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, 11 insertions, 7 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/UnCurry.scala b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
index e98bf519fe..aae9625e2d 100644
--- a/src/compiler/scala/tools/nsc/transform/UnCurry.scala
+++ b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
@@ -35,8 +35,8 @@ import language.postfixOps
  *  - convert non-local returns to throws with enclosing try statements.
  *  - convert try-catch expressions in contexts where there might be values on the stack to
  *      a local method and a call to it (since an exception empties the evaluation stack):
- *      
- *      meth(x_1,..., try { x_i } catch { ..}, .. x_b0) ==> 
+ *
+ *      meth(x_1,..., try { x_i } catch { ..}, .. x_b0) ==>
  *        {
  *          def liftedTry$1 = try { x_i } catch { .. }
  *          meth(x_1, .., liftedTry$1(), .. )
@@ -271,7 +271,7 @@ abstract class UnCurry extends InfoTransform
 
           localTyper.typedPos(fun.pos) {
             Block(
-              List(ClassDef(anonClass, NoMods, List(List()), List(List()), List(applyMethodDef), fun.pos)),
+              List(ClassDef(anonClass, NoMods, ListOfNil, ListOfNil, List(applyMethodDef), fun.pos)),
               Typed(New(anonClass.tpe), TypeTree(fun.tpe)))
           }
 
@@ -396,7 +396,7 @@ abstract class UnCurry extends InfoTransform
 
       localTyper.typedPos(fun.pos) {
         Block(
-          List(ClassDef(anonClass, NoMods, List(List()), List(List()), List(applyOrElseMethodDef, isDefinedAtMethodDef), fun.pos)),
+          List(ClassDef(anonClass, NoMods, ListOfNil, ListOfNil, List(applyOrElseMethodDef, isDefinedAtMethodDef), fun.pos)),
           Typed(New(anonClass.tpe), TypeTree(fun.tpe)))
       }
     }
@@ -558,7 +558,7 @@ abstract class UnCurry extends InfoTransform
         sym.setInfo(MethodType(List(), tree.tpe))
         tree.changeOwner(currentOwner -> sym)
         localTyper.typedPos(tree.pos)(Block(
-          List(DefDef(sym, List(Nil), tree)),
+          List(DefDef(sym, ListOfNil, tree)),
           Apply(Ident(sym), Nil)
         ))
       }
@@ -641,7 +641,7 @@ abstract class UnCurry extends InfoTransform
           case ret @ Return(_) if (isNonLocalReturn(ret)) =>
             withNeedLift(true) { super.transform(ret) }
 
-          case Try(_, Nil, _) => 
+          case Try(_, Nil, _) =>
             // try-finally does not need lifting: lifting is needed only for try-catch
             // expressions that are evaluated in a context where the stack might not be empty.
             // `finally` does not attempt to continue evaluation after an exception, so the fact
@@ -711,8 +711,12 @@ abstract class UnCurry extends InfoTransform
           }
 
         case dd @ DefDef(_, _, _, vparamss0, _, rhs0) =>
+          val vparamss1 = vparamss0 match {
+            case _ :: Nil  => vparamss0
+            case _         => vparamss0.flatten :: Nil
+          }
           val flatdd = copyDefDef(dd)(
-            vparamss = List(vparamss0.flatten),
+            vparamss = vparamss1,
             rhs = nonLocalReturnKeys get dd.symbol match {
               case Some(k) => atPos(rhs0.pos)(nonLocalReturnTry(rhs0, k, dd.symbol))
               case None    => rhs0