Align all equality signs in a source file

main.rs

use std::env;
use std::error::Error;
use std::fs::File;
use std::io::prelude::*;
use std::process;

fn main() {
    let args: Vec<String> = env::args().collect();

    if let Err(e) = doit(&args[1]) {
        println!("Application error: {}", e);
        process::exit(1);
    }
}

fn doit(filename: &str) -> Result<(), Box<Error>> {
    let mut f = File::open(filename)?;
    let mut contents = String::new();

    f.read_to_string(&mut contents)?;

    let max_index = contents.lines()
        .filter_map(|line| length_before_equal_sign(line))
        .max()
        .ok_or("no max index found")?;

    for line in contents.lines() {
        if let Some(n) = line.find('=') {
            let lhs = &line[..n].trim_right();
            let rhs = &line[n..];
            let spacing = " ".repeat(max_index - lhs.len());

            println!("{}{}{}", lhs, spacing, rhs);
        }
        else {
            println!("{}", line);
        }
    }

    Ok(())
}

fn length_before_equal_sign(line: &str) -> Option<usize> {
    if let Some(n) = line.find('=') {
        return Some(line[..n].trim_right().len());
    }

    None
}