<===> input.scss
@mixin foo($x: 1) {
  foo-sel {
    @content;
  }
}

div {
  $x: hey;
  @include foo() {
    bar {
      color: red;
      hux {
        msg: $x;
      }
    }
  }
}
<===> output.css
div foo-sel bar {
  color: red;
}
div foo-sel bar hux {
  msg: hey;
}