...
let rule = match &reserved_rule {
    Rule::String(s) | Rule::Pattern(s, _) | Rule::NamedSymbol(s) => {
        format!("'{s}'")
    }
    Rule::Blank => "`blank`".to_string(),
    Rule::Choice(c) => {
        if c.len() == 2 && matches!((&c[0], &c[1]), (Rule::Repeat(_), Rule::Blank))
        {
            "`repeat`".to_string()
        } else if c.len() == 2 && matches!(&c[1], Rule::Blank) {
            "`optional`".to_string()
        } else {
            "`choice`".to_string()
        }
    }
    Rule::Repeat(_) => "`repeat1`".to_string(),
    Rule::Seq(_) => "`seq`".to_string(),
    Rule::Metadata { params, .. } => {
        if params.alias.is_some() {
            "`alias`".to_string()
        } else if params.field_name.is_some() {
            "`field`".to_string()
        } else if params.associativity.is_some()
            || params.precedence != Precedence::None
            || params.dynamic_precedence != 0
        {
            "`prec`".to_string()
        } else if params.is_token {
            "`token`".to_string()
        } else {
            "<unexpected rule>".to_string() // unreachable?
        }
    }
    // unreachable as well?
    Rule::Symbol(_) | Rule::Reserved { .. } => "<unexpected rule>".to_string(),
};
Err(ExtractTokensError::NonTokenReservedWord(rule))?;
...