Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.0.rc.1)
// Compass (v1.0.0.alpha.20)
// ----

// Reference: https://www.youtube.com/watch?v=o4ok4gsiQSw
@mixin insert-context($target, $add-contexts...) {
  $new-contexts: ();
  $new-selector: ();

  // 新しいコンテキストを追加する
  @each $context in $add-contexts {
    $new-contexts: append($new-contexts, selector-nest($target, $context), comma);
  }

  // "&"内のコンテキストを変更する
  @each $context in $new-contexts {
    $new-selector: append($new-selector, selector-replace(&, $target, $context), comma);
  }
  
  // 出力
  @at-root #{$new-selector}{
    @content;
  }
}

.nav{
  a{
    background-color: #000;
    &:hover,
    &.active{
      background-color: #eee;
    }
  }
  .en{
    color: #fff;
    @include insert-context(".nav", "a:hover", "a.active"){
      color: #333;
    }
  }
}

SassMeister-output.css

.nav a {
  background-color: #000;
}
.nav a:hover, .nav a.active {
  background-color: #eee;
}
.nav .en {
  color: #fff;
}
.nav a:hover .en, .nav a.active .en {
  color: #333;
}