Function meaning_key 

pub fn meaning_key(token: &Token) -> SymStr

The key under which a token’s meaning is stored. All \special_relax-family tokens (\noexpand’d forms — the bare \special_relax and every \special_relax\x01<shadowed>) resolve under the bare \special_relax name: they share its \relax meaning, faithful to TeX where a \noexpand’d token has relax meaning regardless of which token it shadows. The shadowed identity is recovered separately via Token::noexpand_shadowed (delimited matching only). Use this anywhere a token’s name keys a meaning lookup or a “same control sequence?” comparison. Cheap on the common path: non-CS tokens short-circuit before any string access.