macro_rules! env_or {
($env_name:expr) => { ... };
($env_name:expr, $default:expr) => { ... };
($($tokens:tt)*) => { ... };
}
This macro returns environment variable by name and converts variable to desired type
or returns default value.
If you don't pass default value, macro will panic
let url: String = env_or!("DATABASE_URL", "127.0.0.1".to_string());
assert_eq!(url, "127.0.0.1".to_string());