perf: Optimize source apply when empty or one elem seq

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowFlatMapPrefixSpec.scala

@@ -20,6 +20,7 @@ import akka.stream.Attributes.Attribute
 import akka.stream.stage.{ GraphStage, GraphStageLogic, InHandler, OutHandler }
 import akka.stream.testkit.{ StreamSpec, TestPublisher, TestSubscriber }
 import akka.stream.testkit.Utils.TE
+import akka.stream.testkit.scaladsl.TestSource
 
 // Debug loglevel to diagnose https://github.com/akka/akka-core/issues/30469
 class FlowFlatMapPrefixSpec extends StreamSpec("akka.loglevel = debug") {
@@ -654,13 +655,14 @@ class FlowFlatMapPrefixSpec extends StreamSpec("akka.loglevel = debug") {
       }
 
       "complete when downstream cancels before pulling and upstream does not produce" in {
-        val fSeq = Source(List.empty[Int])
+        val (probe, fSeq) = TestSource[Int]()
           .flatMapPrefixMat(1) { prefix =>
             Flow[Int].mapMaterializedValue(_ => prefix)
-          }(Keep.right)
-          .to(Sink.cancelled)
+          }(Keep.both)
+          .toMat(Sink.cancelled)(Keep.left)
           .withAttributes(attributes)
           .run()
+        probe.sendComplete()
 
         if (att.propagateToNestedMaterialization) {
           fSeq.futureValue should equal(Nil)
@@ -670,7 +672,7 @@ class FlowFlatMapPrefixSpec extends StreamSpec("akka.loglevel = debug") {
       }
 
       "complete when downstream cancels before pulling and upstream does not produce, prefix=0" in {
-        val fSeq = Source(List.empty[Int])
+        val fSeq = TestSource[Int]()
           .flatMapPrefixMat(0) { prefix =>
             Flow[Int].mapMaterializedValue(_ => prefix)
           }(Keep.right)

akka-stream-tests/src/test/scala/akka/stream/scaladsl/SourceSpec.scala

@@ -84,6 +84,35 @@ class SourceSpec extends StreamSpec with DefaultTimeout {
     }
   }
 
+  "Source from iterable" must {
+    "produce optimized source for no elements" in {
+      val source = Source(Nil)
+      source should ===(Source.empty)
+      val result = source.runWith(Sink.seq)
+      result.futureValue should ===(Seq.empty)
+    }
+
+    "produce optimized source for one element Vector" in {
+      val source = Source(Vector(1))
+      val result = source.runWith(Sink.seq)
+      result.futureValue should ===(immutable.Seq(1))
+      source.getAttributes.nameLifted should ===(Some("singleSource"))
+    }
+
+    "produce optimized source for one element List" in {
+      val source = Source(List(1))
+      val result = source.runWith(Sink.seq)
+      result.futureValue should ===(immutable.Seq(1))
+      source.getAttributes.nameLifted should ===(Some("singleSource"))
+    }
+
+    "produce all elements fed to it" in {
+      val source = Source(List(1, 2, 3))
+      val result = source.runWith(Sink.seq)
+      result.futureValue should ===(immutable.Seq(1, 2, 3))
+    }
+  }
+
   "Composite Source" must {
     "merge from many inputs" in {
       val probes = immutable.Seq.fill(5)(TestPublisher.manualProbe[Int]())

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

@@ -353,7 +353,17 @@ object Source {
    * beginning) regardless of when they subscribed.
    */
   def apply[T](iterable: immutable.Iterable[T]): Source[T, NotUsed] =
-    fromGraph(new IterableSource[T](iterable))
+    iterable match {
+      case Nil         => empty
+      case head :: Nil => single(head)
+      case s: IterableOnce[T] =>
+        s.knownSize match {
+          case 0 => empty
+          case 1 => single(s.head)
+          case _ => fromGraph(new IterableSource[T](iterable))
+        }
+      case _ => fromGraph(new IterableSource[T](iterable))
+    }
 
   /**
    * Starts a new `Source` from the given `Future`. The stream will consist of