Optimisations: avoid List allocations in the RefChecks.

We add some small optimisations to the code in the RefChecks
- We replace the `map length` calls with the utility methods in the
  Collections traits.
- We replace a combined use of flatten, map, zip, and filter with
  the use of an iterator and a special iterator function.
- We add to the Collections utils a function to create a special
  iterator, which combines zip, filter, and collect functions.

src/compiler/scala/tools/nsc/typechecker/RefChecks.scala

@@ -686,16 +686,20 @@ abstract class RefChecks extends Transform {
               val matchingArity      = matchingName filter { m =>
                 !m.isDeferred &&
                 (m.name == underlying.name) &&
-                (m.paramLists.length == abstractParamLists.length) &&
-                (m.paramLists.map(_.length).sum == abstractParamLists.map(_.length).sum) &&
-                (m.tpe.typeParams.size == underlying.tpe.typeParams.size)
+                sameLength(m.paramLists, abstractParamLists) &&
+                sumSize(m.paramLists, 0) == sumSize(abstractParamLists, 0) &&
+                sameLength(m.tpe.typeParams, underlying.tpe.typeParams)
               }
 
               matchingArity match {
                 // So far so good: only one candidate method
                 case Scope(concrete)   =>
-                  val mismatches  = abstractParamLists.flatten.map(_.tpe) zip concrete.paramLists.flatten.map(_.tpe) filterNot { case (x, y) => x =:= y }
-                  mismatches match {
+                  val aplIter = abstractParamLists .iterator.flatten
+                  val cplIter = concrete.paramLists.iterator.flatten
+                  def mismatch(apl: Symbol, cpl: Symbol): Option[(Type, Type)] =
+                    if (apl.tpe =:= cpl.tpe) None else Some(apl.tpe -> cpl.tpe)
+
+                  mapFilter2(aplIter, cplIter)(mismatch).take(2).toList match {
                     // Only one mismatched parameter: say something useful.
                     case (pa, pc) :: Nil  =>
                       val abstractSym = pa.typeSymbol
@@ -724,7 +728,7 @@ abstract class RefChecks extends Transform {
                       )
 
                       undefined("\n(Note that %s does not match %s%s)".format(pa, pc, addendum))
-                    case xs =>
+                    case _ =>
                       undefined("")
                   }
                 case _ =>

src/reflect/scala/reflect/internal/util/Collections.scala

@@ -16,6 +16,7 @@ package reflect.internal.util
 import scala.collection.{ mutable, immutable }
 import scala.annotation.tailrec
 import mutable.ListBuffer
+import java.util.NoSuchElementException
 
 /** Profiler driven changes.
  *  TODO - inlining doesn't work from here because of the bug that
@@ -308,6 +309,29 @@ trait Collections {
     true
   }
 
+  final def mapFilter2[A, B, C](itA: Iterator[A], itB: Iterator[B])(f: (A, B) => Option[C]): Iterator[C] =
+    new Iterator[C] {
+      private[this] var head: Option[C] = None
+      private[this] def advanceHead(): Unit =
+        while (head.isEmpty && itA.hasNext && itB.hasNext) {
+          val x = itA.next
+          val y = itB.next
+          head = f(x, y)
+        }
+
+      def hasNext: Boolean = {
+        advanceHead()
+        ! head.isEmpty
+      }
+
+      def next(): C = {
+        advanceHead()
+        val res = head getOrElse (throw new NoSuchElementException("next on empty Iterator"))
+        head = None
+        res
+      }
+    }
+
   // "Opt" suffix or traverse clashes with the various traversers' traverses
   final def sequenceOpt[A](as: List[Option[A]]): Option[List[A]] = traverseOpt(as)(identity)
   final def traverseOpt[A, B](as: List[A])(f: A => Option[B]): Option[List[B]] =