Function check_nargs_params 

pub(crate) fn check_nargs_params(
    inputs: &Punctuated<FnArg, Comma>,
    has_options: impl Fn(&str) -> bool,
) -> Result<()>

Check the NArgs parameters of a signature (#1860): at most one, and none that a per-parameter option names (has_options, keyed by Rust name). The parameter is no R argument (the wrapper fills it with nargs()), so an option would have nothing to act on.

Shared by standalone functions (lib.rs) and impl / trait methods (finalize_method_param_attrs). Both errors point at the parameter, also for a method, whose options are written on the method.