Constant rustc_expand::mbe::quoted::VALID_FRAGMENT_NAMES_MSG_2021

const VALID_FRAGMENT_NAMES_MSG_2021: &str = "valid fragment specifiers are \
                                             `ident`, `block`, `stmt`, `expr`, `expr_2021`, `pat`, \
                                             `ty`, `lifetime`, `literal`, `path`, `meta`, `tt`, \
                                             `item` and `vis`";