GameCult.Thing.Contract: every empty memory is an empty RecordPayload; wire nil is refused on read

Present(ReadOnlyMemory<byte>) is deleted. The constructor copies any view, default
included, as an empty payload; a nil on the wire (which decodes to a default view) is
noted by the constructor and refused in OnAfterDeserialize, so the nil-positions rows hold.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/GameCult.Thing.Contract/Command.cs

@@ -104,16 +104,32 @@ public sealed class CommandDecl
     }
 }
 
+/// <summary>
+/// A typed record as its MessagePack bytes. Every empty memory (<c>ReadOnlyMemory&lt;byte&gt;.Empty</c>, a default view,
+/// an empty array) is an empty payload, as Rust's empty <c>Vec&lt;u8&gt;</c> is. A nil on the wire is not an empty
+/// payload: it decodes to a default view the constructor cannot tell from an empty one, so the constructor notes it and
+/// the decode door refuses it once the value is read.
+/// </summary>
 [MessagePackObject]
-public sealed class RecordPayload
+public sealed class RecordPayload : IMessagePackSerializationCallbackReceiver
 {
+    private readonly bool _viewWasDefault;
+
     [Key(0)] public string Schema { get; }
     [Key(1)] public ReadOnlyMemory<byte> Bytes { get; }
 
     public RecordPayload(string schema, ReadOnlyMemory<byte> bytes)
     {
         Schema = ThingDomain.Present(schema, "Schema");
-        Bytes = ThingDomain.Present(bytes, "Bytes").ToArray();
+        _viewWasDefault = bytes.Equals(default(ReadOnlyMemory<byte>));
+        Bytes = bytes.ToArray();
+    }
+
+    void IMessagePackSerializationCallbackReceiver.OnBeforeSerialize() { }
+
+    void IMessagePackSerializationCallbackReceiver.OnAfterDeserialize()
+    {
+        if (_viewWasDefault) throw new ThingDomainException("Bytes", "present");
     }
 }
 

packages/GameCult.Thing.Contract/Domain.cs

@@ -49,15 +49,6 @@ public static class ThingDomain
     public static T Present<T>(T? value, string slot) where T : class =>
         value ?? throw new ThingDomainException(slot, "present");
 
-    /// <summary>
-    /// A required byte slot. A null on the wire decodes to a default view with no backing store, so the default view is
-    /// refused as nil; an empty array is a present, empty payload.
-    /// </summary>
-    public static ReadOnlyMemory<byte> Present(ReadOnlyMemory<byte> value, string slot) =>
-        value.Equals(default(ReadOnlyMemory<byte>))
-            ? throw new ThingDomainException(slot, "present")
-            : value;
-
     /// <summary>
     /// A required list slot: a read-only copy, so the caller's list cannot change the value afterwards. A null list or
     /// a null item is refused naming the slot.

tests/GameCult.Thing.Contract.Tests/ReadOnlyListTests.cs

@@ -1,3 +1,4 @@
+using MessagePack;
 using System.Collections;
 using System.Reflection;
 
@@ -165,11 +166,31 @@ public sealed class ReadOnlyListTests
     }
 
     [Test]
-    public void ANilPayloadIsRefusedAndAnEmptyOneIsKeptThroughTheWire()
+    public void EveryEmptyMemoryIsAnEmptyPayloadThatWritesAsRustsEmptyVec()
     {
-        var error = Assert.Throws<ThingDomainException>(() => new RecordPayload("s", default))!;
-        Assert.That(error.Domain, Is.EqualTo("Bytes"));
-        Assert.That(error.Bound, Is.EqualTo("present"));
+        // Rust's empty Vec<u8> is bin8 of length 0 (c4 00); a RecordPayload is [schema, bytes].
+        var rust = new byte[] { 0x92, 0xA1, (byte)'s', 0xC4, 0x00 };
+        var empties = new Dictionary<string, ReadOnlyMemory<byte>>
+        {
+            ["ReadOnlyMemory.Empty"] = ReadOnlyMemory<byte>.Empty,
+            ["Memory.Empty"] = Memory<byte>.Empty,
+            ["default"] = default(ReadOnlyMemory<byte>),
+            ["Array.Empty"] = Array.Empty<byte>(),
+            ["new byte[0]"] = new byte[0],
+        };
+        foreach (var (name, memory) in empties)
+        {
+            var payload = new RecordPayload("s", memory);
+            Assert.That(payload.Bytes.Length, Is.EqualTo(0), name);
+            Assert.That(Wire.Write(payload), Is.EqualTo(rust), name);
+            Assert.That(Wire.Write(Wire.Read(typeof(RecordPayload), rust)), Is.EqualTo(rust), name);
+        }
+    }
+
+    [Test]
+    public void ANilPayloadOnTheWireIsRefusedAndAnEmptyOneIsKeptThroughTheWire()
+    {
+        Assert.Throws<MessagePackSerializationException>(() => Wire.Read(typeof(RecordPayload), new byte[] { 0x92, 0xA1, (byte)'s', 0xC0 }));
 
         var empty = new CommandInvocation("i", "k", 1, "go", new RecordPayload("s", Array.Empty<byte>()), new List<CapturedInput>(), "c", "t", null);
         var decoded = (CommandInvocation)Wire.Read(typeof(CommandInvocation), Wire.Write(empty));