unstable --playground-url, add test code

src/librustdoc/lib.rs

@@ -162,10 +162,10 @@ pub fn opts() -> Vec<RustcOptGroup> {
         unstable(optmulti("Z", "",
                           "internal and debugging options (only on nightly build)", "FLAG")),
         stable(optopt("", "sysroot", "Override the system root", "PATH")),
-        stable(optopt("", "playground-url",
-                      "URL to send code snippets to, may be reset by --markdown-playground-url \
-                       or `#![doc(html_playground_url=...)]`",
-                      "URL")),
+        unstable(optopt("", "playground-url",
+                        "URL to send code snippets to, may be reset by --markdown-playground-url \
+                         or `#![doc(html_playground_url=...)]`",
+                        "URL")),
     ]
 }
 

src/test/rustdoc/playground-arg.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: --playground-url=https://example.com/ -Z unstable-options
+// ignore-tidy-linelength
+
+#![crate_name = "foo"]
+
+//! ```
+//! use foo::dummy;
+//! dummy();
+//! ```
+
+pub fn dummy() {}
+
+// ensure that `extern crate foo;` was inserted into code snips automatically:
+// @matches foo/index.html '//a[@class="test-arrow"][@href="https://example.com/?code=extern%20crate%20foo%3B%0Afn%20main()%20%7B%0Ause%20foo%3A%3Adummy%3B%0Adummy()%3B%0A%7D"]' "Run"