perf: skip span hashing in non-incremental builds

[xmakro]

Span stable-hashing only matters when the hashes outlive the session, i.e. for incremental fingerprints and coverage source hashes. This PR changes it to be off otherwise, which leads to ~2% fewer instructions on non-incr full builds.

[Kobzol]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: f2d657e (f2d657ea99c440e3e341998de02b54cba54d84bc)
Base parent: 1498f99 (1498f9968f75e8e5ded6892b9c97bfe66b5a966f)

[rust-timer]

Finished benchmarking commit (f2d657e): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.2%, 0.3%]	16
Improvements ✅ (primary)	-3.2%	[-6.7%, -0.5%]	71
Improvements ✅ (secondary)	-2.9%	[-10.3%, -0.3%]	105
All ❌✅ (primary)	-3.2%	[-6.7%, -0.5%]	71

Max RSS (memory usage)

Results (primary -2.4%, secondary -3.0%)

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)	2.1%	[2.1%, 2.1%]	2
Improvements ✅ (primary)	-2.4%	[-3.8%, -1.1%]	14
Improvements ✅ (secondary)	-3.3%	[-6.3%, -1.6%]	46
All ❌✅ (primary)	-2.4%	[-3.8%, -1.1%]	14

Cycles

Results (primary -3.9%, secondary -4.5%)

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)	-3.9%	[-7.1%, -2.0%]	31
Improvements ✅ (secondary)	-4.5%	[-10.5%, -1.8%]	46
All ❌✅ (primary)	-3.9%	[-7.1%, -2.0%]	31

Binary size

Results (primary 0.0%, secondary 0.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	3
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.0%]	3

Bootstrap: 486.272s -> 487.245s (0.20%)
Artifact size: 387.67 MiB -> 388.11 MiB (0.11%)

[bjorn3]

with_stable_hashing_context is also used in the legacy symbol mangling, for type id hashes and most importantly the crate hash which should depend on all compiler inputs that could potentially affect the output, which includes spans.

View changes since the review