Highlighting a Chunk of Code within a lstlisting

You can use \colorbox and an escape character inside your listing:

Add to your preamble

  \usepackage{color}

  \definecolor{light-gray}{gray}{0.80}

then use it like this in your document:

  \begin{lstlisting}[escapechar=!]
  def mult(m: Matrix[Int], n: Matrix[Int]) {
    val p = !\colorbox{light-gray}{new MatrixInt}!(m.rows, n.cols)
  }
  \end{lstlisting}