Add a type="search" to the search bar in the documentation.

src/librustdoc/html/layout.rs

@@ -60,7 +60,8 @@ pub fn render<T: fmt::Default, S: fmt::Default>(
         <form class=\"search-form js-only\">
             <input class=\"search-input\" name=\"search\"
                    autocomplete=\"off\"
-                   placeholder=\"Search documentation...\" />
+                   placeholder=\"Search documentation...\"
+                   type=\"search\" />
             <button class=\"do-search\">Search</button>
         </form>
     </nav>