It was almost literally JSX (except with Scala syntax instead of Javascript, of course).

An XML literal in Scala would have been:

  // XML literals (to be dropped)
  val mails1 = for (from, to, heading, body) <- todoList yield
    <message>
      <from>{from}</from><to>{to}</to>
      <heading>{heading}</heading><body>{body}</body>
    </message>
  println(mails1)

This was replaced with "XML string interpolation", which

  // XML string interpolation
  val mails2 = for (from, to, heading, body) <- todoList yield xml"""
    <message>
      <from>${from}</from><to>${to}</to>
      <heading>${heading}</heading><body>${body}</body>
    </message>"""
  println(mails2)