JsonSerialize.cs

public ref struct JsonSerializer
{
   private Span<byte> _buffer;
   private int _loc; // increments as we write
   ...
   public void WriteToBuffer(string s) => ...; // moves _loc
   
   public Seq SerializeEnumerable(int? count) => new Seq(ref this);
   
   public ref struct Seq
   {
      private ref JsonSerializer _serializer; // must be ref to preserve mutations
      private bool _start = true;
      
      public Seq(ref JsonSerializer serializer)
      {
        _serializer = serializer;
        serializer.WriteToBuffer("{");
      }
      
      public void WriteItem<T>(T item)
      {
         if (!_start) _serializer.WriteToBuffer(",");
         _serializer.WriteToBuffer(item.ToString());
         _start = false;
      }
      public void End() => _serializer.WriteToBuffer("}");
   }
}