Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.20)
// Compass (v1.0.3)
// ----

@mixin using-at($parent) {
  #{$parent}[data-foo] & {
    foo: bar
  }
}

button__inner {
  @include using-at("button");
}

/*
 * produces 
 *  button[data-foo] button--jumbo button__inner
 * you want
 *  button[data-foo]button--jumbo button__inner 
 */

button--jumbo {
  button__inner {
    @include using-at("button");
  }
}

SassMeister-output.css

button[data-foo] button__inner {
  foo: bar;
}

/*
 * produces 
 *  button[data-foo] button--jumbo button__inner
 * you want
 *  button[data-foo]button--jumbo button__inner 
 */
button[data-foo] button--jumbo button__inner {
  foo: bar;
}