        '[Extensible] enum E { [Default] kA, [MinVersion=1] kB };')
 
   def testNewExtensibleEnumValueWithMinVersion(self):
     """Adding a new and properly [MinVersion]'d value to an [Extensible] enum
     is a backward-compatible change. Note that it is irrelevant whether or not
     the new enum is marked [Extensible]."""
-    self.assertBackwardCompatible('[Extensible] enum E { kA, kB };',
+    self.assertBackwardCompatible('[Extensible] enum E { [Default] kA, kB };',
                                   'enum E { kA, kB, [MinVersion=1] kC };')
     self.assertBackwardCompatible(
-        '[Extensible] enum E { kA, kB };',
-        '[Extensible] enum E { kA, kB, [MinVersion=1] kC };')
+        '[Extensible] enum E { [Default] kA, kB };',
+        '[Extensible] enum E { [Default] kA, kB, [MinVersion=1] kC };')
     self.assertBackwardCompatible(
-        '[Extensible] enum E { kA, [MinVersion=1] kB };',
-        '[Extensible] enum E { kA, [MinVersion=1] kB, [MinVersion=2] kC };')
+        '[Extensible] enum E { [Default] kA, [MinVersion=1] kB };',
+        """[Extensible] enum E {
+          [Default] kA,
+          [MinVersion=1] kB,
+          [MinVersion=2] kC };""")
 
   def testRenameEnumValue(self):
     """Renaming an enum value does not affect backward-compatibility. Only
@@ -161,14 +172,17 @@ class VersionCompatibilityTest(MojomParserTestCase):
         'struct S {}; struct T { S s; };',
         'struct S { [MinVersion=1] int32 x; }; struct T { S s; };')
     self.assertBackwardCompatible(
-        '[Extensible] enum E { kA }; struct S { E e; };',
-        '[Extensible] enum E { kA, [MinVersion=1] kB }; struct S { E e; };')
+        '[Extensible] enum E { [Default] kA }; struct S { E e; };',
+        """[Extensible] enum E {
+          [Default] kA,
+          [MinVersion=1] kB };
+          struct S { E e; };""")
     self.assertNotBackwardCompatible(
         'struct S {}; struct T { S s; };',
         'struct S { int32 x; }; struct T { S s; };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kA }; struct S { E e; };',
-        '[Extensible] enum E { kA, kB }; struct S { E e; };')
+        '[Extensible] enum E { [Default] kA }; struct S { E e; };',
+        '[Extensible] enum E { [Default] kA, kB }; struct S { E e; };')
 
   def testNewStructFieldWithInvalidMinVersion(self):
     """Adding a new field using an existing MinVersion breaks backward-
@@ -305,14 +319,17 @@ class VersionCompatibilityTest(MojomParserTestCase):
         'struct S {}; union U { S s; };',
         'struct S { [MinVersion=1] int32 x; }; union U { S s; };')
     self.assertBackwardCompatible(
-        '[Extensible] enum E { kA }; union U { E e; };',
-        '[Extensible] enum E { kA, [MinVersion=1] kB }; union U { E e; };')
+        '[Extensible] enum E { [Default] kA }; union U { E e; };',
+        """[Extensible] enum E {
+          [Default] kA,
+          [MinVersion=1] kB };
+          union U { E e; };""")
     self.assertNotBackwardCompatible(
         'struct S {}; union U { S s; };',
         'struct S { int32 x; }; union U { S s; };')
     self.assertNotBackwardCompatible(
-        '[Extensible] enum E { kA }; union U { E e; };',
-        '[Extensible] enum E { kA, kB }; union U { E e; };')
+        '[Extensible] enum E { [Default] kA }; union U { E e; };',
+        '[Extensible] enum E { [Default] kA, kB }; union U { E e; };')
 
   def testNewUnionFieldWithInvalidMinVersion(self):
     """Adding a new field using an existing MinVersion breaks backward-