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).

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- [ ] 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, 4 insertions, 2 deletions

diff --git a/components/style/custom_properties.rs b/components/style/custom_properties.rs
index 80aed5c10c3..957e5899486 100644
--- a/components/style/custom_properties.rs
+++ b/components/style/custom_properties.rs
@@ -23,7 +23,8 @@ pub fn parse_name(s: &str) -> Result<&str, ()> {
     }
 }
 
-#[derive(Clone, PartialEq, Debug, HeapSizeOf)]
+#[derive(Clone, PartialEq, Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct SpecifiedValue {
     css: String,
 
@@ -41,7 +42,8 @@ pub struct BorrowedSpecifiedValue<'a> {
     references: Option<&'a HashSet<Name>>,
 }
 
-#[derive(Clone, HeapSizeOf, Debug)]
+#[derive(Clone, Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct ComputedValue {
     css: String,
     first_token_type: TokenSerializationType,