Optimization: avoid call to exists in PlainFile#iterator

If we just established that the given path is a File or a Directory,
we can assume it continues to exist.

Before
----------------------------------
File.isFile calls           : 7620
File.isDirectory calls      : 2319
File.exists calls           : 5770

After
----------------------------------
File.isFile calls           : 7620
File.isDirectory calls      : 2319
File.exists calls           : 345

1 files changed, 7 insertions, 1 deletions

diff --git a/src/reflect/scala/reflect/io/PlainFile.scala b/src/reflect/scala/reflect/io/PlainFile.scala
index 31df78f995..b892fe7cef 100644
--- a/src/reflect/scala/reflect/io/PlainFile.scala
+++ b/src/reflect/scala/reflect/io/PlainFile.scala
@@ -56,8 +56,14 @@ class PlainFile(val givenPath: Path) extends AbstractFile {
 
   /** Returns all abstract subfiles of this abstract directory. */
   def iterator: Iterator[AbstractFile] = {
+    // Optimization: Assume that the file was not deleted and did not have permissions changed
+    // between the call to `list` and the iteration. This saves a call to `exists`.
+    def existsFast(path: Path) = path match {
+      case (_: Directory | _: io.File) => true
+      case _                           => path.exists
+    }
     if (!isDirectory) Iterator.empty
-    else givenPath.toDirectory.list filter (_.exists) map (new PlainFile(_))
+    else givenPath.toDirectory.list filter existsFast map (new PlainFile(_))
   }
 
   /**