Runtime support for Protobuf Editions in C#.

PiperOrigin-RevId: 611246775

csharp/generate_protos.sh

@@ -73,6 +73,8 @@ $PROTOC -Isrc -I. \
     src/google/protobuf/unittest_well_known_types.proto \
     src/google/protobuf/test_messages_proto3.proto \
     src/google/protobuf/test_messages_proto2.proto \
+    src/google/protobuf/unittest_features.proto \
+    src/google/protobuf/unittest_legacy_features.proto \
     src/google/protobuf/unittest_proto3_optional.proto \
     src/google/protobuf/unittest_retention.proto
 

csharp/src/Google.Protobuf.Test/Reflection/DescriptorsTest.cs

@@ -8,12 +8,15 @@
 #endregion
 
 using Google.Protobuf.TestProtos;
+using LegacyFeaturesUnittest;
 using NUnit.Framework;
 using ProtobufUnittest;
 using System;
 using System.Collections.Generic;
 using System.Linq;
 using UnitTest.Issues.TestProtos;
+using static Google.Protobuf.Reflection.FeatureSet.Types;
+using proto2 = Google.Protobuf.TestProtos.Proto2;
 
 namespace Google.Protobuf.Reflection
 {
@@ -454,6 +457,65 @@ namespace Google.Protobuf.Reflection
               UnittestRetentionExtensions.SourceRetentionOption));
         }
 
+        [Test]
+        public void GetOptionsStripsFeatures()
+        {
+            var messageDescriptor = TestEditionsMessage.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("required_field");
+            // Note: ideally we'd test GetOptions() for other descriptor types as well, but that requires
+            // non-fields with features applied.
+            Assert.Null(fieldDescriptor.GetOptions().Features);
+        }
+
+        [Test]
+        public void LegacyRequiredTransform()
+        {
+            var messageDescriptor = TestEditionsMessage.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("required_field");
+            Assert.True(fieldDescriptor.IsRequired);
+        }
+
+        [Test]
+        public void LegacyGroupTransform()
+        {
+            var messageDescriptor = TestEditionsMessage.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("delimited_field");
+            Assert.AreEqual(FieldType.Group, fieldDescriptor.FieldType);
+        }
+
+        [Test]
+        public void LegacyInferRequired()
+        {
+            var messageDescriptor = proto2::TestRequired.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("a");
+            Assert.AreEqual(FieldPresence.LegacyRequired, fieldDescriptor.Features.FieldPresence);
+        }
+
+        [Test]
+        public void LegacyInferGroup()
+        {
+            var messageDescriptor = proto2::TestAllTypes.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("optionalgroup");
+            Assert.AreEqual(MessageEncoding.Delimited, fieldDescriptor.Features.MessageEncoding);
+        }
+
+        [Test]
+        public void LegacyInferProto2Packed()
+        {
+            var messageDescriptor = proto2::TestPackedTypes.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("packed_int32");
+            Assert.AreEqual(RepeatedFieldEncoding.Packed, fieldDescriptor.Features.RepeatedFieldEncoding);
+        }
+
+        [Test]
+        public void LegacyInferProto3Expanded()
+        {
+            var messageDescriptor = TestUnpackedTypes.Descriptor;
+            var fieldDescriptor = messageDescriptor.FindFieldByName("unpacked_int32");
+            Assert.NotNull(fieldDescriptor);
+            Assert.AreEqual(RepeatedFieldEncoding.Expanded, fieldDescriptor.Features.RepeatedFieldEncoding);
+        }
+
         private static void TestDescriptorToProto(Func<IMessage> toProtoFunction, IMessage expectedProto)
         {
             var clone1 = toProtoFunction();

