Enable polonius alpha on nightly

[jackh726]

See rust-lang/compiler-team#

The first commit here adds an -Zpolonius=nll argument for tests and so people can disable alpha on nightly.

[jackh726]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 24e1a22 (24e1a22664233bce1f9b5dc6eee7c1f74327e28c)
Base parent: df1ae0f (df1ae0f7dd168513de2d89beb2cb7a95d4fdc118)

[jackh726]

@craterbot check p=1

(priority bump b/c we want to get this enabled for testing)

[craterbot]

👌 Experiment pr-159343 created and queued.
🤖 Automatically detected try build 24e1a22
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[rust-timer]

Finished benchmarking commit (24e1a22): 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 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)	3.0%	[0.3%, 15.3%]	88
Regressions ❌ (secondary)	2.1%	[0.2%, 15.1%]	102
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.1%	[-1.2%, -1.0%]	6
All ❌✅ (primary)	3.0%	[0.3%, 15.3%]	88

Max RSS (memory usage)

Results (primary 4.2%, secondary 3.2%)

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

	mean	range	count
Regressions ❌ (primary)	4.2%	[2.1%, 7.7%]	6
Regressions ❌ (secondary)	6.2%	[2.0%, 11.7%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.3%	[-6.2%, -2.4%]	2
All ❌✅ (primary)	4.2%	[2.1%, 7.7%]	6

Cycles

Results (primary 4.2%, secondary 4.6%)

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

	mean	range	count
Regressions ❌ (primary)	4.2%	[1.8%, 8.4%]	25
Regressions ❌ (secondary)	4.6%	[2.1%, 7.1%]	21
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.2%	[1.8%, 8.4%]	25

Binary size

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

Bootstrap: 487.651s -> 492.438s (0.98%)
Artifact size: 389.37 MiB -> 389.64 MiB (0.07%)

[Kobzol]

The way we enabled nightly-only for LLD was through a build configuration in bootstrap, I think that's a more explicit way, and will be easier to revert/modify on beta/stable, than to have that code in the compiler. If we did it like this PR does currently, then Polonius would be used by default if you use RUSTC_BOOTSTRAP=1 on stable, which is not ideal, I think. I'm pretty sure we set that flag kinda randomly all over bootstrap even on stable/beta toolchains, that would produce a lot of confusion.

If we did it through bootstrap, then there would be an env variable or a cfg that would be set by bootstrap at rustc build time, which would select the default mode and hardcode it when we build rustc.

[jackh726]

@lqd said something similar to me. I'll take stab at it. As to whether RUSTC_BOOTSTRAP=1 should enable this, idk. Probably not,

[craterbot]

🚧 Experiment pr-159343 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-159343 is completed!
📊 2 regressed and 18 fixed (1026208 total)
📊 5682 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-159343/retry-regressed-list.txt ↩

[jackh726]

@craterbot check p=2 crates=https://crater-reports.s3.amazonaws.com/pr-159343/retry-regressed-list.txt name=pr-159343-retry

Let's recheck this to see how we're doing. Wondering if these are performance-related.

[craterbot]

👌 Experiment pr-159343-retry created and queued.
🤖 Automatically detected try build 24e1a22
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-159343-retry is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🎉 Experiment pr-159343-retry is completed!
📊 0 regressed and 0 fixed (5632 total)
📊 773 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-159343-retry/retry-regressed-list.txt ↩

[rust-bors]

☔ The latest upstream changes (presumably #159748) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.