@mixin foo($out: false) {
  @if $out {
    @at-root { @content; }
  }
}

@mixin bar() {
  @at-root { @content; }
}

@mixin baz($string) {
  @at-root .#{$string} { @content; }
}

.test {
  @include foo(true) {
    .nest1 {
      color: red;
    }
  }
  @include bar() {
    .nest2 {
      color: green;
    }
  }
  @include baz('nest3') {
    color: blue;
  }
  @at-root {
    .nest4 {
      color: yellow;
    }
  }
}