What is the best variant for appending a new line

问题:

I am using this code to append a new line to the end of a file:

let text = "New line".to_string();

let mut option = OpenOptions::new();
option.read(true);
option.write(true);
option.create(true);

match option.open("foo.txt") {
    Err(e) => {
        println!("Error");
    }
    Ok(mut f) => {
        println!("File opened");
        let size = f.seek(SeekFrom::End(0)).unwrap();
        let n_text = match size {
            0 => text.clone(),
            _ => format!("\n{}", text),
        };
        match f.write_all(n_text.as_bytes()) {
            Err(e) => {
                println!("Write error");
            }
            Ok(_) => {
                println!("Write success");
            }
        }

        f.sync_all();
    }
}

It works, but I think it's too difficult. I found option.append(true);, but if I use it instead of option.write(true); I get "Write error".

回答1:

Using OpenOptions::append is the clearest way to append to a file:

use std::fs::OpenOptions;
use std::io::prelude::*;

fn main() {
    let mut file = OpenOptions::new()
        .write(true)
        .append(true)
        .open("my-file")
        .unwrap();

    if let Err(e) = writeln!(file, "A new line!") {
        eprintln!("Couldn't write to file: {}", e);
    }
}

As of Rust 1.8.0 (commit) and RFC 1252, append(true) implies write(true). This should not be a problem anymore.

Before Rust 1.8.0, you must use both write and append — the first one allows you to write bytes into a file, the second specifies where the bytes will be written.