Generated by SassMeister.com.

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@mixin admin-bar {
  $selector: &;
  @at-root {
    body.admin-bar {
      #{$selector} {
        @content;
      } 
    } 
  }
}

.dit-is-een-selector {
  @include admin-bar {
    color: red;
  }
}

SassMeister-output.css

body.admin-bar .dit-is-een-selector {
  color: red;
}