Function check_r_formals 

pub(crate) fn check_r_formals(
    inputs: &Punctuated<FnArg, Comma>,
    formals: &Formals,
    reserved: &[(String, String)],
) -> Result<()>

Check the R formals a signature produces, before any wrapper is generated.

Each parameter becomes its formal (Formals::of: the declared formal = "..", else normalize_r_arg_string of its name); the &Dots parameter becomes ..., wherever it sits, and is skipped. A formal must be an R name (not a reserved word such as if or in, not starting with a digit; a declared one must pass declared_formal_problem), distinct from the others (x and _x both become x), and not one of the reserved names the generated wrapper binds itself (a method’s receiver, R6’s self and private), each paired with what it is. Any of these would break the wrapper: a parse error in the wrappers file, or a parameter that hides the object the method works on.

Every case is a compile error rather than a rename: callers pass arguments by name, so the R formal has to stay the parameter’s own name unless the author declares another. An error points at the declaring literal for a declared formal, else at the parameter. Every problem of the signature is reported, in one combined error.