Rust environment config with defaults and validation

use std::env;

#[derive(Debug)]
struct Config {
    host: String,
    port: u16,
    debug: bool,
}

fn load() -> Result<Config, String> {
    let host = env::var("APP_HOST").unwrap_or_else(|_| "127.0.0.1".into());
    let port = env::var("APP_PORT")
        .unwrap_or_else(|_| "8080".into())
        .parse::<u16>()
        .map_err(|e| format!("APP_PORT: {e}"))?;
    let debug = env::var("APP_DEBUG").map(|v| v == "1" || v == "true").unwrap_or(false);
    if port == 0 {
        return Err("APP_PORT must be between 1 and 65535".into());
    }
    Ok(Config { host, port, debug })
}