Warn users who set non_exhaustive_omitted_patterns lint level on a match arm

[Nadrieril]

Before #116734, the recommended usage of the non_exhaustive_omitted_patterns lint was:

match Bar::A {
    Bar::A => {},
    #[warn(non_exhaustive_omitted_patterns)]
    _ => {},
}

After #116734 this no longer makes sense, and recommended usage is now:

#[warn(non_exhaustive_omitted_patterns)]
match Bar::A {
    Bar::A => {},
    _ => {},
}

As you can guess, this silently breaks all uses of the lint that used the previous form. This is a problem in particular because syn recommends usage of this lint to its users in the old way. This PR emits a warning when the previous form is used so users can update.

r? @cjgillot

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[Nadrieril]

(the only clippy change is a test output)

[Nadrieril]

I realized this will likely regress the match-stress bench because it has 8000 arms. Very much a case where this bench is not representative of the world

@bors try @rust-timer queue

[bors]

⌛ Trying commit d53f545 with merge c6ca96d...

[bors]

☀️ Try build successful - checks-actions
Build commit: c6ca96d (c6ca96d0edee56cc12810efaafa59dfc8fb1e2b1)

[rust-timer]

Finished benchmarking commit (c6ca96d): 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)	0.2%	[0.2%, 0.2%]	2
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	2

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)	-	-	0
Regressions ❌ (secondary)	0.8%	[0.8%, 0.8%]	1
Improvements ✅ (primary)	-1.0%	[-1.0%, -1.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.0%	[-1.0%, -1.0%]	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)	2.4%	[2.0%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 636.074s -> 636.231s (0.02%)
Artifact size: 304.47 MiB -> 304.47 MiB (-0.00%)

[cjgillot]

Should we add a suggestion to move the attribute on the match itself?

[Nadrieril]

Oh yes good idea, I'll look into it!

[Nadrieril]

Done!

[Nadrieril]

Well, I added a suggestion to add the lint on the match, I didn't find how to add a suggestion to remove the lint on the arm. Should I do this too?

[cjgillot]

You need to suggest an empty string in place of the current lint attribute's span.

[Nadrieril]

Hmm, I can get the span for the lint level, but it's not the span of the whole attribute; removing it does:

-         #[deny(non_exhaustive_omitted_patterns)]
+         #[deny()]
           _ => {}

It seems lint_level_at_node does not return anything else than this span to identify where the lint was set. Can I use the span to get to the span of the whole attribute somehow? Or use the arm HirId? It's not helping that rustc internal docs are missing atm :'(

[Nadrieril]

Turns out that even with a better span, it's pretty hard to make clear that I'm suggesting to remove the line. Instead I added a "remove this" span label, it's a lot clearer ^^

[bors]

☔ The latest upstream changes (presumably #117564) made this pull request unmergeable. Please resolve the merge conflicts.

[cjgillot]

@bors r+

[bors]

📌 Commit f0e8330 has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit f0e8330 with merge f81d6f0...

[Nadrieril]

Thank u!

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing f81d6f0 to master...

[rust-timer]

Finished benchmarking commit (f81d6f0): 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)	0.2%	[0.2%, 0.3%]	3
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.3%]	3

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)	0.6%	[0.6%, 0.7%]	2
Regressions ❌ (secondary)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-1.2%	[-2.1%, -0.4%]	4
Improvements ✅ (secondary)	-1.8%	[-3.2%, -1.2%]	4
All ❌✅ (primary)	-0.6%	[-2.1%, 0.7%]	6

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)	0.6%	[0.4%, 0.7%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[0.4%, 0.7%]	3

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 635.472s -> 635.521s (0.01%)
Artifact size: 304.37 MiB -> 304.34 MiB (-0.01%)

[pnkfelix]

• regressed check incr-full for cranelift-codegen (0.25%), cargo (0.22%) and hyper (0.21%).
• this small amount does not appear to be noise
• but it shouldn't have any (significant) effect if the non_exhaustive_omitted_patterns lint is not in use.
• marking as triaged

@rustbot label: +perf-regression-triaged