Generated by SassMeister.com.

SassMeister-input.scss

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

@mixin prepend-selector($pre-selector) {
  $selector: &;
  @at-root {
    #{$pre-selector} {
      #{$selector} {
        @content;
      } 
    } 
  }
}

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

SassMeister-output.css

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