csharp/src/Google.Protobuf.Test/Reflection/FeatureInheritanceTest.cs

@@ -0,0 +1,373 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+#endregion
+
+using Google.Protobuf.Reflection;
+using NUnit.Framework;
+using Pb;
+using static Google.Protobuf.Reflection.FieldDescriptorProto.Types;
+using Type = Google.Protobuf.Reflection.FieldDescriptorProto.Types.Type;
+
+namespace Google.Protobuf.Test.Reflection;
+
+public class FeatureInheritanceTest
+{
+    // Note: there's no test for file defaults, as we don't have the same access to modify the
+    // global defaults in C# that exists in Java.
+
+    [Test]
+    public void FileOverrides()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.Features));
+    }
+
+    [Test]
+    public void FileMessageInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].Features));
+    }
+
+    [Test]
+    public void FileMessageOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        SetTestFeature(fileProto.MessageType[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].Features));
+    }
+
+    [Test]
+    public void FileEnumInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.EnumTypes[0].Features));
+    }
+
+    [Test]
+    public void FileEnumOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        SetTestFeature(fileProto.EnumType[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.EnumTypes[0].Features));
+    }
+
+    [Test]
+    public void FileExtensionInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.EnumTypes[0].Features));
+    }
+
+    [Test]
+    public void FileExtensionOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        SetTestFeature(fileProto.Extension[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.Extensions.UnorderedExtensions[0].Features));
+    }
+
+    [Test]
+    public void FileServiceInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.Services[0].Features));
+    }
+
+    [Test]
+    public void FileServiceOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto, 3);
+        SetTestFeature(fileProto.Service[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.Services[0].Features));
+    }
+
+    [Test]
+    public void MessageFieldInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].Fields.InFieldNumberOrder()[0].Features));
+    }
+
+    [Test]
+    public void MessageFieldOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].Field[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].Fields.InFieldNumberOrder()[0].Features));
+    }
+
+    [Test]
+    public void MessageEnumInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].EnumTypes[0].Features));
+    }
+
+    [Test]
+    public void MessageEnumOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].EnumType[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].EnumTypes[0].Features));
+    }
+
+    [Test]
+    public void MessageMessageInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].NestedTypes[0].Features));
+    }
+
+    [Test]
+    public void MessageMessageOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].NestedType[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].NestedTypes[0].Features));
+    }
+
+    [Test]
+    public void MessageExtensionInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].Extensions.UnorderedExtensions[0].Features));
+    }
+
+    [Test]
+    public void MessageExtensionOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].Extension[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].Extensions.UnorderedExtensions[0].Features));
+    }
+
+    [Test]
+    public void MessageOneofInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].Oneofs[0].Features));
+    }
+
+    [Test]
+    public void MessageOneofOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].OneofDecl[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].Oneofs[0].Fields[0].Features));
+    }
+
+    [Test]
+    public void OneofFieldInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.MessageTypes[0].Oneofs[0].Fields[0].Features));
+    }
+
+    [Test]
+    public void OneofFieldOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.MessageType[0].OneofDecl[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.MessageTypes[0].Oneofs[0].Features));
+    }
+
+    [Test]
+    public void EnumValueInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.EnumType[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.EnumTypes[0].Values[0].Features));
+    }
+
+    [Test]
+    public void EnumValueOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.MessageType[0], 3);
+        SetTestFeature(fileProto.EnumType[0].Value[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.EnumTypes[0].Values[0].Features));
+    }
+
+    [Test]
+    public void ServiceMethodInherit()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.Service[0], 3);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(3, GetTestFeature(fileDescriptor.Services[0].Methods[0].Features));
+    }
+
+    [Test]
+    public void ServiceMethodOverride()
+    {
+        var fileProto = CreateFileProto();
+        SetTestFeature(fileProto.Service[0], 3);
+        SetTestFeature(fileProto.Service[0].Method[0], 5);
+        var fileDescriptor = Build(fileProto);
+        Assert.AreEqual(5, GetTestFeature(fileDescriptor.Services[0].Methods[0].Features));
+    }
+
+    private static int GetTestFeature(FeatureSetDescriptor features) =>
+        (features.Proto.GetExtension(UnittestFeaturesExtensions.Test) ?? new TestFeatures()).IntMultipleFeature;
+
+    private static void SetTestFeature(FileDescriptorProto proto, int value)
+    {
+        proto.Options ??= new FileOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(DescriptorProto proto, int value)
+    {
+        proto.Options ??= new MessageOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(EnumDescriptorProto proto, int value)
+    {
+        proto.Options ??= new EnumOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(EnumValueDescriptorProto proto, int value)
+    {
+        proto.Options ??= new EnumValueOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(FieldDescriptorProto proto, int value)
+    {
+        proto.Options ??= new FieldOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(ServiceDescriptorProto proto, int value)
+    {
+        proto.Options ??= new ServiceOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(OneofDescriptorProto proto, int value)
+    {
+        proto.Options ??= new OneofOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(MethodDescriptorProto proto, int value)
+    {
+        proto.Options ??= new MethodOptions();
+        proto.Options.Features ??= new FeatureSet();
+        SetTestFeature(proto.Options.Features, value);
+    }
+
+    private static void SetTestFeature(FeatureSet features, int value) =>
+        features.SetExtension(UnittestFeaturesExtensions.Test, new TestFeatures { IntMultipleFeature = value });
+
+    private static FileDescriptor Build(FileDescriptorProto fileProto) =>
+        FileDescriptor.BuildFromByteStrings(new[] { fileProto.ToByteString() }, new ExtensionRegistry { UnittestFeaturesExtensions.Test })[0];
+
+    private static FileDescriptorProto CreateFileProto() => new FileDescriptorProto
+    {
+        Name = "some/filename/some.proto",
+        Package = "proto2_unittest",
+        Edition = Edition._2023,
+        Syntax = "editions",
+        Extension =
+        {
+            new FieldDescriptorProto { Name = "top_extension", Number = 10, Type = Type.Int32, Label = Label.Optional, Extendee = ".proto2_unittest.TopMessage" }
+        },
+        EnumType =
+        {
+            new EnumDescriptorProto { Name = "TopEnum", Value = { new EnumValueDescriptorProto { Name = "TOP_VALUE", Number = 0 } } }
+        },
+        MessageType =
+        {
+            new DescriptorProto
+            {
+                Name = "TopMessage",
+                Field =
+                {
+                    new FieldDescriptorProto { Name = "field", Number = 1, Type = Type.Int32, Label = Label.Optional },
+                    new FieldDescriptorProto { Name = "oneof_field", Number = 2, Type = Type.Int32, Label = Label.Optional, OneofIndex = 0 }
+                },
+                Extension =
+                {
+                    new FieldDescriptorProto { Name = "nested_extension", Number = 11, Type = Type.Int32, Label = Label.Optional, Extendee = ".proto2_unittest.TopMessage" }
+                },
+                NestedType =
+                {
+                    new DescriptorProto { Name = "NestedMessage" },
+                },
+                EnumType =
+                {
+                    new EnumDescriptorProto { Name = "NestedEnum", Value = { new EnumValueDescriptorProto { Name = "NESTED_VALUE", Number = 0 } } }
+                },
+                OneofDecl = { new OneofDescriptorProto { Name = "Oneof" } }
+            }
+        },
+        Service =
+        {
+            new ServiceDescriptorProto
+            {
+                Name = "TestService",
+                Method = { new MethodDescriptorProto { Name = "CallMethod", InputType = ".proto2_unittest.TopMessage", OutputType = ".proto2_unittest.TopMessage" } }
+            }
+        }
+    };
+}

