Function fill_fn_params 

fn fill_fn_params(
    ctx: &CompletionContext<'_, '_>,
    function: &Fn,
    param_list: &ParamList,
    current_param: &Param,
    impl_or_trait: &Option<Either<Impl, Trait>>,
    add_new_item_to_acc: impl FnMut(&str),
)