ExplicitLocaleLinter.scala

class ExplicitLocale extends SyntacticRule("ExplicitLocale") {
  override def fix(implicit doc: SyntacticDocument): Patch =
    doc.tree.collect {
      case t @ Term.Select(_, tn) if targets.contains(tn.value) =>
        t.parent match {
          case Some(Term.Apply(Term.Select(_, _), args)) if args.nonEmpty =>
            Patch.empty
          case _ =>
            Patch.lint(ExplicitLocaleWarn(t.pos))
        }
      case _ =>
        Patch.empty
    }.asPatch
}
object ExplicitLocale {
  val targets = Seq("toLowerCase", "toUpperCase")
}

final case class ExplicitLocaleWarn(override val position: Position) extends Diagnostic {
  override def severity: LintSeverity = LintSeverity.Warning
  override def message: String        = ExplicitLocale.targets.mkString("", ", ", " は Locale を明示的に指定しましょう。")
}