csharp/src/Google.Protobuf.Test/Reflection/FeatureSetDescriptorTest.cs

@@ -0,0 +1,37 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+#endregion
+
+using Google.Protobuf.Reflection;
+using NUnit.Framework;
+using System;
+using System.Linq;
+
+namespace Google.Protobuf.Test.Reflection;
+
+public class FeatureSetDescriptorTest
+{
+    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
+    // TODO: Update this automatically.
+    private const string DefaultsBase64 = "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";
+
+    [Test]
+    [TestCase(Edition.Proto2)]
+    [TestCase(Edition.Proto3)]
+    [TestCase(Edition._2023)]
+    public void DefaultsMatchCanonicalSerializedForm(Edition edition)
+    {
+        var canonicalDefaults = FeatureSetDefaults.Parser
+          .WithDiscardUnknownFields(true) // Discard language-specific extensions.
+          .ParseFrom(Convert.FromBase64String(DefaultsBase64));
+        var canonicalEditionDefaults = canonicalDefaults.Defaults.Single(def => def.Edition == edition).Features;
+        var candidateEditionDefaults = FeatureSetDescriptor.GetEditionDefaults(edition).Proto;
+
+        Assert.AreEqual(canonicalEditionDefaults, candidateEditionDefaults);
+    }
+}

csharp/src/Google.Protobuf/MessageExtensions.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 //
@@ -175,11 +175,11 @@ namespace Google.Protobuf
         }
 
         /// <summary>
-        /// Checks if all required fields in a message have values set. For proto3 messages, this returns true
+        /// Checks if all required fields in a message have values set. For proto3 messages, this returns true.
         /// </summary>
         public static bool IsInitialized(this IMessage message)
         {
-            if (message.Descriptor.File.Syntax == Syntax.Proto3)
+            if (message.Descriptor.File.Edition == Edition.Proto3)
             {
                 return true;
             }

csharp/src/Google.Protobuf/Reflection/DescriptorBase.cs

@@ -12,19 +12,31 @@ using System.Diagnostics;
 
 namespace Google.Protobuf.Reflection
 {
+    // Implementation note: The descriptors which don't derive from this class are FileDescriptor
+    // and FeatureSetDescriptor - the latter of which isn't a descriptor in exactly the same way
+    // that the others are anyway.
+
     /// <summary>
     /// Base class for nearly all descriptors, providing common functionality.
     /// </summary>
     [DebuggerDisplay("Type = {GetType().Name,nq}, FullName = {FullName}")]
     public abstract class DescriptorBase : IDescriptor
     {
-        internal DescriptorBase(FileDescriptor file, string fullName, int index)
+        internal DescriptorBase(FileDescriptor file, string fullName, int index, FeatureSetDescriptor features)
         {
             File = file;
             FullName = fullName;
             Index = index;
+            Features = features;
         }
 
