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, 8 insertions, 4 deletions

diff --git a/components/style/restyle_hints.rs b/components/style/restyle_hints.rs
index 3a90852c13a..361d7110d04 100644
--- a/components/style/restyle_hints.rs
+++ b/components/style/restyle_hints.rs
@@ -50,7 +50,8 @@ bitflags! {
 /// now to reduce complexity, but it's worth measuring the performance impact (if any) of the
 /// mStateMask approach.
 
-#[derive(HeapSizeOf, Clone)]
+#[derive(Clone)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct ElementSnapshot {
     pub state: Option<ElementState>,
     pub attrs: Option<Vec<(AttrIdentifier, AttrValue)>>,
@@ -225,7 +226,8 @@ fn combinator_to_restyle_hint(combinator: Option<Combinator>) -> RestyleHint {
     }
 }
 
-#[derive(Debug, HeapSizeOf)]
+#[derive(Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 struct Sensitivities {
     pub states: ElementState,
     pub attrs: bool,
@@ -262,14 +264,16 @@ impl Sensitivities {
 // us to quickly scan through the dependency sites of all style rules and determine the
 // maximum effect that a given state or attribute change may have on the style of
 // elements in the document.
-#[derive(Debug, HeapSizeOf)]
+#[derive(Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 struct Dependency<Impl: SelectorImplExt> {
     selector: Arc<CompoundSelector<Impl>>,
     combinator: Option<Combinator>,
     sensitivities: Sensitivities,
 }
 
-#[derive(Debug, HeapSizeOf)]
+#[derive(Debug)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub struct DependencySet<Impl: SelectorImplExt> {
     deps: Vec<Dependency<Impl>>,
 }