title: Why do I get the error "no `block_on` in `executor`" when attempting to block on an async function?
body: <p>I am following Rust's <a href="https://rust-lang.github.io/async-book/01_getting_started/04_async_await_primer.html" rel="nofollow noreferrer">async/await primer</a> but am having trouble running the hello world program shown below.</p>
  
  <pre><code>use futures::executor::block_on;
  
  async fn hello_world() {
      println!("hello, world!");
  }
  
  fn main() {
      let future = hello_world(); // Nothing is printed
      block_on(future); // `future` is run and "hello, world!" is printed
  }
  </code></pre>
  
  <p>I receive the error:</p>
  
  <pre class="lang-none prettyprint-override"><code>1 | use futures::executor::block_on;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ no `block_on` in `executor`
  </code></pre>
  
  <p>Is there an import required to use this function?</p>