Generated by SassMeister.com.

SassMeister-input.scss

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

$placeholders: ();

@mixin placeholder($name) {
  $name: "%placeholder-#{$name}";
  @if not index($placeholders, $name) {
    $placeholders: append($placeholders, $name) !global;
    @at-root #{$name} {
      @content;
    }
  }
  @extend #{$name};
}

@mixin test {
  @include placeholder(test) {
    color: red;
  }
}

.test .testa {
  @include test;
}
.test2 {
  @include test;
}
.test3 {
  @include test;
}

SassMeister-output.css

.test .testa .testa, .test .testa .test2, .test .testa .test3 {
  color: red;
}