Put message conversion into a method

rust/coffi/src/main.rs

    }
}
impl PNGImage {
    fn message(&self) -> String {
        String::from_utf8(self.message.iter().map(|&c| c as u8).collect()).unwrap()
    }
}
impl std::fmt::Display for PNGImage {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        fn get_message(msg: [libc::c_char; 64]) -> String {
            String::from_utf8(msg.iter().map(|&c| c as u8).collect()).unwrap()
        }
        write!(f, "{}x{} {} {} {} {} {} {}", self.width, self.height, self.version,
               self.format, self.flags, self.colormap_entries, self.warning_or_error,
               get_message(self.message))
               self.message())
        }
}