If you like loops, here it is with a for-comprehension:

  def complete_items(completed_binindex: Int, bin: ItemBin, nonterminal: Nonterminal, value: Value): MoreItems = {
    for {
      (item, v) <- bin.items
      _ <- expectedSymbol(item)
      nextValue <- grammar.nextValue(document, item.origin, bin.binindex, completed_binindex,
        nonterminal,item.ruleindex, item.dot, v, value)
    } yield {
      item.inc -> nextvalue
    }