Generated by SassMeister.com.

SassMeister-input-HTML.jade

.something
  .selector
  
.svg
  .selector
  
.backgroundsize
  .selector
  
.svg.backgroundsize
  .selector

SassMeister-input.scss

// ----
// libsass (v3.2.2)
// ----

// Source: http://hugogiraudel.com/2014/01/27/casting-types-in-sass/
@function to-string($value) {
  @return inspect($value);
}

// Source: http://hugogiraudel.com/2014/01/13/sass-string-replacement-function/
@function str-replace($string, $search, $replace: '') {
  $index: str-index($string, $search);
  
  @if $index {
    @return str-slice($string, 1, $index - 1) + $replace + str-replace(str-slice($string, $index + str-length($search)), $search, $replace);
  }
  
  @return $string;
}

@mixin feature-enhancement($feature...) {
  @if length($feature) > 1 {
    $feature-list: ();
    @each $class in $feature {
      $selector: str-insert($class, ".", 1);
      $feature-list: append($feature-list, $selector);
    }
    $feature-list: to-string($feature-list);
    $feature-list: str-replace($feature-list, " ");
    
    #{$feature-list} & {
      @content;
    }
  }
  @else {
    #{$feature} & {
      @content;
    }
  }
}

.selector {
  &:before {
    content: "nope";
    display: inline-block;
  }
  @include feature-enhancement(svg, backgroundsize) {
    &:before {
      background-color: yellowgreen;
      content: "boop";
    }
  }
}

SassMeister-output.css

.selector:before {
  content: "nope";
  display: inline-block;
}

.svg.backgroundsize .selector:before {
  background-color: yellowgreen;
  content: "boop";
}

SassMeister-rendered.html

<div class="something">
  <div class="selector"></div>
</div>
<div class="svg">
  <div class="selector"></div>
</div>
<div class="backgroundsize">
  <div class="selector"></div>
</div>
<div class="svg backgroundsize">
  <div class="selector"></div>
</div>