Avoid reflection when constructing row in bsatn deserialization

Author: joshua-spacetime

crates/bindings-csharp/BSATN.Runtime/BSATN/Runtime.cs

@@ -2,6 +2,12 @@ namespace SpacetimeDB.BSATN;
 
 using System.Text;
 
+internal static class StructuralReadWrite<T>
+    where T : IStructuralReadWrite, new()
+{
+    public static readonly System.Func<T> Constructor = () => new T();
+}
+
 public interface IStructuralReadWrite
 {
     void ReadFields(BinaryReader reader);
@@ -11,10 +17,16 @@ public interface IStructuralReadWrite
     static T Read<T>(BinaryReader reader)
         where T : IStructuralReadWrite, new()
     {
-        // TODO: use `RuntimeHelpers.GetUninitializedObject` as an optimisation here.
-        // We tried but couldn't do this because BitCraft relies on being able
-        // to add and initialize custom fields on autogenerated classes.
-        var result = new T();
+        // Previously we attempted to use `GetUninitializedObject` as an optimisation here.
+        // This is because `new T()` can use reflection depending on the runtime, compiler, etc.
+        // And this is a cost that we must pay per deserialized row.
+        // Unfortunately BitCraft relies on being able to add and initialize custom fields on autogenerated classes.
+        // Therefore we couldn't use `GetUninitializedObject`.
+        //
+        // Instead we use a lambda inside a generic static class.
+        // Supposedly this always result in monomorphization.
+        // This may not be the case for generic methods depending on the runtime.
+        var result = StructuralReadWrite<T>.Constructor();
         result.ReadFields(reader);
         return result;
     }