scala-Wperformance-option

$ scala --version
Scala code runner version 2.13.10 -- Copyright 2002-2022, LAMP/EPFL and Lightbend, Inc.
$ scala -Wperformance -Xprint:jvm -e "def foo: Int = { var a = 10; @annotation.tailrec def loop(): Int = if (a == 0) 3 else { a -= 1 ; loop() } ; loop() }"
/var/folders/nz/vb2z3s8j7719gg71zgz9gj_40000gn/T/scalacmd2181293633408592115.scala:1: warning: Modification of variable a within a closure causes it to be boxed.
def foo: Int = { var a = 10; @annotation.tailrec def loop(): Int = if (a == 0) 3 else { a -= 1 ; loop() } ; loop() }
                     ^
[[syntax trees at end of                       jvm]] // scalacmd2181293633408592115.scala
package <empty> {
  object Main extends Object {
    def main(args: Array[String]): Unit = {
      new <$anon: Object>();
      ()
    };
    def <init>(): Main.type = {
      Main.super.<init>();
      ()
    }
  };
  final class anon$1 extends Object {
    private def foo(): Int = {
      var a: runtime.IntRef = scala.runtime.IntRef.create(10);
      anon$1.this.loop$1(a)
    };
    @scala.annotation.tailrec final private[this] def loop$1(a$1: runtime.IntRef): Int = {
      <synthetic> val _$this: <$anon: Object> = anon$1.this;
      _loop(_$this: <$anon: Object>){
        (if (a$1.elem.==(0))
          3
        else
          {
            a$1.elem = a$1.elem.-(1);
            _loop(anon$1.this)
          }: Int)
      }
    };
    def <init>(): <$anon: Object> = {
      anon$1.super.<init>();
      ()
    }
  }
}