email_newsletter_api/tests/health_check.rs

// tokio spins up a new async runtime every time 
// at the beginning of each test case and shutdown at
// the end of each test case
// the spawn_app() function therefore only survives as long as the runtime
#[tokio::test]
async fn health_check_works(){
    spawn_app();

    let client = reqwest::Client::new();

    let response = client.get("http://127.0.0.1:8000/health_check").send().await.expect("Failed to execute health_check request.");

    assert!(response.status().is_success());
    assert_eq!(Some(0), response.content_length());
}

fn spawn_app() {
    let server = email_newsletter_api::run().expect("Failed to bind address");
    // run() returns an instance of HttpServer that will run forever.
    // We don't want this behavior
    // Therefore we want to spawn our server, run our test logic
    // and then tear down the entire test suite
    let _ = tokio::spawn(server);
}