an amazing discovery made by Iulian

Traces were stalling macro expansions by evaluating messages even when
-Ymacro-debug-* flags were disabled.

src/compiler/scala/tools/nsc/util/SimpleTracer.scala

@@ -10,7 +10,7 @@ import java.io.PrintStream
  *  @param enabled: A condition that must be true for trace info to be produced.
  */
 class SimpleTracer(out: PrintStream, enabled: Boolean = true) {
-  def apply[T](msg: String)(value: T): T = {
+  def apply[T](msg: => String)(value: T): T = {
     if (enabled) out.println(msg+value)
     value
   }