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Remove T: Debug bound on UnsafeCell Debug impl#83707

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exrook:unsafecell
Apr 13, 2021
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Remove T: Debug bound on UnsafeCell Debug impl#83707
bors merged 1 commit intorust-lang:masterfrom
exrook:unsafecell

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@exrook exrook commented Mar 31, 2021 •
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Prior art: #65013

@bjorn3 bjorn3 requested a review from Mark-Simulacrum March 31, 2021 16:18
@bjorn3 bjorn3 removed the request for review from Mark-Simulacrum March 31, 2021 16:19
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bjorn3 commented Mar 31, 2021

Assigned to a random member of the libs team as highfive didn't respond.

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Mar 31, 2021
@JohnTitor JohnTitor added the T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. label Mar 31, 2021
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Mark-Simulacrum commented Apr 2, 2021

Not a member of the libs team :)

cc @rust-lang/libs, do we want to do a full FCP here? It seems clear we can't print the value in a safe way, so there's not much we can do with the Debug bound I think, even if we have it.

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Amanieu commented Apr 2, 2021

It doesn't hurt to be sure. I can't see any reason why this would cause any issues though.

@rfcbot merge

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rfcbot commented Apr 2, 2021 •
edited by m-ou-se
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Team member @Amanieu 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!

See this document for info about what commands tagged team members can give me.

@rfcbot 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 2, 2021
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I agree -- we can't possibly go from &UnsafeCell<T> to &T in order to invoke T's Debug impl, so the T: Debug bound is not useful.

Conceivably Debug could grow a defaulted method which takes an argument of type &UnsafeCell<Self>, which this PR rules out, but I am comfortable ruling that out because there isn't a type which could meaningfully format an arbitrary &UnsafeCell<Self>.

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joshtriplett commented Apr 2, 2021

This seems reasonable to me as well. Because of the rules regarding trait impls, nobody could have an impl Debug for UnsafeCell<SomeType>, even for a type or trait defined in their crate, without having a wrapper type.

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rfcbot commented Apr 2, 2021

🔔 This is now entering its final comment period, as per the review above. 🔔

@rfcbot rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Apr 2, 2021
@rfcbot rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Apr 12, 2021
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rfcbot commented Apr 12, 2021

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

The RFC will be merged soon.

@rfcbot rfcbot added the to-announce Announce this issue on triage meeting label Apr 12, 2021
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m-ou-se commented Apr 12, 2021

@bors r+ rolllup

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bors commented Apr 12, 2021

📌 Commit 58218f6 has been approved by m-ou-se

@bors 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-review Status: Awaiting review from the assignee but also interested parties. labels Apr 12, 2021
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m-ou-se commented Apr 12, 2021

@bors rollup

Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this pull request Apr 12, 2021
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Rollup merge of rust-lang#83707 - exrook:unsafecell, r=m-ou-se
abb93d3 
Remove `T: Debug` bound on UnsafeCell Debug impl

Prior art: rust-lang#65013
@Dylan-DPC-zz Dylan-DPC-zz mentioned this pull request Apr 12, 2021
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@Dylan-DPC-zz Dylan-DPC-zz mentioned this pull request Apr 13, 2021
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bors added a commit to rust-lang-ci/rust that referenced this pull request Apr 13, 2021
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Auto merge of rust-lang#84153 - Dylan-DPC:rollup-5jiqrwu, r=Dylan-DPC
5258a74 
Rollup of 6 pull requests

Successful merges:

 - rust-lang#83438 (Update RELEASES.md)
 - rust-lang#83707 (Remove `T: Debug` bound on UnsafeCell Debug impl)
 - rust-lang#84084 (Stabilize duration_zero.)
 - rust-lang#84121 (Stabilize BTree{Map,Set}::retain)
 - rust-lang#84140 (Don't call bump in check_mistyped_turbofish_with_multiple_type_params)
 - rust-lang#84141 (Fix typo in error message)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit 1cc4b6d into rust-lang:master Apr 13, 2021
@rustbot rustbot added this to the 1.53.0 milestone Apr 13, 2021
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Apr 15, 2021
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