title: What is the plus symbol (+) in generics?
body: <p>I am on <a href="https://doc.rust-lang.org/book/ch19-00-advanced-features.html" rel="nofollow noreferrer">chapter 19 of <em>The Rust Programming Language</em></a> when I saw this code</p>
  
  <pre><code>Box&lt;dyn Fn() + Send + 'static&gt;
  </code></pre>
  
  <p>What does it do and why is there the plus sign (<code>+</code>) in there? I don't remember encountering this in a previous chapter.</p>
  
  <p>How is <code>Box::new(|| println!("hi"))</code> a <code>Box&lt;dyn Fn() + Send + 'static&gt;</code>?</p>