Macros.scala

package macros

import language.experimental.macros
import scala.reflect.macros.Macro

object Macros {
  def swap(a: _, b: _) = macro Impl.swap
}

trait Impl extends Macro {

  import c.universe._
  import c.universe.Flag._

  def swap(a: c.Tree, b: c.Tree): c.Tree = {
    val tmp = TermName(c.freshName("tmp"))
    q"{ var $tmp = $a; $a = $b; $b = $tmp }"
    /*
    // without quasiquotes
    Block(
      List(
        ValDef(Modifiers(MUTABLE), tmp, TypeTree(), a),
        Assign(a, b),
        Assign(b, Ident(tmp))
      ),
      Literal(Constant(()))
    )
    */
  }
}

Test.scala

import macros.Macros.swap

object Test extends App {
  var a = 10
  var b = 5
  println((a, b))
  swap(a, b)
  println((a, b))
}