Function message 

pub(crate) fn message(name: Option<&str>) -> String

The diagnostic for a ... parameter that is not last, named name (None for unnamed dots).

Unnamed dots get _dots: _: &Dots would work on a function (its _ parameters are renamed) but not on a method, where the &Dots parameter needs a plain name. No spelling of Rust ... parses before another parameter, so the hint names &Dots, never _: ....