Was sind asynchrone Streams in C# 8.0?

static IEnumerable<string> Message(){
   yield return "Hello!";
   yield return "Hello!";
}
Can be replaced by IAsyncEnumerable
static async IAsyncEnumerable<string> MessageAsync(){
   await Task.Delay(2000);
   yield return "Hello!";
   await Task.Delay(2000);
   yield return "Hello!";
}

class Program{
   public static async Task Main(){
      await foreach (var item in MessageAsync()){
         System.Console.WriteLine(item);
      }
      Console.ReadLine();
   }
   static async IAsyncEnumerable<string> MessageAsync(){
      await Task.Delay(2000);
      yield return "Hello!";
      await Task.Delay(2000);
      yield return "Hello!";
   }
}

Hello!
Hello!