Add desugar Printer

Aim: better diagnostic for what is desugared

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -15,6 +15,7 @@ import reporting.diagnostic.messages._
 import reporting.trace
 import annotation.constructorOnly
 import printing.Formatting.hl
+import config.Printers
 
 import scala.annotation.internal.sharable
 
@@ -51,7 +52,7 @@ object desugar {
   private type VarInfo = (NameTree, Tree)
 
   /** Is `name` the name of a method that can be invalidated as a compiler-generated
-   *  case class method that clashes with a user-defined method?
+   *  case class method if it clashes with a user-defined method?
    */
   def isRetractableCaseClassMethodName(name: Name)(implicit ctx: Context): Boolean = name match {
     case nme.apply | nme.unapply | nme.unapplySeq | nme.copy => true
@@ -765,7 +766,7 @@ object desugar {
     }
 
     flatTree(cdef1 :: companions ::: implicitWrappers)
-  }
+  }.reporting(res => i"desugared: $res", Printers.desugar)
 
   /** Expand
    *

compiler/src/dotty/tools/dotc/config/Printers.scala

@@ -19,6 +19,7 @@ object Printers {
   val cyclicErrors: Printer = noPrinter
   val debug = noPrinter             // no type annotation here to force inlining
   val derive: Printer = noPrinter
+  val desugar: Printer = noPrinter
   val dottydoc: Printer = noPrinter
   val exhaustivity: Printer = noPrinter
   val gadts: Printer = noPrinter