Function per_param_in_signature_order 

pub(crate) fn per_param_in_signature_order<'a>(
    inputs: &'a Punctuated<FnArg, Comma>,
    per_param: &'a HashMap<String, ParamAttrs>,
) -> impl Iterator<Item = (&'a String, &'a ParamAttrs)>

The entries of per_param in the order their parameters appear in inputs. Code that emits one line or item per parameter walks this, never the map itself: a HashMap’s order differs between maps and between compiler runs, so the generated R wrapper (and the Rust items) would too.