Function substitute_one_reference 

fn substitute_one_reference<'a>(
    css: &'a str,
    url_data: &UrlExtraData,
    substitution_functions: &'a ComputedSubstitutionFunctions,
    reference: &'a SubstitutionFunctionReference,
    stylist: &Stylist,
    computed_context: &Context<'_>,
    attribute_tracker: &mut AttributeTracker<'_>,
    seen: &mut SmallVec<[&'a Name; 8]>,
) -> Result<Substitution<'a>, ()>