[一次性读取文件内容](https://play.rust-lang.org/?version=stable&mode=debug&edition=2015&code=use%20std%3A%3Aio%3A%3Aprelude%3A%3A*%3B%0Ause%20std%3A%3Afs%3A%3AFile%3B%0Ause%20std%3A%3Aio%3A%3AResult%3B%0Ause%20std%3A%3Aio%3A%3A%7BBufReader%7D%3B%0Afn%20main()%20-%3EResult%3C()%3E%20%7B%0A%20%20%20%20let%20hmm_file%20%3D%20File%3A%3Aopen(%22src%2Fdata%2F1.txt%22).expect(%22cannot%20open%201.txt%22)%3B%0A%20%20%20%20let%20mut%20reader%20%3D%20BufReader%3A%3Anew(hmm_file)%3B%0A%20%20%20%20let%20mut%20buffer%20%3D%20String%3A%3Anew()%3B%0A%20%20%20%20reader.read_to_string(%26mut%20buffer)%3F%3B%0A%20%20%20%20println!(%22First%20line%20is%20%7B%7D%20bytes%20long%22%2C%20buffer)%3B%0A%20%20%20%20Ok(())%0A%7D%0A)
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