Hunting down eliminable :: allocations.

With this commit, the number of :: allocations logged in total
after individually compiling each scala file in src/compiler
drops from 190,766,642 to 170,679,925.  Twenty million fewer
colon-colons in the world, it's a start.

For some heavily used lists like List(List()) I made vals so
we can reuse the same one every time, e.g.

  val ListOfNil = List(Nil)

The modifications in this patch were informed by logging call
frequency to List.apply and examining the heaviest users.

>> Origins tag 'listApply' logged 3041128 calls from 318 distinguished sources.

1497759   scala.reflect.internal.Definitions$ValueClassDefinitions$class.ScalaValueClasses(Definitions.scala:149)
 173737   scala.reflect.internal.Symbols$Symbol.alternatives(Symbols.scala:1525)
 148642   scala.tools.nsc.typechecker.SuperAccessors$SuperAccTransformer.transform(SuperAccessors.scala:306)
 141676   scala.tools.nsc.transform.SpecializeTypes$$anonfun$scala$tools$nsc$transform$SpecializeTypes$$specializedOn$3.apply(SpecializeTypes.scala:114)
  69049   scala.tools.nsc.transform.LazyVals$LazyValues$$anonfun$1.apply(LazyVals.scala:79)
  62854   scala.tools.nsc.transform.SpecializeTypes.specializedTypeVars(SpecializeTypes.scala:427)
  54781   scala.tools.nsc.typechecker.SuperAccessors$SuperAccTransformer.transform(SuperAccessors.scala:293)
  54486   scala.reflect.internal.Symbols$Symbol.newSyntheticValueParams(Symbols.scala:334)
  53843   scala.tools.nsc.backend.icode.Opcodes$opcodes$CZJUMP.<init>(Opcodes.scala:562)
  ...     etc.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
index 049157dcf1..30cab66677 100644
--- a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
+++ b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
@@ -111,7 +111,7 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
             case TypeRef(_, GroupOfSpecializable, arg :: Nil) =>
               arg.typeArgs map (_.typeSymbol)
             case _ =>
-              List(tp.typeSymbol)
+              tp.typeSymbol :: Nil
           }
         }
       }
@@ -362,7 +362,7 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
     // creating each permutation of concrete types
     def loop(ctypes: List[List[Type]]): List[List[Type]] = ctypes match {
       case Nil         => Nil
-      case set :: Nil  => set map (x => List(x))
+      case set :: Nil  => set map (_ :: Nil)
       case set :: sets => for (x <- set ; xs <- loop(sets)) yield x :: xs
     }
     // zip the keys with each permutation to create a TypeEnv.
@@ -424,7 +424,7 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
     case MethodType(argSyms, resTpe) => specializedTypeVars(resTpe :: argSyms.map(_.tpe))
     case ExistentialType(_, res)     => specializedTypeVars(res)
     case AnnotatedType(_, tp, _)     => specializedTypeVars(tp)
-    case TypeBounds(lo, hi)          => specializedTypeVars(List(lo, hi))
+    case TypeBounds(lo, hi)          => specializedTypeVars(lo :: hi :: Nil)
     case RefinedType(parents, _)     => parents flatMap specializedTypeVars toSet
     case _                           => Set()
   }