Compile-time computation of prefix size on stable Rust

main.rs

const PREFIX_TYPES: &[&str] = &["Compiling", "Downloading", "Fetching", "Printing", "Doing"];

const fn max_prefix_width() -> usize {
    let mut max_width = 0;

    let mut i = 0;
    loop {
        if i == PREFIX_TYPES.len() {
            break;
        }

        let msg = PREFIX_TYPES[i];
        if msg.len() > max_width {
            max_width = msg.len();
        }

        i += 1;
    }

    max_width
}

const _: () = assert!(max_prefix_width() != 0, "No messages");

const BOLD: &str = "\x1b[1m";
const BLUE: &str = "\x1b[34m";
const RESET: &str = "\x1b[0m";

fn msg(prefix: &str, msg: &str) {
    println!(
        "{BOLD}{BLUE}{prefix: >width$}{RESET} {msg}",
        width = max_prefix_width() + 2
    );
}

fn main() {
    msg("Compiling", "main.rs");
    msg("Downloading", "rustc");
    msg("Fetching", "crates");
    msg("Printing", "output");
    msg("Doing", "stuff");
}

/*
    Compiling main.rs
  Downloading rustc
     Fetching crates
     Printing output
        Doing stuff
*/