Function check_lazy_dots_params 

pub(crate) fn check_lazy_dots_params(
    inputs: &Punctuated<FnArg, Comma>,
    has_options: impl Fn(&str) -> bool,
) -> Result<()>

Check the LazyDots parameters of a signature (#1892): LazyDots is the parameter’s whole type, taken by value (&LazyDots, Option<LazyDots>, Vec<LazyDots> are refused; Missing<LazyDots> is refused by validate_param_type), and no per-parameter option names it (has_options, keyed by Rust name): the wrapper passes its frame, and every option would force the dots or has nothing to act on.

Shared by standalone functions (lib.rs) and impl methods (finalize_method_param_attrs). A function takes at most one ... either way (find_dots_param).