+        /// <summary>
+        /// The feature set for this descriptor, including inherited features.
+        /// This is internal as external users should use the properties on individual
+        /// descriptor types (e.g. FieldDescriptor.IsPacked) rather than querying features directly.
+        /// </summary>
+        internal FeatureSetDescriptor Features { get; }
+
         /// <value>
         /// The index of this descriptor within its parent descriptor.
         /// </value>

csharp/src/Google.Protobuf/Reflection/EnumDescriptor.cs

@@ -19,7 +19,7 @@ namespace Google.Protobuf.Reflection
     public sealed class EnumDescriptor : DescriptorBase
     {
         internal EnumDescriptor(EnumDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index, Type clrType)
-            : base(file, file.ComputeFullName(parent, proto.Name), index)
+            : base(file, file.ComputeFullName(parent, proto.Name), index, (parent?.Features ?? file.Features).MergedWith(proto.Options?.Features))
         {
             Proto = proto;
             ClrType = clrType;
@@ -107,7 +107,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public EnumOptions GetOptions() => Proto.Options?.Clone();
+        public EnumOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value enum option for this descriptor

csharp/src/Google.Protobuf/Reflection/EnumValueDescriptor.cs

@@ -19,7 +19,7 @@ namespace Google.Protobuf.Reflection
     {
         internal EnumValueDescriptor(EnumValueDescriptorProto proto, FileDescriptor file,
                                      EnumDescriptor parent, int index)
-            : base(file, parent.FullName + "." + proto.Name, index)
+            : base(file, parent.FullName + "." + proto.Name, index, parent.Features.MergedWith(proto.Options?.Features))
         {
             Proto = proto;
             EnumDescriptor = parent;
@@ -64,7 +64,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public EnumValueOptions GetOptions() => Proto.Options?.Clone();
+        public EnumValueOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value enum value option for this descriptor
@@ -86,4 +97,3 @@ namespace Google.Protobuf.Reflection
         }
     }
 }
- 

