gistfile1.scala

import scala.language.experimental.macros
import scala.reflect.macros.whitebox.Context

class Foo {
  override def toString: String = macro Foo.toStringImpl
}

object Foo {
  def toStringImpl(c: Context): c.Expr[String] = {
    import c.universe._
    c.Expr[String](q"new ToStringNotAllowed")
  }
}