Auto merge of #11816 - servo:stable-style, r=nox

Make the style crate (almost) build with a stable compiler

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The bulk of this is adding cargo features to make derived implementations of `heapsize` and `serde` traits optional.

"Almost" because `std::intrinsics::discriminant_value` is currently unstable and doesn’t have any stable replacement that I know of. For now, this PR conditionally replaces it with `unimplemented!()`.

r? @nox

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes <s>fix</s> are part of #11815 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [x] These changes do not <s>require tests</s> *have tests yet* because that requires https://github.com/servo/servo/issues/11806, but I still want to land this before it bitrots. (Tests should check that the build succeeds with a stable compiler.)

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1 files changed, 22 insertions, 34 deletions

diff --git a/components/util/lib.rs b/components/util/lib.rs
index 6417fe40a45..ff1a18ab712 100644
--- a/components/util/lib.rs
+++ b/components/util/lib.rs
@@ -2,39 +2,31 @@
  * 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/. */
 
-#![feature(box_syntax)]
-#![feature(core_intrinsics)]
-#![feature(custom_derive)]
-#![feature(fnbox)]
-#![feature(plugin)]
-#![feature(reflect_marker)]
-#![feature(step_by)]
-
-#![plugin(heapsize_plugin, plugins, serde_macros)]
+#![cfg_attr(feature = "servo", feature(core_intrinsics))]
+#![cfg_attr(feature = "servo", feature(custom_derive))]
+#![cfg_attr(feature = "servo", feature(fnbox))]
+#![cfg_attr(feature = "servo", feature(plugin))]
+#![cfg_attr(feature = "servo", feature(reflect_marker))]
+#![cfg_attr(feature = "servo", plugin(serde_macros))]
 
 #![deny(unsafe_code)]
 
 extern crate app_units;
-extern crate backtrace;
-#[allow(unused_extern_crates)]
-#[macro_use]
-extern crate bitflags;
+#[cfg(feature = "servo")] extern crate backtrace;
+#[allow(unused_extern_crates)] #[macro_use] extern crate bitflags;
 extern crate deque;
 extern crate euclid;
 extern crate getopts;
-extern crate heapsize;
-extern crate ipc_channel;
-#[allow(unused_extern_crates)]
-#[macro_use]
-extern crate lazy_static;
+#[macro_use] extern crate heapsize;
+#[cfg(feature = "servo")] extern crate ipc_channel;
+#[allow(unused_extern_crates)] #[macro_use] extern crate lazy_static;
 extern crate libc;
-#[macro_use]
-extern crate log;
+#[macro_use] extern crate log;
 extern crate num_cpus;
 extern crate num_traits;
 extern crate rand;
 extern crate rustc_serialize;
-extern crate serde;
+#[cfg(feature = "servo")] extern crate serde;
 extern crate smallvec;
 extern crate url;
 #[cfg(all(unix, not(target_os = "macos"), not(target_os = "ios"), not(target_os = "android")))]
@@ -44,28 +36,24 @@ use std::sync::Arc;
 
 pub mod basedir;
 pub mod cache;
-#[allow(unsafe_code)]
-pub mod debug_utils;
+#[allow(unsafe_code)] pub mod debug_utils;
 pub mod geometry;
-#[allow(unsafe_code)]
-pub mod ipc;
-pub mod linked_list;
-#[allow(unsafe_code)]
-pub mod opts;
-pub mod panicking;
+#[cfg(feature = "servo")] #[allow(unsafe_code)] pub mod ipc;
+#[cfg(feature = "servo")] pub mod linked_list;
+#[allow(unsafe_code)] pub mod opts;
+#[cfg(feature = "servo")] pub mod panicking;
 pub mod prefs;
-pub mod print_tree;
+#[cfg(feature = "servo")] pub mod print_tree;
 pub mod resource_files;
-#[allow(unsafe_code)]
 pub mod str;
 pub mod thread;
 pub mod thread_state;
 pub mod tid;
-pub mod time;
+#[cfg(feature = "servo")] pub mod time;
 pub mod vec;
-#[allow(unsafe_code)]
-pub mod workqueue;
+#[allow(unsafe_code)] pub mod workqueue;
 
+#[cfg(feature = "servo")]
 #[allow(unsafe_code)]
 pub fn breakpoint() {
     unsafe { ::std::intrinsics::breakpoint() };