Merge pull request #19754 from akka/wip-19732-async-RK

nicer declarative .async boundaries and remove Module.nest()

akka-stream/src/main/scala/akka/stream/javadsl/SubSource.scala

@@ -82,27 +82,6 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def via[T, M](flow: Graph[FlowShape[Out, T], M]): SubSource[T, Mat] =
     new SubSource(delegate.via(flow))
 
-  /**
-   * Transform this [[SubSource]] by appending the given processing steps, ensuring
-   * that an `asyncBoundary` attribute is set around those steps.
-   * {{{
-   *     +----------------------------+
-   *     | Resulting Source           |
-   *     |                            |
-   *     |  +------+        +------+  |
-   *     |  |      |        |      |  |
-   *     |  | this | ~Out~> | flow | ~~> T
-   *     |  |      |        |      |  |
-   *     |  +------+        +------+  |
-   *     +----------------------------+
-   * }}}
-   * The materialized value of the combined [[Flow]] will be the materialized
-   * value of the current flow (ignoring the other Flow’s value), use
-   * [[Flow#viaMat viaMat]] if a different strategy is needed.
-   */
-  def viaAsync[T, M](flow: Graph[FlowShape[Out, T], M]): SubSource[T, Mat] =
-    new SubSource(delegate.viaAsync(flow))
-
   /**
    * Connect this [[SubSource]] to a [[Sink]], concatenating the processing steps of both.
    * This means that all sub-flows that result from the previous sub-stream operator
@@ -1208,6 +1187,12 @@ class SubSource[+Out, +Mat](delegate: scaladsl.SubFlow[Out, Mat, scaladsl.Source
   def named(name: String): SubSource[Out, Mat] =
     new SubSource(delegate.named(name))
 
+  /**
+   * Put an asynchronous boundary around this `SubSource`
+   */
+  def async: SubSource[Out, Mat] =
+    new SubSource(delegate.async)
+
   /**
    * Logs elements flowing through the stream as well as completion and erroring.
    *