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, 4 deletions

diff --git a/components/util/opts.rs b/components/util/opts.rs
index 323abc19d62..711782ab3ea 100644
--- a/components/util/opts.rs
+++ b/components/util/opts.rs
@@ -23,7 +23,8 @@ use url::{self, Url};
 
 
 /// Global flags for Servo, currently set on the command line.
-#[derive(Clone, Deserialize, Serialize)]
+#[derive(Clone)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 pub struct Opts {
     pub is_running_problem_test: bool,
 
@@ -379,7 +380,8 @@ pub fn print_debug_usage(app: &str) -> ! {
     process::exit(0)
 }
 
-#[derive(Clone, Deserialize, Serialize)]
+#[derive(Clone)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 pub enum OutputOptions {
     FileName(String),
     Stdout(f64)
@@ -404,7 +406,8 @@ enum UserAgent {
     Android,
 }
 
-#[derive(Clone, Debug, Eq, Deserialize, PartialEq, Serialize)]
+#[derive(Clone, Debug, Eq, PartialEq)]
+#[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 pub enum RenderApi {
     GL,
     ES2,
@@ -858,7 +861,7 @@ pub fn set_defaults(opts: Opts) {
     unsafe {
         assert!(DEFAULT_OPTIONS.is_null());
         assert!(DEFAULT_OPTIONS != INVALID_OPTIONS);
-        let box_opts = box opts;
+        let box_opts = Box::new(opts);
         DEFAULT_OPTIONS = Box::into_raw(box_opts);
     }
 }