Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints #124097

compiler-errors · 2024-04-17T21:10:35Z

Adds a similar method probe opt-out mechanism to the [T;N]: IntoIterator implementation for edition 2021.
Adjusts the relevant lints (shadowed .into_iter() calls, new source of method ambiguity).
Adds some tests.
Took the liberty to rework the logic in the ARRAY_INTO_ITER lint, since it was kind of confusing.

Based mostly off of #116607.

ACP: rust-lang/libs-team#263
References #59878
Tracking for Rust 2024: #123759

Crater run was done here: #116607 (comment)
Consensus afaict was that there is too much breakage, so let's do this in an edition-dependent way much like [T; N]: IntoIterator.

rustbot · 2024-04-17T21:10:43Z

r? @oli-obk

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rustbot · 2024-04-17T21:10:45Z

This PR changes Stable MIR

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

compiler-errors · 2024-04-17T21:11:28Z

Needs a T-compiler review of the impl first, then can go into FCP.

compiler-errors · 2024-04-17T21:14:04Z

compiler/rustc_ast/src/ptr.rs

@@ -184,7 +184,7 @@ impl<'a, T> IntoIterator for &'a P<[T]> {
    type Item = &'a T;
    type IntoIter = slice::Iter<'a, T>;
    fn into_iter(self) -> Self::IntoIter {
-        self.ptr.into_iter()
+        self.ptr.iter()


Very funny to see somewhere in the compiler where we were relying on this behavior.

tmandry · 2024-04-17T23:11:26Z

I looked over the PR and I think we can kick off FCP in tandem with code review. The implementation includes a full migration that behaves as I'd expect.

@rust-lang/lang Are we okay with applying the same method resolution hack used for slices in 2021 to boxed slices in 2024?

@rust-lang/libs-api Are we okay with insta-stabilizing impl<T> IntoIterator for Box<[T]> and applying the method resolution hack?

@rfcbot fcp merge

rfcbot · 2024-04-17T23:11:28Z

Team member @tmandry has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

BurntSushi · 2024-04-17T23:36:37Z

The IntoIterator impl using vec::IntoIter<I, A> for Box<[T]> strikes me as a little odd, since Box<[T]> isn't a Vec. But I also can't necessarily come up with a specific concern with reusing the type other than "it seems weird."

rustbot · 2024-04-17T23:57:33Z

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

compiler-errors · 2024-04-18T00:05:32Z

For FCP folks, see some of the discussion in #116607 -- apparently reusing vec::IntoIter was discussed in a T-libs-api meeting and the consensus was that it was not desired to add a new type.

Also, this PR doesn't implement IntoIterator for Box<[T; N]>. That impl seems slightly less useful, since calling Box::new([1, 2, 3]).into_iter() already uses the IntoIterator for [T; N] impl via autoderef, so we already get an impl Iterator<Item = T>.

If anyone has a strong opinion, I could either add to this PR, or (ideally) add to this PR in a follow-up. It would only cause a breaking change when the user is relying on the exact array::Iter output type of [T; N], rather than the much larger fallout of this change.

clarfonthey · 2024-04-18T01:55:29Z

Also, this PR doesn't implement IntoIterator for Box<[T; N]>. That impl seems slightly less useful, since calling Box::new([1, 2, 3]).into_iter() already uses the IntoIterator for [T; N] impl via autoderef, so we already get an impl Iterator<Item = T>.

If anyone has a strong opinion, I could either add to this PR, or (ideally) add to this PR in a follow-up. It would only cause a breaking change when the user is relying on the exact array::Iter output type of [T; N], rather than the much larger fallout of this change.

My personal opinion here is that it would be desirable to have a heap-based iterator here, since using the array implementation would copy the entire array onto the stack before iterating. It also helps avoid the weird discrepancy between Box<[T]> and Box<[T; N]> having different performance implications.

Worth noting that a lot of people will use Box<[T; N]> also instead of [T; N] when N is large because they want to avoid storing the array on the stack, and iterating over it by copying it to the stack would go against these desires.

…=<try> [crate] Add `Box<[T; N]>: IntoIterator` without any method dispatch hacks **Unlike** `Box<[T]>` (rust-lang#116607 (comment)), there's a much higher chance that this will not conflict with existing usages since it produces an iterator with the same type before/after this change, but let's test that theory with crater. Ideally we have fewer migrations that are tangled up in hacks like `rustc_skip_during_method_dispatch`, so if this crater comes back clean, I'd strongly suggest landing this as-is. As for the rationale for having this impl at all, I agree (as `@clarfonthey` pointed out in rust-lang#124097 (comment)) that it is generally better for any user to not require moving the array *out* of the box just to turn it into an iterator.

rustbot · 2024-05-14T19:54:57Z

Some changes occurred in src/tools/cargo

cc @ehuss

WaffleLapkin

r=me with the nits

compiler/rustc_hir_typeck/src/method/prelude_edition_lints.rs

WaffleLapkin · 2024-05-15T00:39:10Z

compiler/rustc_hir_typeck/src/method/prelude_edition_lints.rs

+                    && !span.at_least_rust_2024()
+                {
+                    // In this case, it wasn't really a prelude addition that was the problem.
+                    // Instead, the problem is that the array-into_iter hack will no longer apply in Rust 2021.


The comment is still the same?

