doc: set playground to nightly for unstable code

Author: tsurai

src/librustdoc/html/static/playpen.js

@@ -16,6 +16,7 @@ document.addEventListener('DOMContentLoaded', function() {
         return;
     }
 
+    var featureRegexp = new RegExp('^\s*#!\\[feature\\(\.*?\\)\\]');
     var elements = document.querySelectorAll('pre.rust');
 
     Array.prototype.forEach.call(elements, function(el) {
@@ -29,8 +30,14 @@ document.addEventListener('DOMContentLoaded', function() {
             a.setAttribute('class', 'test-arrow');
 
             var code = el.previousElementSibling.textContent;
+
+            var channel = '';
+            if (featureRegexp.test(code)) {
+                channel = '&version=nightly';
+            }
+
             a.setAttribute('href', window.playgroundUrl + '?code=' +
-                           encodeURIComponent(code));
+                           encodeURIComponent(code) + channel);
             a.setAttribute('target', '_blank');
 
             el.appendChild(a);