ListMap.last should not generate additional garbage

Use Iterator.empty when iterating IndexedSeq and LinearSeq which are cheaply provably empty

Author: mkeskells

library/src/scala/collection/IndexedSeqLike.scala

@@ -87,7 +87,11 @@ trait IndexedSeqLike[+A, +Repr] extends Any with SeqLike[A, Repr] {
   }
 
   override /*IterableLike*/
-  def iterator: Iterator[A] = new Elements(0, length)
+  def iterator: Iterator[A] = {
+    val len = length
+    if (len == 0) Iterator.empty
+    else new Elements(0, length)
+  }
 
   /* Overridden for efficiency */
   override def toBuffer[A1 >: A]: mutable.Buffer[A1] = {

library/src/scala/collection/LinearSeqLike.scala

@@ -41,7 +41,7 @@ trait LinearSeqLike[+A, +Repr <: LinearSeqLike[A, Repr]] extends SeqLike[A, Repr
   override def hashCode()= scala.util.hashing.MurmurHash3.seqHash(seq) // TODO - can we get faster via "linearSeqHash" ?
 
   override /*IterableLike*/
-  def iterator: Iterator[A] = new AbstractIterator[A] {
+  def iterator: Iterator[A] = if (self.isEmpty) Iterator.empty else new AbstractIterator[A] {
     var these = self
     def hasNext: Boolean = !these.isEmpty
     def next(): A =