Generated by SassMeister.com.

SassMeister-input.scss

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

$bem-element-separator: '__';
$bem-modifier-separator: '--';

@function selectorToString($selector) {
  $selector: inspect($selector); //cast to string
  $selector: str-slice($selector, 2, -2); //remove brackets
  @return $selector;
}

@function lastIndex($str, $needle, $from: 0) {
  $index: str-index($str, $needle);

  @if(not $index) {
    @return $from;
  }
  @else {
    $str: str-slice($str, $index + 1);
    @return lastIndex($str, $needle, $from + $index);
  }
}

@function containsModifier($selector) {
  $selector: selectorToString($selector);

  @if(str-index($selector, $bem-modifier-separator)) {
    @return true;
  }
  @else {
    @return false;
  }
}

@function lastElement($str) {
  $last-index: lastIndex($str, " ");
  $str: if($last-index, str-slice($str, $last-index + 1), $str);
  @return $str;
}

@function getBlock($selector) {
  $selector: selectorToString($selector);
  $element: lastElement($selector);
  @debug($element);
  $modifierStart: str-index($element, $bem-modifier-separator) - 1;
  @return str-slice($element, 0, $modifierStart);
}

@mixin block($block) {
  .#{$block} {
    @content;
  }
}

@mixin element($element) {
  $selector: &;

  @if(containsModifier($selector)) {
    $block: getBlock($selector);
    @debug($block);
    @at-root {
      #{$selector} {
        #{$block+$bem-element-separator+$element} {
          @content;
        }
      }
    }
  }
  @else {
    @at-root {
      #{$selector+$bem-element-separator+$element} {
        @content;
      }
    }
  }
}

@mixin modifier($modifier) {
  @at-root {
    #{&}#{$bem-modifier-separator+$modifier} {
      @content;
    }
  }
}

@include block(test) {
  background: red;
  @include element(element){
    font-size: 14px;

    @include modifier(big) {
      font-size: 18px;
    }
  }
  
  @include modifier(modifier) {
    color: blue;

    @include element(subelement) {
      background: gray;
    }
  }
  
  @include block(nested-block) {
    @include modifier(modifier) {
      @include element(element) {
        color: blue;
      }
    }
  }
}

SassMeister-output.css

.test {
  background: red;
}
.test__element {
  font-size: 14px;
}
.test__element--big {
  font-size: 18px;
}
.test--modifier {
  color: blue;
}
.test--modifier .test__subelement {
  background: gray;
}
.test .nested-block--modifier .nested-block__element {
  color: blue;
}