Add tests verifying fast-path semantics for nullable structs

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/ContextClasses.cs

@@ -46,6 +46,7 @@ public interface ITestContext
         public JsonTypeInfo<StructWithCustomConverterPropertyFactory> StructWithCustomConverterPropertyFactory { get; }
         public JsonTypeInfo<ClassWithBadCustomConverter> ClassWithBadCustomConverter { get; }
         public JsonTypeInfo<StructWithBadCustomConverter> StructWithBadCustomConverter { get; }
+        public JsonTypeInfo<PersonStruct?> NullablePersonStruct { get; }
     }
 
     internal partial class JsonContext : JsonSerializerContext

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MetadataAndSerializationContextTests.cs

@@ -40,6 +40,7 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory))]
     [JsonSerializable(typeof(ClassWithBadCustomConverter))]
     [JsonSerializable(typeof(StructWithBadCustomConverter))]
+    [JsonSerializable(typeof(PersonStruct?))]
     internal partial class MetadataAndSerializationContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Default;
@@ -85,6 +86,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.NotNull(MetadataAndSerializationContext.Default.StructWithCustomConverterPropertyFactory);
             Assert.Throws<InvalidOperationException>(() => MetadataAndSerializationContext.Default.ClassWithBadCustomConverter);
             Assert.Throws<InvalidOperationException>(() => MetadataAndSerializationContext.Default.StructWithBadCustomConverter);
+            Assert.Null(MetadataAndSerializationContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.NotNull(MetadataAndSerializationContext.Default.PersonStruct.SerializeHandler);
         }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MetadataContextTests.cs

@@ -39,6 +39,7 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory), GenerationMode = JsonSourceGenerationMode.Metadata)]
     [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
     [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata)]
+    [JsonSerializable(typeof(PersonStruct?), GenerationMode = JsonSourceGenerationMode.Metadata)]
     internal partial class MetadataWithPerTypeAttributeContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Metadata;
@@ -83,6 +84,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MetadataWithPerTypeAttributeContext.Default.StructWithCustomConverterPropertyFactory.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MetadataWithPerTypeAttributeContext.Default.ClassWithBadCustomConverter.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MetadataWithPerTypeAttributeContext.Default.StructWithBadCustomConverter.SerializeHandler);
+            Assert.Null(MetadataWithPerTypeAttributeContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.Null(MetadataWithPerTypeAttributeContext.Default.PersonStruct.SerializeHandler);
         }
     }
 
@@ -120,6 +123,7 @@ public override void EnsureFastPathGeneratedAsExpected()
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory))]
     [JsonSerializable(typeof(ClassWithBadCustomConverter))]
     [JsonSerializable(typeof(StructWithBadCustomConverter))]
+    [JsonSerializable(typeof(PersonStruct?))]
     internal partial class MetadataContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Metadata;
@@ -187,6 +191,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MetadataContext.Default.StructWithCustomConverterPropertyFactory.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MetadataContext.Default.ClassWithBadCustomConverter.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MetadataContext.Default.StructWithBadCustomConverter.SerializeHandler);
+            Assert.Null(MetadataContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.Null(MetadataContext.Default.PersonStruct.SerializeHandler);
         }
 
         [Fact]

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/MixedModeContextTests.cs

@@ -40,6 +40,7 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(PersonStruct?), GenerationMode = JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization)]
     internal partial class MixedModeContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Metadata | JsonSourceGenerationMode.Serialization;
@@ -85,6 +86,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(MixedModeContext.Default.StructWithCustomConverterPropertyFactory.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MixedModeContext.Default.ClassWithBadCustomConverter.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => MixedModeContext.Default.StructWithBadCustomConverter.SerializeHandler);
+            Assert.Null(MixedModeContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.NotNull(MixedModeContext.Default.PersonStruct.SerializeHandler);
         }
 
         [Fact]

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/RealWorldContextTests.cs

@@ -864,5 +864,22 @@ public void PropertyOrdering()
             string json = JsonSerializer.Serialize(obj, DefaultContext.MyTypeWithPropertyOrdering);
             Assert.Equal("{\"C\":0,\"B\":0,\"A\":0}", json);
         }
+
+        [Fact]
+        public virtual void NullableStruct()
+        {
+            PersonStruct? person = new()
+            {
+                FirstName = "Jane",
+                LastName = "Doe"
+            };
+
+            string json = JsonSerializer.Serialize(person, DefaultContext.NullablePersonStruct);
+            JsonTestHelper.AssertJsonEqual(@"{""FirstName"":""Jane"",""LastName"":""Doe""}", json);
+
+            person = JsonSerializer.Deserialize(json, DefaultContext.NullablePersonStruct);
+            Assert.Equal("Jane", person.Value.FirstName);
+            Assert.Equal("Doe", person.Value.LastName);
+        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/SerializationContextTests.cs

