title: Is there a Cargo environment variable for the workspace directory?
body: <p>I have the following projects in a workspace:</p>
  
  <pre class="lang-none prettyprint-override"><code>Workspacefolder
   |
   +-- Project A
   |    |
   |    +-- build.rs
   |
   +-- Dep
   |    |
   |    +-- test.json  
   |
   +-Cargo.toml
  </code></pre>
  
  <p>In <code>Project A</code>, there is <code>build.rs</code> that wants to open <code>test.json</code> in a way that doesn't rely on platform and that works well with CI.  </p>
  
  <p>I'm looking for a <code>CARGO_WORKSPACE</code> environment variable, because then I can say <code>Path::new(&amp;workspace_dir).join("/Dep/test.json")</code>.</p>