Function parse_method_formal 

pub(crate) fn parse_method_formal(
    meta: &ParseNestedMeta<'_>,
    per_param: &mut HashMap<String, ParamAttrs>,
) -> Result<()>

Parse a method-level formal(conf_int = "conf.int", ...) (#1842): the R formal of each named parameter, the method form of a standalone function’s per-parameter formal = "..". Shared by the inherent-impl parser only: trait methods take their formals from the trait declaration, and the trait parsers refuse it. A parameter named twice is an error; finalize_method_param_attrs refuses a name that is no parameter, and crate::r_wrapper_builder::check_r_formals checks each formal.