Remove Clone impl for StableHashingContext.

[nnethercote]

HashStable::hash_stable takes a &mut Hcx. In contrast, ToStableHashKey::to_stable_hash_key takes a &Hcx. But there are some places where the latter calls the former, and due to the mismatch a clone call is required to get a mutable StableHashingContext.

This commit changes to_stable_hash_key to instead take a &mut Hcx. This eliminates the mismatch, the need for the clones, and the need for the Clone impls.

r? @fee1-dead

[nnethercote]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: b67d172 (b67d172374c3229c5c67970f61c951ccc3d2641b, parent: 55e86c996809902e8bbad512cfb4d2c18be446d9)

[rust-timer]

Finished benchmarking commit (b67d172): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (primary -1.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.0%	[-1.0%, -1.0%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 486.952s -> 486.532s (-0.09%)
Artifact size: 395.04 MiB -> 395.07 MiB (0.01%)

[rustbot]

fee1-dead is not on the review rotation at the moment.
They may take a while to respond.

[rustbot]

The Miri subtree was changed

cc @rust-lang/miri

[nnethercote]

This PR makes hash_stable and to_stable_hash_key match by making them both use &mut Hcx. I also tried making them both use &Hcx (by using interior mutability) in #154735 but it was a slight performance regression.