match self.state {
    ...
    56usize => match self.iter.peek().copied() {
        ...
        Some((char_idx, char)) => match char {
            '\u{5c}' => {
                self.current_match_end += char.len_utf8();
                let _ = self.iter.next();
                self.state = 58usize;
            }
            ...
        },
    },
    ...
    58usize => {
        let rhs: fn(LexerHandle<'_, 'input>) -> LexerAction<Token> =
            |lexer| lexer.switch(LexerRule::StringEscape);
        let str = &self.input[self.current_match_start..self.current_match_end];
        let handle = LexerHandle {
            iter: &mut self.iter,
            match_: str,
            user_state: &mut self.user_state,
        };
        match rhs(handle) {
            LexerAction::Continue => {
                self.state = self.initial_state;
                continue;
            }
            LexerAction::Return(tok) => {
                self.state = self.initial_state;
                return Some(Ok((
                    self.current_match_start,
                    tok,
                    self.current_match_end,
                )));
            }
            LexerAction::Switch(rule_set) => {
                self.switch(rule_set);
                continue;
            }
            LexerAction::SwitchAndReturn(tok, rule_set) => {
                self.switch(rule_set);
                return Some(Ok((
                    self.current_match_start,
                    tok,
                    self.current_match_end,
                )));
            }
        }
    }
    ...
}