title: How do I fix "cannot find derive macro in this scope"?
body: <p>I have this:</p>
  
  <pre><code>#[derive(FromPrimitive)]
  pub enum MyEnum {
      Var1 = 1,
      Var2
  }
  </code></pre>
  
  <p>And an error:</p>
  
  <pre class="lang-none prettyprint-override"><code>error: cannot find derive macro `FromPrimitive` in this scope                                                                                                 
     |                                                                                                                                                          
  38 | #[derive(FromPrimitive)]                                                                                                                                 
     |          ^^^^^^^^^^^^^   
  </code></pre>
  
  <p>Why do I get this? How do I fix it?</p>