title: Cannot compile the ring crate: file not found for module `montgomery`
body: <p>Cargo is not compiling with the following error:</p>
  
  <pre class="lang-none prettyprint-override"><code>$ cargo build
  Compiling ring v0.12.1
  error[E0583]: file not found for module `montgomery`
      --&gt; 
   C:\Users\jmccrae\.cargo\registry\src\github.com1ecc6299db9ec823\ring-0.12.1\src\arithmetic/arithmetic.rs:15:9
     |
  15 | pub mod montgomery;
     |         ^^^^^^^^^^
     |
     = help: name the file either arithmetic\montgomery.rs or arithmetic\montgomery\mod.rs inside the directory 
  "C:\\Users\\jmccrae\\.cargo\\registry\\src\\github.com-1ecc6299db9ec823\\ring-0.12.1\\src\\arithmetic"
  </code></pre>
  
  <p>The project was a new project with Cargo.toml modified to include a dependency to the most recent version (0.12.1) of the <code>ring</code> crate. The Cargo.toml is as follows:</p>
  
  <pre><code>[package]
  name = "testring"
  version = "0.1.0"
  authors = ["John McCrae &lt;john@mccr.ae&gt;"]
  
  [dependencies]
  ring = "0.12.1"
  </code></pre>
  
  <p>The required file seems to actually exist:</p>
  
  <pre class="lang-none prettyprint-override"><code>$ ls  C:\\Users\\jmccrae\\.cargo\\registry\\src\\github.com-1ecc6299db9ec823\\ring-0.12.1\\src\\arithmetic
  arithmetic.rs  montgomery.rs
  </code></pre>
  
  <p>The cargo version is <code>cargo 0.25.0-nightly (930f9d949 2017-12-05)</code> and it is running on MINGW.</p>
  
  <p>Is there anything wrong with the compiler set-up?</p>