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

fn main()  -> std::io::Result<()> {
    let  fieltxt=readtxt();
    writetxt(fieltxt);
    Ok(())
}
///读取指定文件内容
fn readtxt()->String{
    let text = fs::read_to_string("D:\\from.txt").unwrap();
    return text;
}
///写内容到指定文件
fn writetxt(fieltxt:String) -> std::io::Result<()> {
    let mut file = OpenOptions::new()
            .append(true).open("D:\\text.txt")?;
    file.write(fieltxt.as_bytes()).expect("write failed");
    Ok(())
}

临时写一个练习，欢迎完善指教。