chore(generator): guard memorymarshal helpers for unity

Wrap MemoryMarshal.GetArrayDataReference usage behind UNITY_2020_2_OR_NEWER so older Unity runtimes fall back to array indexing.

Co-authored-by: OpenAI Codex <codex@openai.com>

Author: JasonXuDeveloper

src/Nino.Core/TypeCollector.cs

@@ -39,16 +39,16 @@ public class ListView<T>
         [SuppressMessage("ReSharper", "InconsistentNaming")]
         public class DictionaryView<TKey, TValue>
         {
-            public int[]? _buckets; // Do not rename (binary serialization)
-            public Entry[]? _entries; // Do not rename (binary serialization)
+            public int[] _buckets; // Do not rename (binary serialization)
+            public Entry[] _entries; // Do not rename (binary serialization)
             public int _count; // Do not rename (binary serialization)
             public int _version; // Do not rename (binary serialization)
             public int _freeList; // Do not rename (binary serialization)
             public int _freeCount; // Do not rename (binary serialization)
-            public IEqualityComparer<TKey>? _comparer; // Do not rename (binary serialization)
-            public Dictionary<TKey, TValue>.KeyCollection? _keys; // Do not rename (binary serialization)
-            public Dictionary<TKey, TValue>.ValueCollection? _values; // Do not rename (binary serialization)
-            public object? _syncRoot; // Do not rename (binary serialization)
+            public IEqualityComparer<TKey> _comparer; // Do not rename (binary serialization)
+            public Dictionary<TKey, TValue>.KeyCollection _keys; // Do not rename (binary serialization)
+            public Dictionary<TKey, TValue>.ValueCollection _values; // Do not rename (binary serialization)
+            public object _syncRoot; // Do not rename (binary serialization)
 
             [SuppressMessage("ReSharper", "InconsistentNaming")]
             public struct Entry

src/Nino.Generator/BuiltInType/ArrayGenerator.cs

@@ -82,7 +82,15 @@ protected override void GenerateSerializer(ITypeSymbol typeSymbol, Writer writer
             writer.AppendLine("    int cnt = value.Length;");
             writer.AppendLine("    writer.Write(TypeCollector.GetCollectionHeader(cnt));");
             writer.AppendLine();
-            writer.AppendLine($"    ref var cur = ref System.Runtime.InteropServices.MemoryMarshal.GetArrayDataReference(value);");
+            writer.AppendLine("#if !UNITY_2020_2_OR_NEWER");
+            writer.AppendLine("    ref var cur = ref System.Runtime.InteropServices.MemoryMarshal.GetArrayDataReference(value);");
+            writer.AppendLine("#else");
+            writer.AppendLine("    if (cnt == 0)");
+            writer.AppendLine("    {");
+            writer.AppendLine("        return;");
+            writer.AppendLine("    }");
+            writer.AppendLine("    ref var cur = ref value[0];");
+            writer.AppendLine("#endif");
             writer.AppendLine("    ref var end = ref System.Runtime.CompilerServices.Unsafe.Add(ref cur, cnt);");
             writer.AppendLine("    while (!System.Runtime.CompilerServices.Unsafe.AreSame(ref cur, ref end))");
             writer.AppendLine("    {");

src/Nino.Generator/BuiltInType/DictionaryGenerator.cs

@@ -109,8 +109,16 @@ protected override void GenerateSerializer(ITypeSymbol typeSymbol, Writer writer
             writer.AppendLine("        return;");
             writer.AppendLine("    }");
             writer.AppendLine("    int count = dict._count;");
+            writer.AppendLine("    if (count == 0)");
+            writer.AppendLine("    {");
+            writer.AppendLine("        return;");
+            writer.AppendLine("    }");
             writer.AppendLine("    // Iterate entries via direct ref to avoid bounds checks");
+            writer.AppendLine("#if !UNITY_2020_2_OR_NEWER");
             writer.AppendLine("    ref var entryRef = ref System.Runtime.InteropServices.MemoryMarshal.GetArrayDataReference(entries);");
+            writer.AppendLine("#else");
+            writer.AppendLine("    ref var entryRef = ref entries[0];");
+            writer.AppendLine("#endif");
             writer.AppendLine("    int index = 0;");
             writer.AppendLine("    while ((uint)index < (uint)count)");
             writer.AppendLine("    {");