[experiment] Try to make index vec smaller when possible

[panstromek]

This is very ugly and unsound, but I want to check CI and perf to see whether there's at least something to gain.

Notes

• this roundtrips every mutation through vec, which might mask any gains if it's not optimized out properly. I hope I'm gonna see at least some change to see whether it's worth cleaning up.

TODO

• not sure about Dyn[Send|Sync]
• Why I had to change the lifetimes in ProjectedMaybeOwner? (Failed in borrowck in rustc_middle)
• proper desc
• proper commits

[panstromek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 76803df (76803df0625aa385701a24c186e4526355371fd5, parent: a1e52fc1cf67929a7c01ed9c037520e276ec98fd)

[rust-timer]

Finished benchmarking commit (76803df): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.4%	[0.2%, 0.7%]	155
Regressions ❌ (secondary)	0.4%	[0.1%, 1.0%]	107
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.9%, -0.4%]	7
All ❌✅ (primary)	0.4%	[0.2%, 0.7%]	155

Max RSS (memory usage)

Results (primary -0.9%, secondary -1.2%)

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)	-0.9%	[-1.0%, -0.8%]	4
Improvements ✅ (secondary)	-1.2%	[-2.0%, -0.3%]	2
All ❌✅ (primary)	-0.9%	[-1.0%, -0.8%]	4

Cycles

Results (primary 2.2%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 519.113s -> 519.594s (0.09%)
Artifact size: 400.82 MiB -> 400.82 MiB (-0.00%)

[panstromek]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: e8c5fee (e8c5feedca5bbedf67ca37e9d86d33007371e1f6, parent: 1f087276ba7a16d74ec04739ecf582ae495b3b79)

[rust-timer]

Finished benchmarking commit (e8c5fee): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.4%	[0.2%, 0.7%]	162
Regressions ❌ (secondary)	0.4%	[0.2%, 0.8%]	118
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.9%, -0.4%]	7
All ❌✅ (primary)	0.4%	[0.2%, 0.7%]	162

Max RSS (memory usage)

Results (primary -0.9%, secondary -1.9%)

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)	-0.9%	[-1.7%, -0.5%]	8
Improvements ✅ (secondary)	-1.9%	[-2.0%, -1.8%]	5
All ❌✅ (primary)	-0.9%	[-1.7%, -0.5%]	8

Cycles

Results (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)	3.0%	[2.2%, 4.1%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 526.872s -> 519.342s (-1.43%)
Artifact size: 401.48 MiB -> 400.98 MiB (-0.12%)