How can you change a txt file to lowercase using Rust programming language?

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To change a text file to lowercase using Rust programming language, you can use the following code:

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

fn main() {
    let input_file = File::open("input.txt").unwrap();
    let reader = BufReader::new(input_file);
    let mut output_file = File::create("output.txt").unwrap();

    for line_result in reader.lines() {
        let line = line_result.unwrap();
        let lowercase_line = line.to_lowercase();
        writeln!(output_file, "{}", lowercase_line).unwrap();
    }
}

This code opens an input file called "input.txt", reads its contents line by line, converts each line to lowercase using the to_lowercase() method, and writes the lowercased lines to an output file called "output.txt".