Function missing_choice_default_msg 

fn missing_choice_default_msg(param_name: &str) -> String

A default on a choice parameter under Missing<..> (#2161). An omitted argument reaches Rust as Missing::Absent, so the default would pick no choice; on a choices(...) list it would also replace the formal, the choice vector that ParamAttrs::absent_formal compares a supplied value with (#2152).