f"hello, world"  // scala.this.Predef.augmentString("hi").format()

f"$$ordinary"  // no cost
"$ordinary"     // looks like missing interpolator
f"$$ordinary%n"  // requires format, though could be improved

package goodbye

import Predef.{ augmentString => _ }

object Test extends App {

  implicit class `bold format`(s: String) {
    def format(args: Any*): String = {
      Predef require args.isEmpty
      "goodbye, everybody"
    }
  }

  Console println f"hello, world"
}