Sass mixin - rems with px fallback

gistfile1.sass

// 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.
// $unit-default-px 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:
//
// $unit-default-px: 16px;
//
// .margin { @include px-and-rem(margin, 2);}
//
// becomes
//
// .margin {
// 	margin: 32px;
// 	margin: 2rem;
// }

// Example 2:
//
// $unit-default-px: 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_unit_to_px($val) {
	@if unitless($val) {
		@if $val == 0 { @return $val}
		@else { @return $val * $unit-default-px};
	} @else { @return $val};
}

@function fix8_unit_to_rem($val) {
	@if unitless($val) {
		@if $val == 0 { @return $val}
		@else { @return $val+rem};
	} @else { @return $val};
}

@mixin px-and-rem($prop, $val...) {
	$n: length($val);
	$i: 1;

	$px-list: ();
	$rem-list: ();

	@while $i <= $n {
		$px-list: append($px-list, fix8_unit_to_px(nth($val, $i)));
		$rem-list: append($rem-list, fix8_unit_to_rem(nth($val, $i)));
		$i: $i + 1;
	}

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

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