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.cs

@@ -0,0 +1,118 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+#endregion
+
+using System;
+using System.Collections.Concurrent;
+using static Google.Protobuf.Reflection.FeatureSet.Types;
+
+namespace Google.Protobuf.Reflection;
+
+/// <summary>
+/// A resolved set of features for a file, message etc.
+/// </summary>
+/// <remarks>
+/// Only features supported by the C# runtime are exposed; currently
+/// all enums in C# are open, and we never perform UTF-8 validation.
+/// If either of those features are ever implemented in this runtime,
+/// the feature settings will be exposed as properties in this class.
+/// </remarks>
+internal sealed class FeatureSetDescriptor
+{
+    private static readonly ConcurrentDictionary<FeatureSet, FeatureSetDescriptor> cache = new();
+
+    // Note: this approach is deliberately chosen to circumvent bootstrapping issues.
+    // This can still be tested using the binary representation.
+    // TODO: Generate this code (as a partial class) from the binary representation.
+    private static readonly FeatureSetDescriptor edition2023Defaults = new FeatureSetDescriptor(
+        new FeatureSet
+        {
+            EnumType = EnumType.Open,
+            FieldPresence = FieldPresence.Explicit,
+            JsonFormat = JsonFormat.Allow,
+            MessageEncoding = MessageEncoding.LengthPrefixed,
+            RepeatedFieldEncoding = RepeatedFieldEncoding.Packed,
+            Utf8Validation = Utf8Validation.Verify,
+        });
+    private static readonly FeatureSetDescriptor proto2Defaults = new FeatureSetDescriptor(
+        new FeatureSet
+        {
+            EnumType = EnumType.Closed,
+            FieldPresence = FieldPresence.Explicit,
+            JsonFormat = JsonFormat.LegacyBestEffort,
+            MessageEncoding = MessageEncoding.LengthPrefixed,
+            RepeatedFieldEncoding = RepeatedFieldEncoding.Expanded,
+            Utf8Validation = Utf8Validation.None,
+        });
+    private static readonly FeatureSetDescriptor proto3Defaults = new FeatureSetDescriptor(
+        new FeatureSet
+        {
+            EnumType = EnumType.Open,
+            FieldPresence = FieldPresence.Implicit,
+            JsonFormat = JsonFormat.Allow,
+            MessageEncoding = MessageEncoding.LengthPrefixed,
+            RepeatedFieldEncoding = RepeatedFieldEncoding.Packed,
+            Utf8Validation = Utf8Validation.Verify,
+        });
+
+    internal static FeatureSetDescriptor GetEditionDefaults(Edition edition) =>
+        edition switch
+        {
+            Edition.Proto2 => proto2Defaults,
+            Edition.Proto3 => proto3Defaults,
+            Edition._2023 => edition2023Defaults,
+            _ => throw new ArgumentOutOfRangeException($"Unsupported edition: {edition}")
+        };
+
+    // Visible for testing. The underlying feature set proto, usually derived during
+    // feature resolution.
+    internal FeatureSet Proto { get; }
+
+    /// <summary>
+    /// Only relevant to fields. Indicates if a field has explicit presence.
+    /// </summary>
+    internal FieldPresence FieldPresence => Proto.FieldPresence;
+
+    /// <summary>
+    /// Only relevant to fields. Indicates how a repeated field should be encoded.
+    /// </summary>
+    internal RepeatedFieldEncoding RepeatedFieldEncoding => Proto.RepeatedFieldEncoding;
+
+    /// <summary>
+    /// Only relevant to fields. Indicates how a message-valued field should be encoded.
+    /// </summary>
+    internal MessageEncoding MessageEncoding => Proto.MessageEncoding;
+
+    private FeatureSetDescriptor(FeatureSet proto)
+    {
+        Proto = proto;
+    }
+
+    /// <summary>
+    /// Returns a new descriptor based on this one, with the specified overrides.
+    /// Multiple calls to this method that produce equivalent feature sets will return
+    /// the same instance.
+    /// </summary>
+    /// <param name="overrides">The proto representation of the "child" feature set to merge with this
+    /// one. May be null, in which case this descriptor is returned.</param>
+    /// <returns>A descriptor based on the current one, with the given set of overrides.</returns>
+    public FeatureSetDescriptor MergedWith(FeatureSet overrides)
+    {
+        if (overrides is null)
+        {
+            return this;
+        }
+
+        // Note: It would be nice if we could avoid cloning unless
+        // there are actual changes, but this won't happen that often;
+        // it'll be temporary garbage.
+        var clone = Proto.Clone();
+        clone.MergeFrom(overrides);
+        return cache.GetOrAdd(clone, clone => new FeatureSetDescriptor(clone));
+    }
+}

csharp/src/Google.Protobuf/Reflection/FieldDescriptor.cs

@@ -62,9 +62,10 @@ namespace Google.Protobuf.Reflection
             : IsRepeated ? false
             : IsMap ? false
             : FieldType == FieldType.Message ? true
+            : FieldType == FieldType.Group ? true
             // This covers "real oneof members" and "proto3 optional fields"
             : ContainingOneof != null ? true
-            : File.Syntax == Syntax.Proto2;
+            : Features.FieldPresence != FeatureSet.Types.FieldPresence.Implicit;
 
         internal FieldDescriptorProto Proto { get; }
 
@@ -83,10 +84,11 @@ namespace Google.Protobuf.Reflection
 
         internal FieldDescriptor(FieldDescriptorProto proto, FileDescriptor file,
                                  MessageDescriptor parent, int index, string propertyName, Extension extension)
-            : base(file, file.ComputeFullName(parent, proto.Name), index)
+            : base(file, file.ComputeFullName(parent, proto.Name), index,
+                  GetDirectParentFeatures(proto, file, parent).MergedWith(InferFeatures(file, proto)).MergedWith(proto.Options?.Features))
         {
             Proto = proto;
-            if (proto.Type != 0)
+            if (proto.HasType)
             {
                 fieldType = GetFieldTypeFromProtoType(proto.Type);
             }
@@ -117,6 +119,52 @@ namespace Google.Protobuf.Reflection
             JsonName =  Proto.JsonName == "" ? JsonFormatter.ToJsonName(Proto.Name) : Proto.JsonName;
         }
 
