You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mod.rs
    path of file: ./repos/log4rs/src/append/rolling_file
    
    the code of the file until where you have to start completion: //! A rolling file appender.
//!
//! Logging directly to a file can be a dangerous proposition for long running
//! processes. You wouldn't want to start a server up and find out a couple
//! weeks later that the disk is filled with hund
    
    fn get_writer<'a>(&self, writer: &'a mut Option<LogWriter>) -> io::Result<&'a mut LogWriter> {
        if writer.is_none() {
            let file = OpenOptions::new()
                .write(true)
                .append(self.append)
                .truncate(!self.append)
                .create(true)
                .open(&self.path)?;
            let len = if self.append {
                file.metadata()?.len()
            } else {
                0
            };
            *writer = Some(LogWriter {
                file: BufWriter::with_capacity(1024, file),
                len,
            });
        }

        // :( unwrap
        Ok(writer.as_mut().unwrap())
    }