Sass mixin - rems with px fallback

_px-and-rem.scss

// This mixin outputs a property with rem units and a px fallback.
// Values passed without units are used as multipliers for the final
// rem and px values, all other units are output without modification.
// $base-font-size represents the root value of the document font-size
// in pixels.
//
// i.e. html { font-size: 100%;} // -> 16px

// Usage:
// @include px-and-rem([property], [multiplier | explicit value] [, ...]);

// Example 1:
//
// $base-font-size: 16px;
//
// .margin { @include px-and-rem(margin, 2);}
//
// becomes
//
// .margin {
//  margin: 32px;
//  margin: 2rem;
// }

// Example 2:
//
// $base-font-size: 16px;
//
// .padding { @include px-and-rem(padding, 1 2% 1.5em);}
//
// becomes
//
// .padding {
//  padding: 16px 2% 1.5em;
//  padding: 1rem 2% 1.5em;
// }

@function fix8_get_px_and_rem_val($val) {
  $output: ();

  @if type-of($val) == 'string' {
    @if $val == 'auto' or $val == '!important' {
      $output: join($val, $val);
      @return $output;
    }
  } @else {
    @if $val == 0 or $val == 0px {
      $output: join(0, 0);
    } @else if unitless($val) {
      $output: join($val * $base-font-size, $val + rem);
    } @else {
      $output: join($val, $val);
    }
    @return $output;
  }

  @warn "#{$val} is not a valid value";
  @return false;
}

@mixin px-and-rem($prop, $vals) {
  $px-list: ();
  $rem-list: ();

  @each $val in $vals {
    $calcs: fix8_get_px_and_rem_val($val);
    $px-list: append($px-list, nth($calcs, 1));
    $rem-list: append($rem-list, nth($calcs, 2));
  }

  #{$prop}: $px-list;
  #{$prop}: $rem-list;
}

Thanks to @joelhans for the addition for handling the auto value: SASS + mixins + rems = awesome