SI-7981 toSeq on immutable Map and Set return ArrayBuffer

Switched to `Vector` for the default `MapLike`/`SetLike`, then back again in `mutable.MapLike`/`mutable.SetLike`.

Preliminary performance testing indicates an across-the-board improvement due to cutting out a great deal of indirection.  In the case of empty immutable maps or sets, the improvement is enormous: it quickly returns Vector.empty instead of jumping through all sorts of hoops to eventually create an empty `ArrayBuffer`.

One possible downside is that `Vector` is pretty memory-hungry for small collections.  Need to evalute this; rather than abandoning this change, it's probably better to make `Vector` smarter about small collections.

Also fixed test with literal "ArrayBuffer" in output (`s/ArrayBuffer/Vector/g`).

1 files changed, 12 insertions, 3 deletions

diff --git a/src/library/scala/collection/SetLike.scala b/src/library/scala/collection/SetLike.scala
index 3e549f72cd..80a344e6a8 100644
--- a/src/library/scala/collection/SetLike.scala
+++ b/src/library/scala/collection/SetLike.scala
@@ -77,11 +77,20 @@ self =>
 
   protected[this] override def parCombiner = ParSet.newCombiner[A]
 
-  /* Overridden for efficiency. */
-  override def toSeq: Seq[A] = toBuffer[A]
+  // Default collection type appropriate for immutable collections; mutable collections override this
+  override def toSeq: Seq[A] = {
+    if (isEmpty) Vector.empty[A]
+    else {
+      val vb = Vector.newBuilder[A]
+      foreach(vb += _)
+      vb.result
+    }
+  }
+  
   override def toBuffer[A1 >: A]: mutable.Buffer[A1] = {
     val result = new mutable.ArrayBuffer[A1](size)
-    copyToBuffer(result)
+    // Faster to let the map iterate itself than to defer through copyToBuffer
+    foreach(result += _)
     result
   }