Different syntax for SassScript Maps in libsass and rubysass

example.scss

// rubysass

  $icons: (
    pencil-square-o  : $fa-var-pencil-square-o,
    user             : $fa-var-user,
    key              : $fa-var-key,
  );

  @each $icon-value, $icon-key in $icons {
    li.#{$icon-key} a:before {
      content: $icon-value;
    }
  }
  
// libsass

  $icons: (
    pencil-square-o   $fa-var-pencil-square-o,
    user              $fa-var-user,
    key               $fa-var-key
  );

  @each $icon-key in map-keys($icons) {
    $icon-value: map-get($icons, $icon-key);
    li.#{$icon-key} a:before {
      content: $icon-value;
    }
  }