Function r_formal_names 

pub(crate) fn r_formal_names<'a>(
    inputs: &'a Punctuated<FnArg, Comma>,
    formals: &'a Formals,
) -> impl Iterator<Item = (String, &'a Ident)>

The R formal each parameter of a signature becomes, with the parameter’s ident: Formals::of its name. The &Dots / LazyDots parameter becomes ... wherever it sits and is skipped, as are receivers, non-ident patterns and an NArgs parameter, which is no formal (#1860).

check_r_formals checks these names, and the shadowing pass (formal_names) qualifies the calls they would shadow, so the two agree on what each formal is named.