Lint against iterator functions that panic when N is zero

[Urgau]

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This PR extends the deny-by-default unconditional_panic lint, by linting on iterator functions that panics when N (chunks/windows size) is zero¹.

Those methods are documented to panic if N is zero.

error: this operation will panic at runtime
  --> $DIR/const-n-is-zero.rs:11:13
   |
LL |     let _ = s.array_windows::<0>();
   |             ^^^^^^^^^^^^^^^^^^^^^^ const parameter `N` is zero
   |
   = note: `#[deny(unconditional_panic)]` on by default

cc @rust-lang/libs-api

Footnotes

1. this is done by introducing a new internal attribute on the const parameter: #[rustc_panics_when_zero] ↩

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

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[RalfJung]

This PR adds new kind of variant to the deny-by-default unconditional_panic lint, by linting on iterator functions that panics when N (chunks/windows size) is zero.

This seems to do much more than that. It adds a new language primitive, a new kind of built-in assertion. Could you motivate that?

[Urgau]

This seems to do much more than that. It adds a new language primitive, a new kind of built-in assertion. Could you motivate that?

The code is unfortunately tangled, it doesn't add a new language primitive per se, but report_assert_as_lint requires a AssertKind variant even if it's just for linting in one place.

I could probably modify that function so it doesn't take an AssertKind, but I wasn't sure about that.

[RalfJung]

Given that AssertKind is part of the MIR syntax, I feel very strongly that we shouldn't modify it for non-MIR/opsem purposes.

[Urgau]

Understandable, I've reworked the PR to avoid touching at AssertKind.

[JohnTitor]

I love the idea but it should better for someone more familiar with this topic other than me to review. @RalfJung Could you take over? Otherwise I'd reroll.

View changes since this review

[RalfJung]

I'm afraid not, I don't have capacity at the moment.

@rustbot reroll

[fee1-dead]

I think the overall idea is good and I could see myself approving down the line, however I think we should see if we can have an attribute on the generic parameter itself instead of applying rustc_panics_when_n_is_zero to the enclosing function. If not, I'd still want something more verbose, such as #[rustc_panics_when_const_param_is_zero(N)]

I don't think this should be touching AssertLint. I feel like it might be simpler to just add a separate error for this rather than using existing mechanisms.

View changes since this review

[Urgau]

I had to add the encoding of const-params in rmeta, but otherwise I've moved the attribute directly to the const generic.

I've also removed any interaction with AssertLint, but kept the reporting behind the unconditional_panic lint.

@rustbot ready

[fee1-dead]

overall looks good, would like someone else to sign off with me on this together though

@rustbot reroll

[Urgau]

@fee1-dead also approved it in this comment.

@bors r=fee1-dead,ShoyuVanilla

[rust-bors]

📌 Commit 93b8cf2 has been approved by fee1-dead,ShoyuVanilla

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

[JonathanBrouwer]

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: https://github.com/rust-lang/rust/actions/runs/30255157007/job/89942047433?pr=160002

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#160002), which was unapproved.

View changes since this unapproval

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[ShoyuVanilla]

@bors r=fee1-dead,ShoyuVanilla

[rust-bors]

📌 Commit 24e4698 has been approved by fee1-dead,ShoyuVanilla

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

[jhpratt]

@rust-timer build 4020bb0

[rust-timer]

Finished benchmarking commit (4020bb0): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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
Regressions ❌ (secondary)	0.1%	[0.1%, 0.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -9.3%, secondary 0.2%)

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)	3.8%	[2.1%, 5.5%]	2
Improvements ✅ (primary)	-9.3%	[-9.3%, -9.3%]	1
Improvements ✅ (secondary)	-2.3%	[-4.2%, -0.6%]	3
All ❌✅ (primary)	-9.3%	[-9.3%, -9.3%]	1

Cycles

Results (primary -2.5%, secondary 5.9%)

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)	14.6%	[2.2%, 39.3%]	6
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-4.5%	[-6.3%, -2.7%]	5
All ❌✅ (primary)	-2.5%	[-2.5%, -2.5%]	1

Binary size

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

Bootstrap: 487.856s -> 488.493s (0.13%)
Artifact size: 388.05 MiB -> 390.08 MiB (0.52%)