Make usage of unstable feature on_unimplemented optional.

components/script/Cargo.toml

@@ -13,6 +13,7 @@ path = "lib.rs"
 
 [features]
 debugmozjs = ['js/debugmozjs']
+unstable = []
 
 [build-dependencies]
 cmake = "0.1"

components/script/dom/bindings/conversions.rs

@@ -67,7 +67,8 @@ pub trait IDLInterface {
 }
 
 /// A trait to mark an IDL interface as deriving from another one.
-#[rustc_on_unimplemented = "The IDL interface `{Self}` is not derived from `{T}`."]
+#[cfg_attr(feature = "unstable",
+           rustc_on_unimplemented = "The IDL interface `{Self}` is not derived from `{T}`.")]
 pub trait DerivedFrom<T: Castable>: Castable {}
 
 // https://heycam.github.io/webidl/#es-object

components/script/lib.rs

@@ -2,11 +2,11 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+#![cfg_attr(feature = "unstable", feature(on_unimplemented))]
 #![feature(conservative_impl_trait)]
 #![feature(const_fn)]
 #![feature(core_intrinsics)]
 #![feature(mpsc_select)]
-#![feature(on_unimplemented)]
 #![feature(plugin)]
 #![feature(proc_macro)]
 

components/servo/Cargo.toml

@@ -23,6 +23,7 @@ unstable = [
     "gfx/unstable",
     "msg/unstable",
     "profile/unstable",
+    "script/unstable",
 ]
 
 [dependencies]