1 files changed, 31 insertions, 3 deletions
diff --git a/src/reflect/scala/reflect/internal/util/Collections.scala b/src/reflect/scala/reflect/internal/util/Collections.scala
index 7cc2952c96..d128521be8 100644
--- a/src/reflect/scala/reflect/internal/util/Collections.scala
+++ b/src/reflect/scala/reflect/internal/util/Collections.scala
@@ -47,6 +47,30 @@ trait Collections {
   final def mforeach[A](xss: List[List[A]])(f: A => Unit) = xss foreach (_ foreach f)
   final def mforeach[A](xss: Traversable[Traversable[A]])(f: A => Unit) = xss foreach (_ foreach f)
 
+  /** A version of List#map, specialized for List, and optimized to avoid allocation if `as` is empty */
+  final def mapList[A, B](as: List[A])(f: A => B): List[B] = if (as eq Nil) Nil else {
+    val head = new ::[B](f(as.head), Nil)
+    var tail: ::[B] = head
+    var rest = as.tail
+    while (rest ne Nil) {
+      val next = new ::(f(rest.head), Nil)
+      tail.tl = next
+      tail = next
+      rest = rest.tail
+    }
+    head
+  }
+
+  final def collectFirst[A, B](as: List[A])(pf: PartialFunction[A, B]): Option[B] = {
+    @tailrec
+    def loop(rest: List[A]): Option[B] = rest match {
+      case Nil => None
+      case a :: as if pf.isDefinedAt(a) => Some(pf(a))
+      case a :: as => loop(as)
+    }
+    loop(as)
+  }
+
   final def map2[A, B, C](xs1: List[A], xs2: List[B])(f: (A, B) => C): List[C] = {
     val lb = new ListBuffer[C]
     var ys1 = xs1
@@ -99,15 +123,19 @@ trait Collections {
     else f(xs1.head, xs2.head, xs3.head) :: map3(xs1.tail, xs2.tail, xs3.tail)(f)
   }
   final def flatMap2[A, B, C](xs1: List[A], xs2: List[B])(f: (A, B) => List[C]): List[C] = {
-    val lb = new ListBuffer[C]
+    var lb: ListBuffer[C] = null
     var ys1 = xs1
     var ys2 = xs2
     while (!ys1.isEmpty && !ys2.isEmpty) {
-      lb ++= f(ys1.head, ys2.head)
+      val cs = f(ys1.head, ys2.head)
+      if (cs ne Nil) {
+        if (lb eq null) lb = new ListBuffer[C]
+        lb ++= cs
+      }
       ys1 = ys1.tail
       ys2 = ys2.tail
     }
-    lb.toList
+    if (lb eq null) Nil else lb.result
   }
 
   final def flatCollect[A, B](elems: List[A])(pf: PartialFunction[A, Traversable[B]]): List[B] = {