Constant IMPL_OPTIONS_HELP 

const IMPL_OPTIONS_HELP: &str = "env, env(...), r6, r6(...), s3, s3(...), s4, s4(...), s7, \
     s7(...), vctrs, vctrs(...) (class system), class = \"...\" (R class name), \
     blanket, strict, no_strict, preconditions, no_preconditions, internal, noexport";

Every impl-block option, for the “unknown impl block option” errors of the bare and the key = value forms.