Generated by SassMeister.com.

SassMeister-input-HTML.html

<div class="header">
  <a href="#0">Link</a>
  <a href="#0" class="btn  foo">Button</a>
</div>

SassMeister-input.scss

// ----
// Sass (v3.4.14)
// Compass (v1.0.3)
// ----

@mixin bump() {
  /**
   * Increase specificity of `#{&}`.
   */
  &#{&} {
    @content;
  }
}

.header {
  background: red;
}

.header a {
  color: white;
}

.btn {
  background: white;

  @include bump() {
    color: red;
  }

}

SassMeister-output.css

.header {
  background: red;
}

.header a {
  color: white;
}

.btn {
  background: white;
  /**
   * Increase specificity of `.btn`.
   */
}
.btn.btn {
  color: red;
}

SassMeister-rendered.html

<div class="header">
  <a href="#0">Link</a>
  <a href="#0" class="btn  foo">Button</a>
</div>