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/MapLike.scala b/src/library/scala/collection/MapLike.scala
index 3de30c2c8b..38a598321f 100644
--- a/src/library/scala/collection/MapLike.scala
+++ b/src/library/scala/collection/MapLike.scala
@@ -323,11 +323,20 @@ self =>
     res
   }
 
-  /* Overridden for efficiency. */
-  override def toSeq: Seq[(A, B)] = toBuffer[(A, B)]
+  override def toSeq: Seq[(A, B)] = {
+    if (isEmpty) Vector.empty[(A, B)]
+    else {
+      // Default appropriate for immutable collections; mutable collections override this
+      val vb = Vector.newBuilder[(A, B)]
+      foreach(vb += _)
+      vb.result
+    }
+  }  
+  
   override def toBuffer[C >: (A, B)]: mutable.Buffer[C] = {
     val result = new mutable.ArrayBuffer[C](size)
-    copyToBuffer(result)
+    // Faster to let the map iterate itself than to defer through copyToBuffer
+    foreach(result += _)
     result
   }