title: Inline `cfg_attr(feature = x, path = "file.rs")` doesn't work
body: <p>According to the <a href="https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute" rel="nofollow noreferrer">documentation for cfg_attr</a>, it is possible to do both of these:</p>
  
  <pre><code>#[cfg_attr(windows, path = "windows.rs")]
  #[cfg_attr(feature = "magic", sparkles, crackles)]
  </code></pre>
  
  <p>However, I'm seeing an error when trying to use them together.</p>
  
  <pre><code>// this works
  #[cfg(connection = "websocket")]
  #[path = "websocket.rs"]
  mod protocol;
  
  // file not found for module `protocol`
  #[cfg_attr(connection = "websocket", path = "websocket.rs")]
  mod protocol ;
  </code></pre>