Golfed about 20 lines into the sand trap.

And it's a nice golf clinic and all, but let's remove our
golf gloves and take in some film.

  for (stat <- defBuf.iterator ++ auxConstructorBuf.iterator)

A quick count:

 - defBuf is a ListBuffer (1 mutant)
 - auxConstructorBuf is a ListBuffer (2 mutants)
 - two mutable iterators over mutable sequences (3, 4 mutants)
 - Iterator.++ joins them and is BY-NAME (4 mutants, 1 tragedy in waiting)
 - the joined Iterator is a new mutable structure (5 mutants, now 3 deep)
 - omittables is a mutable Set (6 mutants)
 - the 5-layer-3-deep iterator mutates omittables as it walks

[The following is a public service breakdown. The letter
sequence y-o-u is a local variable which should be replaced
with your name, whoever "you" are, if you commit any code in
these parts.]

Hear my plea! YOU DON'T HAVE TO DO IT THIS WAY! It isn't simpler,
faster, easier, more satisfying, shorter, more pixelated, there
just isn't any advantage to it, even if you're lazy! Especially
if you're lazy! Whatever combination of virtues and vices exist
in your personal petri dish, this will never be a hilltop!

PLEASE COME ENJOY A DRINK WITH ME AND MY FRIEND 'VAL' !!
I'LL INTRODUCE YOU! I THINK YOU WILL REALLY LIKE HER! I HOPE
YOU WILL SEE A LOT OF ONE ANOTHER! REMEMBER THAT NAME, 'VAL' !!
SHE'LL HAVE HER EYE OUT FOR YOU!

Author: paulp

src/compiler/scala/tools/nsc/transform/Constructors.scala

@@ -173,50 +173,33 @@ abstract class Constructors extends Transform with ast.TreeDSL {
       omittables ++= paramCandidatesForElision
       omittables ++= outerCandidatesForElision
 
-      val bodyOfOuterAccessor: Map[Symbol, DefDef] = {
-        val outers = (defBuf collect { case dd: DefDef if outerCandidatesForElision.contains(dd.symbol) => dd })
-        Map(outers.map { dd => (dd.symbol, dd) } : _*)
-      }
-
-      class UsagesDetector extends Traverser {
-        var done = false
-        override def traverse(tree: Tree) {
-          if (done) { return }
+      val bodyOfOuterAccessor: Map[Symbol, DefDef] =
+        defBuf collect { case dd: DefDef if outerCandidatesForElision(dd.symbol) => dd.symbol -> dd } toMap
+
+      // no point traversing further once omittables is empty, all candidates ruled out already.
+      object detectUsages extends Traverser {
+        private def markUsage(sym: Symbol) {
+          omittables -= debuglogResult("omittables -= ")(sym)
+          // recursive call to mark as needed the field supporting the outer-accessor-method.
+          bodyOfOuterAccessor get sym foreach (this traverse _.rhs)
+        }
+        override def traverse(tree: Tree): Unit = if (omittables.nonEmpty) {
+          def sym = tree.symbol
           tree match {
-            case DefDef(_, _, _, _, _, body) if outerCandidatesForElision.contains(tree.symbol) =>
-              () // don't mark as "needed" the field supporting this outer-accessor, ie not just yet.
-            case Select(_, _) =>
-              val sym = tree.symbol
-              if (omittables contains sym) {
-                debuglog("omittables -= " + sym.fullName)
-                omittables   -= sym
-                bodyOfOuterAccessor.get(sym) match {
-                  case Some(dd) => traverse(dd.rhs) // recursive call to mark as needed the field supporting the outer-accessor-method.
-                  case _        => ()
-                }
-                if (omittables.isEmpty) {
-                  done = true
-                  return // no point traversing further, all candidates ruled out already.
-                }
-              }
-              super.traverse(tree)
-            case _ =>
-              super.traverse(tree)
+            // don't mark as "needed" the field supporting this outer-accessor, ie not just yet.
+            case _: DefDef if outerCandidatesForElision(sym) => ()
+            case _: Select if omittables(sym)                => markUsage(sym) ; super.traverse(tree)
+            case _                                           => super.traverse(tree)
           }
         }
+        def walk(xs: Seq[Tree]) = xs.iterator foreach traverse
       }
-
       if (omittables.nonEmpty) {
-        val usagesDetector = new UsagesDetector
-
-        for (stat <- defBuf.iterator ++ auxConstructorBuf.iterator) {
-          usagesDetector.traverse(stat)
-        }
+        detectUsages walk defBuf
+        detectUsages walk auxConstructorBuf
       }
-
     }
-
-    def mustbeKept(sym: Symbol) = (!omittables(sym))
+    def mustbeKept(sym: Symbol) = !omittables(sym)
 
   } // OmittablesHelper