Hello.scala

package jto.demo

case class Log(v: String){
  def log = print("[LOG] " + v)
}

trait LogConv{
  implicit def toLog(v: String): Log = Log(v)
}

object HelloWorld extends LogConv{
  
  def main(args: Array[String]) {
    println("Hello, world!") 
    // ==> "Hello, world!".log
  }
}

NoPrint.scala

package jto.scala.compiler.plugins

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

/**
* Replace "println" in AST by "log"
*/
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 DropPrint extends Transformer{
      override def transform(tree: Tree) = {
          tree match {
            case Apply( id @ Ident(name), params) if(name == newTermName("println")) => {
              reporter.info(id.pos, "println found!", true)
              Select(params.head, newTermName("log"))
            }
            case _ => super.transform(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) = {
              //NoPrint.this.global.treeBrowser.browse(unit.body)
              unit.body = DropPrint.transform(unit.body)
              NoPrint.this.global.treeBrowser.browse(unit.body)
            }
        }
   }
}