Performance improvement: Avoid unncecessary allocations of ListBuffer

There were a lot in StoreReporter, as we are creating about 0.5M new ones per self-compile.

Author: odersky

bin/dotc

@@ -282,7 +282,7 @@ class Definitions {
 
   object FunctionType {
     def apply(args: List[Type], resultType: Type) =
-      FunctionClass(args.length).typeRef.appliedTo(args :+ resultType)
+      FunctionClass(args.length).typeRef.appliedTo(args ::: resultType :: Nil)
     def unapply(ft: Type): Option[(List[Type], Type)] = { // Dotty deviation: Type annotation needed because inferred type
                                                           // is Some[(List[Type], Type)] | None, which is not a legal unapply type.
       val tsym = ft.typeSymbol

src/dotty/tools/dotc/core/Definitions.scala

@@ -12,13 +12,14 @@ import config.Printers._
  */
 class StoreReporter extends Reporter {
 
-  val infos = new mutable.ListBuffer[Diagnostic]
+  private var infos: mutable.ListBuffer[Diagnostic] = null
 
   protected def doReport(d: Diagnostic)(implicit ctx: Context): Unit = {
     typr.println(s">>>> StoredError: ${d.msg}") // !!! DEBUG
+    if (infos == null) infos = new mutable.ListBuffer
     infos += d
   }
 
   override def flush()(implicit ctx: Context) =
-    infos foreach ctx.reporter.report
+    if (infos != null) infos foreach ctx.reporter.report
 }