Generated by SassMeister.com, the Sass playground.

SassMeister-input.scss

@function rem($rem, $base: 16px) {
    $property: font-size; // need property inspection
    @if unitless($rem) == false {
        $rem: $rem / 1rem;
    }
    $pixels: $rem * $base;
    @return ($rem * 1rem) + unquote(";") $property + unquote(":") $pixels ;
}

foo {
    font-size: rem(1);
}

SassMeister-output.css

foo {
  font-size: 1rem; font-size: 16px;
}