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, 2 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/TreeDSL.scala b/src/compiler/scala/tools/nsc/ast/TreeDSL.scala
index 2ee38d4b91..e90d779885 100644
--- a/src/compiler/scala/tools/nsc/ast/TreeDSL.scala
+++ b/src/compiler/scala/tools/nsc/ast/TreeDSL.scala
@@ -95,7 +95,7 @@ trait TreeDSL {
       def INT_>=  (other: Tree)     = fn(target, getMember(IntClass, nme.GE), other)
       def INT_==  (other: Tree)     = fn(target, getMember(IntClass, nme.EQ), other)
       def INT_!=  (other: Tree)     = fn(target, getMember(IntClass, nme.NE), other)
-      
+
       // generic operations on ByteClass, IntClass, LongClass
       def GEN_|   (other: Tree, kind: ClassSymbol)  = fn(target, getMember(kind, nme.OR), other)
       def GEN_&   (other: Tree, kind: ClassSymbol)  = fn(target, getMember(kind, nme.AND), other)
@@ -234,7 +234,7 @@ trait TreeDSL {
     }
     class DefTreeStart(val name: Name) extends TreeVODDStart with DefCreator {
       def tparams: List[TypeDef] = Nil
-      def vparamss: List[List[ValDef]] = List(Nil)
+      def vparamss: List[List[ValDef]] = ListOfNil
     }
 
     class IfStart(cond: Tree, thenp: Tree) {