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/typechecker/MethodSynthesis.scala b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
index f7ba189e0f..4f597f97c9 100644
--- a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
@@ -242,7 +242,7 @@ trait MethodSynthesis {
             abort("No synthetics for " + meth + ": synthetics contains " + context.unit.synthetics.keys.mkString(", "))
         }
       case _ =>
-        List(stat)
+        stat :: Nil
       }
 
     def standardAccessors(vd: ValDef): List[DerivedFromValDef] = (
@@ -491,7 +491,7 @@ trait MethodSynthesis {
       // Derives a tree without attempting to use the original tree's symbol.
       override def derivedTree = {
         atPos(tree.pos.focus) {
-          DefDef(derivedMods, name, Nil, List(Nil), tree.tpt.duplicate,
+          DefDef(derivedMods, name, Nil, ListOfNil, tree.tpt.duplicate,
             if (isDeferred) EmptyTree else Select(This(owner), tree.name)
           )
         }