+str #16547 "lazyEmpty" source, which allows external triggering of completion

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -177,6 +177,17 @@ object Source {
   def empty[T](): Source[T] = _empty
   private[this] val _empty: Source[Nothing] = apply(EmptyPublisher)
 
+  /**
+   * Create a `Source` with no elements, which does not complete its downstream,
+   * until externally triggered to do so.
+   *
+   * It materializes a [[scala.concurrent.Promise]] which will be completed
+   * when the downstream stage of this source cancels. This promise can also
+   * be used to externally trigger completion, which the source then signalls
+   * to its downstream.
+   */
+  def lazyEmpty[T]() = LazyEmptySource[T]()
+
   /**
    * Create a `Source` that immediately ends the stream with the `cause` error to every connected `Sink`.
    */