There are enough functions in the std::env standard module to work with environment variables. It contains the var function to read external values. This function returns a Result with a String value of the variable if it exists, or a VarError error if it doesn't exist. Add the import of the env module to your main.rs file:

use std::env;

We need to replace the following line:

let addr = ([127, 0, 0, 1], 8080).into();

Replace it with the following:

let addr = env::var("ADDRESS")
    .unwrap_or_else(|_| "127.0.0.1:8080".into())
    .parse()
    .expect("can't parse ADDRESS variable");

This new code reads the ADDRESS value. If this value doesn't exist, we won't let the code throw a panic. Instead, we will replace it with the default value, "127.0.0.1:8080", using the unwrap_or_else method call. As the var function returns a String, we also have to convert &'static str into a String instance with the into method call.

If we can't parse an address, we will throw a panic in the except method call.

Your server will now use the addr variable, which takes a value from the ADDRESS environment variable or from the default value.

Environment variables are a simple way of configuring your application. They are also widely supported with hosting or cloud platforms and Docker containers.