WaffleLapkin · 2024-05-15T00:47:47Z

library/alloc/src/boxed.rs

+/// `Box` is fundamental, so coherence needs help understanding these impls are okay.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<I, A: Allocator> !Iterator for Box<[I], A> {}
+
+/// `Box` is fundamental, so coherence needs help understanding these impls are okay.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, A: Allocator> !Iterator for &'a Box<[I], A> {}
+
+/// `Box` is fundamental, so coherence needs help understanding these impls are okay.
+#[stable(feature = "boxed_slice_into_iter", since = "CURRENT_RUSTC_VERSION")]
+impl<'a, I, A: Allocator> !Iterator for &'a mut Box<[I], A> {}


Those are user facing docs, maybe we should write something that can be understood by normal users? (this can be a follow up)

fff I thought I fixed these. I'll copy them over from the other PR for Box<[T; N]> where I think did.

WaffleLapkin · 2024-05-15T00:49:37Z

@rustbot author

…not-notable, r=fmease rustdoc: Negative impls are not notable In rust-lang#124097, we add `impl !Iterator for [T]` for coherence reasons, and since `Iterator` is a [notable trait](https://github.com/rust-lang/rust/blob/8387315ab3c26a57a1f53a90f188f0bc88514bca/library/core/src/iter/traits/iterator.rs#L40), this means that all `-> &[_]` now are tagged with a `!Iterator` impl as a notable trait. I "fixed" the failing tests in that PR with 6cbbb8b, where I just blessed the tests, since I didn't want to mix these changes with that PR; however, don't believe negative impls are notable, and this PR aims to prevent these impls from being mentioned. In the standard library, we use negative impls purely to guide coherence. They're not really a signal of anything useful to the end-user. If there ever is a case that we want negative impls to be mentioned as notable, this really should be an opt-in feature.

bors · 2024-05-19T22:12:02Z

☔ The latest upstream changes (presumably #125006) made this pull request unmergeable. Please resolve the merge conflicts.

Co-authored-by: ltdk <usr@ltdk.xyz>

compiler-errors · 2024-05-21T00:00:38Z

@bors r=WaffleLapkin

bors · 2024-05-21T00:00:40Z

📌 Commit 917bb83 has been approved by WaffleLapkin

It is now in the queue for this repository.

bors · 2024-05-21T10:13:57Z

⌛ Testing commit 917bb83 with merge e8fbd99...

bors · 2024-05-21T12:49:01Z

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing e8fbd99 to master...

rust-timer · 2024-05-21T14:03:22Z

Finished benchmarking commit (e8fbd99): comparison URL.

Overall result: ❌ regressions - no action needed

@rustbot label: -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)	0.4%	[0.3%, 0.5%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.1%, secondary -3.4%)

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
Improvements ✅ (primary)	-2.1%	[-2.1%, -2.1%]	1
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	1

Cycles

Results (secondary -4.0%)

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
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.0%	[-4.0%, -4.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 669.761s -> 671.391s (0.24%)
Artifact size: 316.16 MiB -> 315.47 MiB (-0.22%)

rustbot assigned oli-obk Apr 17, 2024

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compiler-errors force-pushed the box-into-iter branch from 0aafa34 to 6fcbd7a Compare April 17, 2024 21:15

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rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Apr 17, 2024

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compiler-errors mentioned this pull request Apr 18, 2024

Add Box<[T; N]>: IntoIterator without any method dispatch hacks #124108

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compiler-errors force-pushed the box-into-iter branch from 03ec16e to 6cbbb8b Compare May 15, 2024 00:21

compiler-errors mentioned this pull request May 15, 2024

rustdoc: Negative impls are not notable #125134

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WaffleLapkin approved these changes May 15, 2024

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Veykril mentioned this pull request May 18, 2024

rustc_skip_array_during_method_dispatch -> rustc_skip_during_method_dispatch rust-lang/rust-analyzer#17256

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dtolnay mentioned this pull request May 19, 2024

Methods for extracting 1 element out of an owned slice and dropping the rest rust-lang/libs-team#376

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compiler-errors and others added 6 commits May 20, 2024 19:16

Backticks

c86a4aa

Add the impls for Box<[T]>: IntoIterator

1a81092

Co-authored-by: ltdk <usr@ltdk.xyz>

Implement BOXED_SLICE_INTO_ITER

a502e7a

Adjust the method ambiguity lint too

bc6b70f

Remove a clippy test that doesn't apply anymore

bd0d306

Fix miri too

917bb83

compiler-errors force-pushed the box-into-iter branch from 6cbbb8b to 917bb83 Compare May 20, 2024 23:21

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels May 21, 2024

bors added the merged-by-bors This PR was explicitly merged by bors. label May 21, 2024

bors merged commit e8fbd99 into rust-lang:master May 21, 2024
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rustc_lint cleanups #125057

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Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints #124097

Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints #124097

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edited by traviscross

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Add IntoIterator for Box<[T]> + edition 2024-specific lints #124097

Add IntoIterator for Box<[T]> + edition 2024-specific lints #124097

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Overall result: ❌ regressions - no action needed

Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints #124097

Add `IntoIterator` for `Box<[T]>` + edition 2024-specific lints #124097

compiler-errors commented Apr 17, 2024 •

edited by traviscross

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edited by m-ou-se

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edited