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Add missing reverse PartialEq impls for Cow - #160973

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kiana1kaslana:cow_partialeq_reverse
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Add missing reverse PartialEq impls for Cow#160973
kiana1kaslana wants to merge 2 commits into
rust-lang:mainfrom
kiana1kaslana:cow_partialeq_reverse

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@kiana1kaslana

@kiana1kaslana kiana1kaslana commented Aug 12, 2026 •
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Closes #152830

Implements the Cow-related cases that #152972 doesn't cover:

  • impl PartialEq<Cow<'_, [U]>> for Vec<T, A>
  • impl PartialEq<Cow<'_, [U]>> for &[T]
  • impl PartialEq<Cow<'_, [U]>> for &mut [T]

Without these, expressions like vec == cow or slice == cow fail to compile.

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 12, 2026
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Thanks for the pull request, and welcome! The Rust Project is excited to review your changes, and you should hear from @clarfonthey (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

  • @rustbot author: the review is finished, PR author should check the comments and take action accordingly
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Why was this reviewer chosen?

The reviewer was selected based on:

  • Owners of files modified in this PR: libs
  • libs expanded to 12 candidates
  • Random selection from JohnTitor, Mark-Simulacrum, clarfonthey, nia-e

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kianalikasana
Add missing reverse PartialEq impls for Cow<'_, [T]>
b3383b6 
Vec, &[T] and &mut [T] already implement PartialEq against Cow<'_, [U]>,
but the reverse direction is missing, so `vec == cow`, `slice == cow`
and `mut_slice == cow` all fail to compile.

Add the three reverse impls to fill the remaining gap from issue 152830,
which was partially addressed by the recent VecDeque companion PR 152972.
@kiana1kaslana
kiana1kaslana force-pushed the cow_partialeq_reverse branch from 86bb69c to b3383b6 Compare August 12, 2026 09:39
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kianalikasana
Bless UI tests for new Cow reverse PartialEq impls
a088a5c 
Adding `impl PartialEq<Cow<'_, [U]>>` for `&[T]` and `&mut [T]` shifts
the trait help message in two UI tests, which now list the new impls
and push two older entries into "N others":
- tests/ui/consts/too_generic_eval_ice.current.stderr
- tests/ui/macros/assert-ne-no-invalid-help-issue-146204.stderr

No source changes; only the expected stderr snapshots were regenerated.
@clarfonthey

clarfonthey commented Aug 12, 2026

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I remember mentioning this during the libs meaning, but don't actually remember why it came up since this isn't nominated.

My general opinion is we should have these, but I do question whether they weren't added initially because of inference failures, and not just because we forgot them. So, it makes sense to do a crater run first.

@bors try

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rust-bors Bot pushed a commit that referenced this pull request Aug 12, 2026
@bors
Auto merge of #160973 - kiana1kaslana:cow_partialeq_reverse, r=<try>
0638027 
Add missing reverse PartialEq impls for Cow
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rust-bors Bot commented Aug 12, 2026

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☀️ Try build successful (CI)
Build commit: 0638027 (0638027fc267e2ac327148556d875936927466d2)
Base parent: ab8058a (ab8058aa09e8c14b86b3d06c08ade66f863d22fe)

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clarfonthey commented Aug 12, 2026

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@craterbot check

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craterbot commented Aug 12, 2026

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👌 Experiment pr-160973 created and queued.
🤖 Automatically detected try build 0638027
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

@craterbot craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 12, 2026
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rust-bors Bot commented Aug 23, 2026

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☔ The latest upstream changes (presumably #161566) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

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clarfonthey commented Aug 23, 2026

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FWIW: The crater runs are delayed since the release crater runs are the ones that are prioritised right now. I don't think we need to cancel the running job since it'll still be useful for whether these impls specifically cause regressions.

@kiana1kaslana

kiana1kaslana commented Aug 23, 2026

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Closing — #156160 (which went in via #161566) added the same impls, so this is no longer needed. Thanks to @clarfonthey for the review and the crater run, sorry for the churn.

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PartialEq implementations are missing for symmetric comparisons between certain collections

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