Lint non_exhaustive_omitted_patterns by columns

[Nadrieril]

This is a rework of the non_exhaustive_omitted_patterns lint to make it more consistent. The intent of the lint is to help consumers of non_exhaustive enums ensure they stay up-to-date with all upstream variants. This rewrite fixes two cases we didn't handle well before:

First, because of details of exhaustiveness checking, the following wouldn't lint Enum::C as missing:

match Some(x) {
    Some(Enum::A) => {}
    Some(Enum::B) => {}
    _ => {}
}

Second, because of the fundamental workings of exhaustiveness checking, the following would treat the true and false cases separately and thus lint about missing variants:

match (true, x) {
    (true, Enum::A) => {}
    (true, Enum::B) => {}
    (false, Enum::C) => {}
    _ => {}
}

Moreover, it would correctly not lint in the case where the pair is flipped, because of asymmetry in how exhaustiveness checking proceeds.

A drawback is that it no longer makes sense to set the lint level per-arm. This will silently break the lint for current users of it (but it's behind a feature gate so that's ok).

The new approach is now independent of the exhaustiveness algorithm; it's a separate pass that looks at patterns column by column. This is another of the motivations for this: I'm glad to move it out of the algorithm, it was akward there.

This PR is almost identical to #111651. cc @eholk who reviewed it at the time. Compared to then, I'm more confident this is the right approach.

[rustbot]

r? @cjgillot

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[Nadrieril]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 493111e with merge 2d1ddb5...

[Nadrieril]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 33a65f2 with merge 81a52d7...

[Nadrieril]

@bors try @rust-timer queue

[bors]

⌛ Trying commit ca869e3 with merge 360c042...

[bors]

☀️ Try build successful - checks-actions
Build commit: 360c042 (360c042a375d7b1717a20b3ee87893b38dede496)

[rust-timer]

Finished benchmarking commit (360c042): comparison URL.

Overall result: ❌ regressions - 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-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.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.4%	[1.2%, 1.5%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	1.4%	[1.0%, 1.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	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: 627.524s -> 629.57s (0.33%)
Artifact size: 271.31 MiB -> 271.24 MiB (-0.02%)

[cjgillot]

A drawback is that it no longer makes sense to set the lint level per-arm. This will silently break the lint for current users of it (but it's behind a feature gate so that's ok).

This check is an global analysis anyway, so I'm not shocked by seeing it anchored to the match itself and not to an arm.

Perf: the only impact is on match-stress, which is a stress test for this code, so 1.5 % there does not indicate a widespread regression.

@bors r+

[bors]

📌 Commit ca869e3 has been approved by cjgillot

It is now in the queue for this repository.

[Nadrieril]

Yay thanks!

[bors]

⌛ Testing commit ca869e3 with merge 786c94a...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 786c94a to master...

[rust-timer]

Finished benchmarking commit (786c94a): comparison URL.

Overall result: ❌ regressions - 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
Regressions ❌ (secondary)	1.5%	[1.5%, 1.6%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	2.9%	[2.2%, 3.6%]	2
Regressions ❌ (secondary)	3.0%	[2.1%, 5.3%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-6.3%, -0.9%]	8
All ❌✅ (primary)	2.9%	[2.2%, 3.6%]	2

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: 629.633s -> 630.361s (0.12%)
Artifact size: 304.07 MiB -> 301.94 MiB (-0.70%)

[rylev]

Given that the impacted benchmark is the stress test for the code in question, I think the small regression is fine.

@rustbot label: +perf-regression-triaged

[lqd]

IIRC @Nadrieril has many in-progress PRs and local changes: does some of this work maybe already reduces or eliminates this small regression?

[Nadrieril]

Unfortunately no. Moreover, these surprising match-stress changes often don't show up on my local perf runs so I'm going blind :/

I just noticed: this has an instruction count regression but no cycle count or wall-time regression. Sounds fine actually