+        /// <summary>
+        /// Returns the features from the direct parent:
+        /// - The file for top-level extensions
+        /// - The oneof for one-of fields
+        /// - Otherwise the message
+        /// </summary>
+        private static FeatureSetDescriptor GetDirectParentFeatures(FieldDescriptorProto proto, FileDescriptor file, MessageDescriptor parent) =>
+            parent is null ? file.Features
+            // Ignore invalid oneof indexes here; they'll be validated later anyway.
+            : proto.OneofIndex >= 0 && proto.OneofIndex < parent.Proto.OneofDecl.Count ? parent.Oneofs[proto.OneofIndex].Features
+            : parent.Features;
+
+        /// <summary>
+        /// Returns a feature set with inferred features for the given field, or null if no features
+        /// need to be inferred.
+        /// </summary>
+        private static FeatureSet InferFeatures(FileDescriptor file, FieldDescriptorProto proto)
+        {
+            if ((int) file.Edition >= (int) Edition._2023)
+            {
+                return null;
+            }
+            // This is lazily initialized, as most fields won't need it.
+            FeatureSet features = null;
+            if (proto.Label == FieldDescriptorProto.Types.Label.Required)
+            {
+                features ??= new FeatureSet();
+                features.FieldPresence = FeatureSet.Types.FieldPresence.LegacyRequired;
+            }
+            if (proto.Type == FieldDescriptorProto.Types.Type.Group)
+            {
+                features ??= new FeatureSet();
+                features.MessageEncoding = FeatureSet.Types.MessageEncoding.Delimited;
+            }
+            if (file.Edition == Edition.Proto2 && (proto.Options?.Packed ?? false))
+            {
+                features ??= new FeatureSet();
+                features.RepeatedFieldEncoding = FeatureSet.Types.RepeatedFieldEncoding.Packed;
+            }
+            if (file.Edition == Edition.Proto3 && !(proto.Options?.Packed ?? true))
+            {
+                features ??= new FeatureSet();
+                features.RepeatedFieldEncoding = FeatureSet.Types.RepeatedFieldEncoding.Expanded;
+            }
+            return features;
+        }
 
         /// <summary>
         /// The brief name of the descriptor's target.
@@ -185,7 +233,7 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// Returns <c>true</c> if this field is a required field; <c>false</c> otherwise.
         /// </summary>
-        public bool IsRequired => Proto.Label == FieldDescriptorProto.Types.Label.Required;
+        public bool IsRequired => Features.FieldPresence == FeatureSet.Types.FieldPresence.LegacyRequired;
 
         /// <summary>
         /// Returns <c>true</c> if this field is a map field; <c>false</c> otherwise.
@@ -195,21 +243,7 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// Returns <c>true</c> if this field is a packed, repeated field; <c>false</c> otherwise.
         /// </summary>
-        public bool IsPacked
-        {
-            get
-            {
-                if (File.Syntax != Syntax.Proto3)
-                {
-                    return Proto.Options?.Packed ?? false;
-                }
-                else
-                {
-                    // Packed by default with proto3
-                    return Proto.Options == null || !Proto.Options.HasPacked || Proto.Options.Packed;
-                }
-            }
-        }
+        public bool IsPacked => Features.RepeatedFieldEncoding == FeatureSet.Types.RepeatedFieldEncoding.Packed;
 
         /// <summary>
         /// Returns <c>true</c> if this field extends another message type; <c>false</c> otherwise.
@@ -219,7 +253,8 @@ namespace Google.Protobuf.Reflection
         /// <summary>
         /// Returns the type of the field.
         /// </summary>
-        public FieldType FieldType => fieldType;
+        public FieldType FieldType => fieldType == FieldType.Message && Features.MessageEncoding == FeatureSet.Types.MessageEncoding.Delimited
+            ? FieldType.Group : fieldType;
 
         /// <summary>
         /// Returns the field number declared in the proto file.
@@ -299,12 +334,23 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public FieldOptions GetOptions() => Proto.Options?.Clone();
+        public FieldOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value field option for this descriptor
         /// </summary>
-         [Obsolete("GetOption is obsolete. Use the GetOptions() method.")]
+        [Obsolete("GetOption is obsolete. Use the GetOptions() method.")]
         public T GetOption<T>(Extension<FieldOptions, T> extension)
         {
             var value = Proto.Options.GetExtension(extension);
@@ -330,7 +376,8 @@ namespace Google.Protobuf.Reflection
                 IDescriptor typeDescriptor =
                     File.DescriptorPool.LookupSymbol(Proto.TypeName, this);
 
