此问题在此处已有答案：

Why do I need to import a trait to use the methods it defines for a type?（1个答案）
昨天关门了。
为什么我们在例子中需要std::io::BufRead，没有它，'lines'将是未定义的方法。
https://riptutorial.com/rust/example/4275/read-a-file-line-by-line

use std::fs::File;
use std::io::{BufRead, BufReader};

fn main() {
    let filename = "src/main.rs";
    // Open the file in read-only mode (ignoring errors).
    let file = File::open(filename).unwrap();
    let reader = BufReader::new(file);

    // Read the file line by line using the lines() iterator from std::io::BufRead.
    for (index, line) in reader.lines().enumerate() {
        let line = line.unwrap(); // Ignore errors.
        // Show the line and its number.
        println!("{}. {}", index + 1, line);
    }
}