Generated by SassMeister.com, the Sass playground.

SassMeister-input.sass

//========== REM dimensions, pixel fallback

$base-font-size: 10px !default

=rem($prop, $rems...)
    @if type-of($prop) == 'list' and nth($prop,1) == position
        position: #{nth($prop,2)}
        @each $rem in $rems
            #{nth($rem,1)}: #{nth($rem,2) * 1rem}
            *#{nth($rem,1)}: #{nth($rem,2) * $base-font-size}
    @else if index((absolute, fixed, relative),$prop)
        position: #{$prop}
        @each $rem in $rems
            #{nth($rem,1)}: #{nth($rem,2) * 1rem}
            *#{nth($rem,1)}: #{nth($rem,2) * $base-font-size}
    @else
        $rm: ()
        $px: ()
        @each $rem in $rems
            @if type-of($rem) == 'string'
                $rm: append($rm, $rem)
                $px: append($px, $rem)
            @else
                $rm: append($rm, $rem * 1rem)
                $px: append($px, $rem * $base-font-size)
        #{$prop}: $rm
        *#{$prop}: $px

.test
    +rem(position relative, bottom 0, right 5)
    +rem(height, 2)
    +rem(width, 1.5)
    +rem(margin, 2, auto, 3)
    +rem(position absolute, bottom 0, right 0)
    +rem(fixed, bottom 0)
    +rem(width, 20)
    +rem(height, 4)
    
.test2
    +rem(fixed, top 0)
    +rem(font-size, 1)


=position($type, $params...)
    position: #{$type}
    @each $param in $params
        #{nth($param,1)}: #{nth($param,2)}

SassMeister-output.css

.test {
  position: relative;
  bottom: 0rem;
  *bottom: 0px;
  right: 5rem;
  *right: 50px;
  height: 2rem;
  *height: 20px;
  width: 1.5rem;
  *width: 15px;
  margin: 2rem auto 3rem;
  *margin: 20px auto 30px;
  position: absolute;
  bottom: 0rem;
  *bottom: 0px;
  right: 0rem;
  *right: 0px;
  position: fixed;
  bottom: 0rem;
  *bottom: 0px;
  width: 20rem;
  *width: 200px;
  height: 4rem;
  *height: 40px;
}

.test2 {
  position: fixed;
  top: 0rem;
  *top: 0px;
  font-size: 1rem;
  *font-size: 10px;
}