-                if (Proto.HasType)
+                // TODO: See how much of this is actually required.
+                if (!Proto.HasType)
                 {
                     // Choose field type based on symbol.
                     if (typeDescriptor is MessageDescriptor)
@@ -423,4 +470,3 @@ namespace Google.Protobuf.Reflection
         }
     }
 }
- 

csharp/src/Google.Protobuf/Reflection/FileDescriptor.cs

@@ -21,6 +21,7 @@ namespace Google.Protobuf.Reflection
     /// <summary>
     /// The syntax of a .proto file
     /// </summary>
+    [Obsolete("Use features instead")]
     public enum Syntax
     {
         /// <summary>
@@ -32,6 +33,10 @@ namespace Google.Protobuf.Reflection
         /// </summary>
         Proto3,
         /// <summary>
+        /// Editions syntax
+        /// </summary>
+        Editions,
+        /// <summary>
         /// An unknown declared syntax
         /// </summary>
         Unknown
@@ -47,7 +52,9 @@ namespace Google.Protobuf.Reflection
         // Prevent linker failures when using IL2CPP with the well-known types.
         static FileDescriptor()
         {
+#pragma warning disable CS0618 // Type or member is obsolete
             ForceReflectionInitialization<Syntax>();
+#pragma warning restore CS0618 // Type or member is obsolete
             ForceReflectionInitialization<NullValue>();
             ForceReflectionInitialization<Field.Types.Cardinality>();
             ForceReflectionInitialization<Field.Types.Kind>();
@@ -61,6 +68,9 @@ namespace Google.Protobuf.Reflection
             SerializedData = descriptorData;
             DescriptorPool = pool;
             Proto = proto;
+            // Note: the Edition property relies on the proto being set first, so this line
+            // has to come after Proto = proto.
+            Features = FeatureSetDescriptor.GetEditionDefaults(Edition).MergedWith(proto.Options?.Features);
             Dependencies = new ReadOnlyCollection<FileDescriptor>(dependencies.ToList());
 
             PublicDependencies = DeterminePublicDependencies(this, proto, dependencies, allowUnknownDependencies);
@@ -82,19 +92,6 @@ namespace Google.Protobuf.Reflection
             Extensions = new ExtensionCollection(this, generatedCodeInfo?.Extensions);
 
             declarations = new Lazy<Dictionary<IDescriptor, DescriptorDeclaration>>(CreateDeclarationMap, LazyThreadSafetyMode.ExecutionAndPublication);
-
-            if (!proto.HasSyntax || proto.Syntax == "proto2")
-            {
-                Syntax = Syntax.Proto2;
-            }
-            else if (proto.Syntax == "proto3")
-            {
-                Syntax = Syntax.Proto3;
-            }
-            else
-            {
-                Syntax = Syntax.Unknown;
-            }
         }
 
         private Dictionary<IDescriptor, DescriptorDeclaration> CreateDeclarationMap()
@@ -226,9 +223,32 @@ namespace Google.Protobuf.Reflection
         public FileDescriptorProto ToProto() => Proto.Clone();
 
         /// <summary>
-        /// The syntax of the file
+        /// The feature set for this file, including inherited features.
         /// </summary>
-        public Syntax Syntax { get; }
+        internal FeatureSetDescriptor Features { get; }
+
+        /// <summary>
+        /// Returns the edition of the file descriptor.
+        /// </summary>
+        internal Edition Edition => Proto.Syntax switch
+        {
+            "editions" => Proto.Edition,
+            "proto3" => Edition.Proto3,
+            _ => Edition.Proto2
+        };
+
+        /// <summary>
+        /// The syntax of the file.
+        /// </summary>
+        [Obsolete("Use features instead of proto syntax.")]
+        public Syntax Syntax => Proto.HasEdition ?  Syntax.Editions
+            : Proto.Syntax switch
+            {
+                "proto3" => Syntax.Proto3,
+                "proto2" => Syntax.Proto2,
+                "" => Syntax.Proto2,
+                _ => throw new InvalidOperationException("No edition or known syntax present")
+            };
 
         /// <value>
         /// The file name.
@@ -546,7 +566,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public FileOptions GetOptions() => Proto.Options?.Clone();
+        public FileOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value file option for this descriptor

csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs

@@ -13,7 +13,6 @@ using System.Collections.ObjectModel;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-using System.Reflection;
 
 namespace Google.Protobuf.Reflection
 {
@@ -42,7 +41,7 @@ namespace Google.Protobuf.Reflection
         private Func<IMessage, bool> extensionSetIsInitialized;
 
         internal MessageDescriptor(DescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int typeIndex, GeneratedClrTypeInfo generatedCodeInfo)
-            : base(file, file.ComputeFullName(parent, proto.Name), typeIndex)
+            : base(file, file.ComputeFullName(parent, proto.Name), typeIndex, (parent?.Features ?? file.Features).MergedWith(proto.Options?.Features))
         {
             Proto = proto;
             Parser = generatedCodeInfo?.Parser;
@@ -280,7 +279,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public MessageOptions GetOptions() => Proto.Options?.Clone();
+        public MessageOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value message option for this descriptor

