twbs--bootstrap/scss/mixins/_utilities.scss

// Utility generator
// Used to generate utilities & print utilities
@mixin generate-utility($utility, $infix, $is-rfs-media-query: false) {
  $values: map-get($utility, values);

  // If the values are a list or string, convert it into a map
  @if type-of($values) == "string" or type-of(nth($values, 1)) != "list" {
    $values: zip($values, $values);
  }

  @each $key, $value in $values {
    $properties: map-get($utility, property);

    // Multiple properties are possible, for example with vertical or horizontal margins or paddings
    @if type-of($properties) == "string" {
      $properties: append((), $properties);
    }

    // Use custom class if present
    $property-class: if(map-has-key($utility, class), map-get($utility, class), nth($properties, 1));
    $property-class: if($property-class == null, "", $property-class);

    $infix: if($property-class == "" and str-slice($infix, 1, 1) == "-", str-slice($infix, 2), $infix);

    // Don't prefix if value key is null (eg. with shadow class)
    $property-class-modifier: if($key, if($property-class == "" and $infix == "", "", "-") + $key, "");

    @if map-get($utility, rfs) {
      // Inside the media query
      @if $is-rfs-media-query {
        $val: rfs-value($value);

        // Do not render anything if fluid and non fluid values are the same
        $value: if($val == rfs-fluid-value($value), null, $val);
      }
      @else {
        $value: rfs-fluid-value($value);
      }
    }

    @if $value != null {
      .#{$property-class + $infix + $property-class-modifier} {
        @each $property in $properties {
          #{$property}: $value if($enable-important-utilities, !important, null);
        }
      }
    }
  }
}