@@ -40,6 +40,7 @@ namespace System.Text.Json.SourceGeneration.Tests
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory))]
     [JsonSerializable(typeof(ClassWithBadCustomConverter))]
     [JsonSerializable(typeof(StructWithBadCustomConverter))]
+    [JsonSerializable(typeof(PersonStruct?))]
     internal partial class SerializationContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Serialization;
@@ -78,6 +79,7 @@ internal partial class SerializationContext : JsonSerializerContext, ITestContex
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(PersonStruct?), GenerationMode = JsonSourceGenerationMode.Serialization)]
     internal partial class SerializationWithPerTypeAttributeContext : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Serialization;
@@ -117,6 +119,7 @@ internal partial class SerializationWithPerTypeAttributeContext : JsonSerializer
     [JsonSerializable(typeof(StructWithCustomConverterPropertyFactory), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(ClassWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
     [JsonSerializable(typeof(StructWithBadCustomConverter), GenerationMode = JsonSourceGenerationMode.Serialization)]
+    [JsonSerializable(typeof(PersonStruct?), GenerationMode = JsonSourceGenerationMode.Serialization)]
     internal partial class SerializationContextWithCamelCase : JsonSerializerContext, ITestContext
     {
         public JsonSourceGenerationMode JsonSourceGenerationMode => JsonSourceGenerationMode.Serialization;
@@ -164,6 +167,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(SerializationContext.Default.StructWithCustomConverterProperty.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => SerializationContext.Default.ClassWithBadCustomConverter.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => SerializationContext.Default.StructWithBadCustomConverter.SerializeHandler);
+            Assert.Null(SerializationContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.NotNull(SerializationContext.Default.PersonStruct.SerializeHandler);
         }
 
         [Fact]
@@ -431,6 +436,21 @@ public void OnSerializeCallbacks()
             Assert.Equal("{\"MyProperty\":\"Before\"}", json);
             Assert.Equal("After", obj.MyProperty);
         }
+
+        [Fact]
+        public override void NullableStruct()
+        {
+            PersonStruct? person = new()
+            {
+                FirstName = "Jane",
+                LastName = "Doe"
+            };
+
+            string json = JsonSerializer.Serialize(person, DefaultContext.NullablePersonStruct);
+            JsonTestHelper.AssertJsonEqual(@"{""FirstName"":""Jane"",""LastName"":""Doe""}", json);
+
+            Assert.Throws<InvalidOperationException>(() => JsonSerializer.Deserialize(json, DefaultContext.NullablePersonStruct));
+        }
     }
 
     public sealed class SerializationWithPerTypeAttributeContextTests : SerializationContextTests
@@ -470,6 +490,8 @@ public override void EnsureFastPathGeneratedAsExpected()
             Assert.Null(SerializationWithPerTypeAttributeContext.Default.StructWithCustomConverterPropertyFactory.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => SerializationWithPerTypeAttributeContext.Default.ClassWithBadCustomConverter.SerializeHandler);
             Assert.Throws<InvalidOperationException>(() => SerializationWithPerTypeAttributeContext.Default.StructWithBadCustomConverter.SerializeHandler);
+            Assert.Null(SerializationWithPerTypeAttributeContext.Default.NullablePersonStruct.SerializeHandler);
+            Assert.NotNull(SerializationWithPerTypeAttributeContext.Default.PersonStruct.SerializeHandler);
         }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/SerializationLogicTests.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Text.Encodings.Web;
 using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
 using Xunit;
 
 namespace System.Text.Json.SourceGeneration.Tests
@@ -123,5 +124,70 @@ public static void WriterIsFlushedAtRootCall()
             Assert.Equal(18, writer.BytesCommitted);
             Assert.Equal(0, writer.BytesPending);
         }
+
+        [Fact]
+        public static void FastPathInvokedForNullableUnderlyingType()
+        {
+            PersonStruct? person = new()
+            {
+                FirstName = "Jane",
+                LastName = "Doe"
+            };
+
+            NullablePersonContext context = new();
+            Assert.False(context.FastPathCalled);
+            string json = JsonSerializer.Serialize(person, context.NullablePersonStruct);
+            Assert.True(context.FastPathCalled);
+            JsonTestHelper.AssertJsonEqual(@"{""FirstName"":""Jane"",""LastName"":""Doe""}", json);
+        }
+
+        internal partial class NullablePersonContext : JsonSerializerContext
+        {
+            private static JsonSerializerOptions s_options = new JsonSerializerOptions();
+
+            public bool FastPathCalled { get; private set; }
+
+            public NullablePersonContext() : base(s_options)
+            {
+            }
+
+            protected override JsonSerializerOptions? GeneratedSerializerOptions => s_options;
+
+            public JsonTypeInfo<PersonStruct?> NullablePersonStruct =>
+                JsonMetadataServices.CreateValueInfo<PersonStruct?>(s_options, JsonMetadataServices.GetNullableConverter(underlyingTypeInfo: PersonStruct));
+
+            public JsonTypeInfo<PersonStruct> PersonStruct
+            {
+                get
+                {
+                    var objectInfo = new JsonObjectInfoValues<PersonStruct>()
+                    {
+                        ObjectCreator = static () => new PersonStruct(),
+                        SerializeHandler = PersonStructSerializeHandler
+                    };
+
+                    return JsonMetadataServices.CreateObjectInfo(s_options, objectInfo);
+                }
+            }
+
+            private void PersonStructSerializeHandler(Utf8JsonWriter writer, PersonStruct value)
+            {
+                FastPathCalled = true;
+                writer.WriteStartObject();
+                writer.WriteString("FirstName", value.FirstName);
+                writer.WriteString("LastName", value.LastName);
+                writer.WriteEndObject();
+            }
+
+            public override JsonTypeInfo? GetTypeInfo(Type type)
+            {
+                if (type == typeof(PersonStruct))
+                {
+                    return PersonStruct;
+                }
+
+                return null;
+            }
+        }
     }
 }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/TestClasses.cs

@@ -151,4 +151,10 @@ public class JsonMessage
     }
 
     internal struct MyStruct { }
+
+    public struct PersonStruct
+    {
+        public string FirstName { get; set; }
+        public string LastName { get; set; }
+    }
 }