title: What are structs with round brackets in Rust for?
body: <p>I found a language construct I don't understand:</p>
  
  <pre><code>pub struct OpenOptions(fs_imp::OpenOptions);
  </code></pre>
  
  <p>I even created a compilable piece of code with those brackets but I still couldn't understand it:</p>
  
  <pre><code>struct Foo {
      bar: i32,
  }
  
  struct Baz(Foo);
  
  fn main() {
      let mut x: Baz = Baz(Foo{ bar: 3 });
  }
  </code></pre>
  
  <p>What are those round brackets for?</p>