A wart rule has to be an object that extends WartTraverser. The object only needs an apply method which takes a WartUniverse and returns a WartUniverse#universe#Traverser.

The WartUniverse has error and warning methods, which both take (WartUniverse#universe#Position, String). They are side-effecting methods for adding errors and warnings.

Most traversers will want a super.traverse call to be able to recursively continue.

package mywarts

import org.wartremover.{ WartTraverser, WartUniverse }

object Unimplemented extends WartTraverser {
  def apply(u: WartUniverse): u.Traverser = {
    import u.universe._
    import scala.reflect.NameTransformer

    val notImplementedName: TermName = TermName(NameTransformer.encode("???"))
    val notImplemented: Symbol = typeOf[Predef.type].member(notImplementedName)
    require(notImplemented != NoSymbol)
    new Traverser {
      override def traverse(tree: Tree): Unit = {
        tree match {
          case rt: RefTree if rt.symbol == notImplemented =>
            error(u)(tree.pos, "There was something left unimplemented")
          case _ =>

Once you have your wart written you can add it to your config using Wart.custom:

wartremoverWarnings += Wart.custom("mywarts.Unimplemented")

You’ll also need to add the package containing your wart to wartremover’s classpath. The usual way to do this relies on adding your package as a library dependency:

libraryDependencies += "myOrg" %% "myWartPackage" % "1.0.0"
wartremoverClasspaths ++= {
  (Compile / dependencyClasspath).value.files

See also example repository