Hope this makes sense? Even if it's possible doubt it'd work with compound selectors though

mixin.scss

$aList: (
  one
  , two
  , three
  , four
);

// So $elem is the name of the selector it was called from (.one, #two etc)
@mixin myMixin($elem) {
	$value: index(&aList, $elem);
}

// Allowing something like this (more abstracted)
.one {
	@include myMixin()
	@include myMixin($elem)
}

// Instead of typing it again
.one {
	@include myMixin(one)
}