Fixes firefox copy paste issue

Author: gstjepan2

src/librustdoc/html/static/js/src-script.js

@@ -206,6 +206,14 @@ const handleSrcHighlight = (function() {
     };
 }());
 
+if (navigator.userAgent.includes('Firefox')) {
+  document.addEventListener('copy', function(e) {
+    const text = nonnull(window.getSelection()).toString();    
+    nonnull(e.clipboardData).setData('text/plain', text);
+    e.preventDefault();
+  });
+}
+
 window.addEventListener("hashchange", highlightSrcLines);
 
 onEachLazy(document.querySelectorAll("a[data-nosnippet]"), el => {