Function parse_method_no_default 

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

Method-level no_default(p, q) on an impl or trait method, whose parameters cannot carry attributes: each entry names a match_arg / choices parameter whose R formal has no default (#1828). Shared by the inherent-impl and trait-impl method parsers; finalize_method_param_attrs refuses a name that is no parameter, no choice or has a default.