csharp/src/Google.Protobuf/Reflection/MethodDescriptor.cs

@@ -17,7 +17,6 @@ namespace Google.Protobuf.Reflection
     /// </summary>
     public sealed class MethodDescriptor : DescriptorBase
     {
-
         /// <value>
         /// The service this method belongs to.
         /// </value>
@@ -55,7 +54,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public MethodOptions GetOptions() => Proto.Options?.Clone();
+        public MethodOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value method option for this descriptor
@@ -78,7 +88,7 @@ namespace Google.Protobuf.Reflection
 
         internal MethodDescriptor(MethodDescriptorProto proto, FileDescriptor file,
                                   ServiceDescriptor parent, int index)
-            : base(file, parent.FullName + "." + proto.Name, index)
+            : base(file, parent.FullName + "." + proto.Name, index, parent.Features.MergedWith(proto.Options?.Features))
         {
             Proto = proto;
             Service = parent;
@@ -118,4 +128,3 @@ namespace Google.Protobuf.Reflection
         }
     }
 }
- 

csharp/src/Google.Protobuf/Reflection/OneofDescriptor.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 //
@@ -27,7 +27,7 @@ namespace Google.Protobuf.Reflection
         private readonly OneofAccessor accessor;
 
         internal OneofDescriptor(OneofDescriptorProto proto, FileDescriptor file, MessageDescriptor parent, int index, string clrName)
-            : base(file, file.ComputeFullName(parent, proto.Name), index)
+            : base(file, file.ComputeFullName(parent, proto.Name), index, parent.Features.MergedWith(proto.Options?.Features))
         {
             this.Proto = proto;
             containingType = parent;
@@ -113,7 +113,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public OneofOptions GetOptions() => Proto.Options?.Clone();
+        public OneofOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value oneof option for this descriptor

csharp/src/Google.Protobuf/Reflection/ServiceDescriptor.cs

@@ -19,7 +19,7 @@ namespace Google.Protobuf.Reflection
     public sealed class ServiceDescriptor : DescriptorBase
     {
         internal ServiceDescriptor(ServiceDescriptorProto proto, FileDescriptor file, int index)
-            : base(file, file.ComputeFullName(null, proto.Name), index)
+            : base(file, file.ComputeFullName(null, proto.Name), index, file.Features.MergedWith(proto.Options?.Features))
         {
             Proto = proto;
             Methods = DescriptorUtil.ConvertAndMakeReadOnly(proto.Method,
@@ -75,7 +75,18 @@ namespace Google.Protobuf.Reflection
         /// Custom options can be retrieved as extensions of the returned message.
         /// NOTE: A defensive copy is created each time this property is retrieved.
         /// </summary>
-        public ServiceOptions GetOptions() => Proto.Options?.Clone();
+        public ServiceOptions GetOptions()
+        {
+            var clone = Proto.Options?.Clone();
+            if (clone is null)
+            {
+                return null;
+            }
+            // Clients should be using feature accessor methods, not accessing features on the
+            // options proto.
+            clone.Features = null;
+            return clone;
+        }
 
         /// <summary>
         /// Gets a single value service option for this descriptor
@@ -105,4 +116,3 @@ namespace Google.Protobuf.Reflection
         }
     }
 }
- 

csharp/src/Google.Protobuf/Reflection/SingleFieldAccessor.cs

@@ -1,4 +1,4 @@
-#region Copyright notice and license
+#region Copyright notice and license
 // Protocol Buffers - Google's data interchange format
 // Copyright 2015 Google Inc.  All rights reserved.
 //
@@ -64,8 +64,9 @@ namespace Google.Protobuf.Reflection
                     }
                 };
             }
-            // Primitive fields always support presence in proto2, and support presence in proto3 for optional fields.
-            else if (descriptor.File.Syntax == Syntax.Proto2 || descriptor.Proto.Proto3Optional)
+            // Anything else that supports presence should have a "HasXyz" property and a "ClearXyz"
+            // method.
+            else if (descriptor.HasPresence)
             {
                 MethodInfo hasMethod = messageType.GetRuntimeProperty("Has" + property.Name).GetMethod;
                 if (hasMethod == null)
@@ -80,8 +81,7 @@ namespace Google.Protobuf.Reflection
                 }
                 clearDelegate = ReflectionUtil.CreateActionIMessage(clearMethod);
             }
-            // What's left?
-            // Primitive proto3 fields without the optional keyword, which aren't in oneofs.
+            // Otherwise, we don't support presence.
             else
             {
                 hasDelegate = message => throw new InvalidOperationException("Presence is not implemented for this field");