Support for mutable structs as BufferWriter.

src/MemoryPack.Core/MemoryPackSerializer.Serialize.cs

@@ -92,11 +92,12 @@ public static unsafe void Serialize<T, TBufferWriter>(in TBufferWriter bufferWri
         where TBufferWriter : class, IBufferWriter<byte>
 #endif
     {
+        ref var bufferWriterRef = ref Unsafe.AsRef(in bufferWriter);
         if (!RuntimeHelpers.IsReferenceOrContainsReferences<T>())
         {
-            var buffer = bufferWriter.GetSpan(Unsafe.SizeOf<T>());
+            var buffer = bufferWriterRef.GetSpan(Unsafe.SizeOf<T>());
             Unsafe.WriteUnaligned(ref MemoryMarshal.GetReference(buffer), value);
-            bufferWriter.Advance(Unsafe.SizeOf<T>());
+            bufferWriterRef.Advance(Unsafe.SizeOf<T>());
             return;
         }
 #if NET7_0_OR_GREATER
@@ -105,29 +106,30 @@ public static unsafe void Serialize<T, TBufferWriter>(in TBufferWriter bufferWri
         {
             if (value == null)
             {
-                var span = bufferWriter.GetSpan(4);
+                var span = bufferWriterRef.GetSpan(4);
                 MemoryPackCode.NullCollectionData.CopyTo(span);
-                bufferWriter.Advance(4);
+                bufferWriterRef.Advance(4);
                 return;
             }
 
             var srcArray = ((Array)(object)value!);
             var length = srcArray.Length;
             if (length == 0)
             {
-                var span = bufferWriter.GetSpan(4);
+                var span = bufferWriterRef.GetSpan(4);
                 MemoryPackCode.ZeroCollectionData.CopyTo(span);
-                bufferWriter.Advance(4);
+                bufferWriterRef.Advance(4);
                 return;
             }
+
             var dataSize = elementSize * length;
-            var destSpan = bufferWriter.GetSpan(dataSize + 4);
+            var destSpan = bufferWriterRef.GetSpan(dataSize + 4);
             ref var head = ref MemoryMarshal.GetReference(destSpan);
 
             Unsafe.WriteUnaligned(ref head, length);
             Unsafe.CopyBlockUnaligned(ref Unsafe.Add(ref head, 4), ref MemoryMarshal.GetArrayDataReference(srcArray), (uint)dataSize);
 
-            bufferWriter.Advance(dataSize + 4);
+            bufferWriterRef.Advance(dataSize + 4);
             return;
         }
 #endif
@@ -141,7 +143,7 @@ public static unsafe void Serialize<T, TBufferWriter>(in TBufferWriter bufferWri
 
         try
         {
-            var writer = new MemoryPackWriter<TBufferWriter>(ref Unsafe.AsRef(in bufferWriter), state);
+            var writer = new MemoryPackWriter<TBufferWriter>(ref bufferWriterRef, state);
             Serialize(ref writer, value);
         }
         finally

tests/MemoryPack.Tests/SerializerStructBufferWriterTest.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Buffers;
+using MemoryPack.Tests.Models;
+
+namespace MemoryPack.Tests;
+
+public class SerializerStructBufferWriterTest
+{
+    [Fact]
+    public void Serialize_ShouldSupportStructAsBufferWriter_WhenValueIsNotReferenceAndNotContainsReferences()
+    {
+        var writer = new TestBufferWriter();
+        MemoryPackSerializer.Serialize(writer, 16);
+        Assert.Equal(4, writer.WrittenSize);
+    }
+
+    [Fact]
+    public void Serialize_ShouldSupportStructAsBufferWriter_WhenValueIsUnmanagedSZArray()
+    {
+        var writer = new TestBufferWriter();
+        MemoryPackSerializer.Serialize(writer, new UnmanagedStruct[] { new() { X = 1, Y = 2, Z = 3 } });
+        Assert.Equal(16, writer.WrittenSize);
+    }
+
+    [Fact]
+    public void Serialize_ShouldSupportStructAsBufferWriter_WhenFormatterRequired()
+    {
+        var writer = new TestBufferWriter();
+        MemoryPackSerializer.Serialize(writer, new TestData(1));
+        Assert.Equal(5, writer.WrittenSize);
+    }
+}
+
+[MemoryPackable]
+public partial record TestData(int A);
+
+public struct TestBufferWriter : IBufferWriter<byte>
+{
+    public int WrittenSize = 0;
+
+    public TestBufferWriter()
+    {
+    }
+
+    public void Advance(int count)
+    {
+        WrittenSize += count;
+    }
+
+    public Memory<byte> GetMemory(int sizeHint = 0)
+    {
+        throw new InvalidOperationException();
+    }
+
+    public Span<byte> GetSpan(int sizeHint = 0)
+    {
+        return new byte[sizeHint];
+    }
+}