Function parse_method_preconditions 

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

Method-level preconditions(p, q) / no_preconditions(p) on an impl or trait method, whose parameters cannot carry attributes: each entry names a parameter that keeps (keep) or drops its type-derived R-side checks. Naming one parameter twice follows the pair rule: the last one written wins. Shared by the inherent-impl and trait-impl method parsers; the bare preconditions / no_preconditions (the whole method) is theirs to read.