> like io::stdout().flush().unwrap(), where you have to unwrap an empty return value, because there's nothing sane to do if it's not Ok(()) other than panic

Try to flush other files, finish off transactions, remove potentially hanging locks, alert on stderr or in logs, etc. I don't think there isn't anything sane to do. Every app should have some global "something's fucked, try to clean up as much as possible" code path.