Variable `counter` passed to a macros behaves as if it was passed by-reference.

example.out

scala> var c = 0
c: Int = 0

scala> inc(c)

scala> c
res1: Int = 1

macro.scala

  def inc(counter: Int): Unit = macro incImpl

  def incImpl(c: Context)(counter: c.Expr[Int]): c.Expr[Unit] = {
    import c.universe._
    c.Expr[Unit](q"$counter += 1")
  }