Compute generator saved locals on MIR

[cjgillot]

Generators are currently type-checked by introducing a witness type variable, which is unified with a GeneratorWitness(captured types) whose purpose is to ensure that the auto traits correctly migrate from the captured types to the witness type. This requires computing the captured types on HIR during type-checking, only to re-do it on MIR later.

This PR proposes to drop the HIR-based computation, and only keep the MIR one. This is done in 3 steps.

1. During type-checking, the witness type variable is never unified. This allows to stall all the obligations that depend on it until the end of type-checking. Then, the stalled obligations are marked as successful, and saved into the typeck results for later verification.
2. At type-checking writeback, witness is replaced by GeneratorWitnessMIR(def_id, substs). From this point on, all trait selection involving GeneratorWitnessMIR will fetch the MIR-computed locals, similar to what opaque types do. There is no lifetime to be preserved here: we consider all the lifetimes appearing in this witness type to be higher-ranked.
3. After borrowck, the stashed obligations are verified against the actually computed types, in the check_generator_obligations query. If any obligation was wrongly marked as fulfilled in step 1, it should be reported here.

There are still many issues:

• I am not too happy having to filter out some locals from the checked bounds, I think this is MIR building that introduces raw pointers polluting the analysis; solved by a check specific to static variables.
• the diagnostics for captured types don't show where they are used/dropped;
• I do not attempt to support chalk.

cc @eholk@jyn514 for the drop-tracking work
r? @oli-obk as you warned me of potential unsoundness

[jyn514]

I don't currently have time to review a PR this large but I think this work is super exciting, thank you for tackling it ❤️

[cjgillot]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 6c9ec2af2224dcbfba350ffdd69a27e5030000c1 with merge 8ea250bdb76176ecb0225baa34aa634b024ae0e2...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8ea250bdb76176ecb0225baa34aa634b024ae0e2 (8ea250bdb76176ecb0225baa34aa634b024ae0e2)

[rust-timer]

Queued 8ea250bdb76176ecb0225baa34aa634b024ae0e2 with parent 0d56e34, future comparison URL.

[rust-timer]

Finished benchmarking commit (8ea250bdb76176ecb0225baa34aa634b024ae0e2): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	0.4%	[0.2%, 0.8%]	42
Regressions ❌ (secondary)	1.1%	[0.3%, 2.8%]	27
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-49.7%	[-86.8%, -0.9%]	11
All ❌✅ (primary)	0.4%	[0.2%, 0.8%]	42

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	1.2%	[0.6%, 1.6%]	4
Regressions ❌ (secondary)	1.5%	[1.5%, 1.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.7%	[-2.6%, -0.7%]	2
All ❌✅ (primary)	1.2%	[0.6%, 1.6%]	4

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean1	range	count2
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.4%, 2.8%]	3
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-21.1%	[-76.4%, -1.6%]	21
All ❌✅ (primary)	-2.5%	[-2.5%, -2.5%]	1

Footnotes

1. the arithmetic mean of the percent change↩↩²↩³

2. number of relevant changes↩↩²↩³

[cjgillot]

@craterbot check

[craterbot]

🎉 Experiment pr-101692 is completed!
📊 11235 regressed and 6 fixed (243276 total)
📰 Open the full report.

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

[cjgillot]

I investigated those failures quite a long time ago, so I can't remember exactly. I'll have to perform a new crater check before stabilisation, so the issue will definitely reappear then.

Simple rebase:
@bors r=oli-obk

[bors]

📌 Commit d3d6269 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit d3d6269 with merge 6cd6bad...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 6cd6bad to master...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 6cd6bad to master...

[rust-timer]

Finished benchmarking commit (6cd6bad): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.4%, 0.6%]	14
Regressions ❌ (secondary)	2.2%	[0.3%, 6.0%]	22
Improvements ✅ (primary)	-0.4%	[-0.7%, -0.2%]	12
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.7%, 0.6%]	26

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.5%	[2.5%, 2.5%]	1
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	2.5%	[2.5%, 2.5%]	1

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[2.3%, 4.8%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

[cjgillot]

perf: I've managed to remove the 5% regression with #107406. Is left a ~1% regression on some incr-unchanged and incr-patched (measured in #107438).

[Mark-Simulacrum]

Thanks! That seems sufficient to me, marking as triaged.

[cjgillot]

#107443 has removed the remaining 1%.