使用scala宏更改变量时打印字符串

import scala.language.experimental.macros
import scala.reflect.macros.blackbox.Context

object PrintMacro {
  def printOnChange[T](expr: T): T = macro printOnChangeImpl[T]

  def printOnChangeImpl[T: c.WeakTypeTag](c: Context)(expr: c.Expr[T]): c.Expr[T] = {
    import c.universe._

    val oldVal = TermName(c.freshName("oldVal"))
    val newVal = TermName(c.freshName("newVal"))

    val printStmt = q"""println("Variable changed: " + $oldVal + " -> " + $newVal)"""

    val result = expr.tree match {
      case q"$mods val $name: $tpt = $rhs" =>
        q"""
          var $oldVal = $rhs
          var $name: $tpt = $rhs
          $printStmt
          $name
        """
      case _ =>
        c.abort(c.enclosingPosition, "Invalid usage of printOnChange macro")
    }

    c.Expr[T](result)
  }
}

object Main extends App {
  var x = 10

  x = PrintMacro.printOnChange {
    x = 20
    x
  }

  println(x) // 输出：Variable changed: 10 -> 20
}