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, 5 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index dd5588e9a6..6a908c6c65 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -27,6 +27,13 @@ trait Contexts { self: Analyzer =>
     override def implicitss: List[List[ImplicitInfo]] = Nil
     override def toString = "NoContext"
   }
+  private object RootImports {
+    import definitions._
+    // Possible lists of root imports
+    val javaList         = JavaLangPackage :: Nil
+    val javaAndScalaList = JavaLangPackage :: ScalaPackage :: Nil
+    val completeList     = JavaLangPackage :: ScalaPackage :: PredefModule :: Nil
+  }
 
   private val startContext = {
     NoContext.make(
@@ -46,13 +53,12 @@ trait Contexts { self: Analyzer =>
    *    among its leading imports, or if the tree is [[scala.Predef]], `Predef` is not imported.
    */
   protected def rootImports(unit: CompilationUnit): List[Symbol] = {
-    import definitions._
-    assert(isDefinitionsInitialized, "definitions uninitialized")
+    assert(definitions.isDefinitionsInitialized, "definitions uninitialized")
 
     if (settings.noimports.value) Nil
-    else if (unit.isJava) List(JavaLangPackage)
-    else if (settings.nopredef.value || treeInfo.noPredefImportForUnit(unit.body)) List(JavaLangPackage, ScalaPackage)
-    else List(JavaLangPackage, ScalaPackage, PredefModule)
+    else if (unit.isJava) RootImports.javaList
+    else if (settings.nopredef.value || treeInfo.noPredefImportForUnit(unit.body)) RootImports.javaAndScalaList
+    else RootImports.completeList
   }
 
   def rootContext(unit: CompilationUnit): Context             = rootContext(unit, EmptyTree, false)