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, 7 insertions, 6 deletions

diff --git a/components/util/thread.rs b/components/util/thread.rs
index 87d8954dcf9..571d3529d9f 100644
--- a/components/util/thread.rs
+++ b/components/util/thread.rs
@@ -2,13 +2,13 @@
  * 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/. */
 
-use backtrace::Backtrace;
-use ipc_channel::ipc::IpcSender;
-use panicking;
-use serde::Serialize;
-use std::any::Any;
+#[cfg(feature = "servo")] use backtrace::Backtrace;
+#[cfg(feature = "servo")] use ipc_channel::ipc::IpcSender;
+#[cfg(feature = "servo")] use panicking;
+#[cfg(feature = "servo")] use serde::Serialize;
+#[cfg(feature = "servo")] use std::any::Any;
+#[cfg(feature = "servo")] use thread_state;
 use std::thread;
-use thread_state;
 
 pub fn spawn_named<F>(name: String, f: F)
     where F: FnOnce() + Send + 'static
@@ -17,6 +17,7 @@ pub fn spawn_named<F>(name: String, f: F)
 }
 
 /// Arrange to send a particular message to a channel if the thread fails.
+#[cfg(feature = "servo")]
 pub fn spawn_named_with_send_on_panic<F, Id>(name: String,
                                              state: thread_state::ThreadState,
                                              f: F,