Reading files in Rust, three ways

Rust gives you several levels of control when reading a file. Here are the three I actually use.

The one-liner

For small files, just slurp the whole thing:

use std::fs;

fn main() -> std::io::Result<()> {
    let contents = fs::read_to_string("notes.txt")?;
    println!("{} bytes", contents.len());
    Ok(())
}

Buffered lines

For larger files, iterate line by line without loading everything:

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

fn main() -> std::io::Result<()> {
    let file = File::open("notes.txt")?;
    for line in BufReader::new(file).lines() {
        let line = line?;
        if line.starts_with('#') {
            continue; // skip comments
        }
        println!("{line}");
    }
    Ok(())
}

Raw bytes

And when the file is not text at all:

1	let bytes = std::fs::read("image.png")?;

That covers 99% of everyday needs. The remaining 1% is memory-mapped IO, which is a story for another post.