Hello.scala

package jto.demo

object HelloWorld {
  def main(args: Array[String]) {
    println("Hello, world!")
  }
}

NoPrint.scala

package jto.scala.compiler.plugins

import scala.tools.nsc._
import scala.tools.nsc.plugins._
import scala.tools.nsc.ast.parser._

/**
* Search "println" in AST
*/
class NoPrint(val global: scala.tools.nsc.Global) extends Plugin with Parsers{

    import global._

    val name = "NoPrint"
    val description = """Find all "println""""
    val components = List[PluginComponent](NoPrintComponent)       

    object PrintFinder extends Traverser{
      override def traverse(tree: Tree) = {
          tree match{
            case id @ Ident(_) if (id.name == newTermName("println"))  => {
              reporter.info(id.pos, "Println found!", true)
            }
            case _ => //Nothing
          }
          super.traverse(tree)
      }
    }

    private object NoPrintComponent extends PluginComponent{
        val global: NoPrint.this.global.type = NoPrint.this.global
        val runsAfter = List[String]()
        val phaseName = NoPrint.this.name
        //We run this just after the parser phase
        override val runsRightAfter= Some("parser")
        def newPhase(_prev: Phase) = new NoPrintPhase(_prev)

        class NoPrintPhase(_prev: Phase) extends StdPhase(_prev){
            override def name = NoPrint.this.name
            def apply(unit: CompilationUnit) = PrintFinder.traverse(unit.body)
        }
   }
}