do not apply DerefMut on union field by RalfJung · Pull Request #75584 · rust-lang/rust

RalfJung · 2020-08-16T08:35:37Z

This implements the part of RFC 2514 about DerefMut. Unlike described in the RFC, we only apply this warning specifically when doing DerefMut of a ManuallyDrop field; that is really the case we are worried about here.

@matthewjasper suggested I patch convert_place_derefs_to_mutable and convert_place_op_to_mutable for this, but I could not find anything to do in convert_place_op_to_mutable and this is sufficient to make the test pass. However, maybe there are some other cases this misses? I have no familiarity with this code.

This is a breaking change in theory, if someone used ManuallyDrop<T> in a union field and relied on automatic DerefMut. But on stable this means T: Copy, so the ManuallyDrop is rather pointless.

Cc #55149

rust-highfive · 2020-08-16T08:35:41Z

r? @petrochenkov

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RalfJung · 2020-08-16T08:36:11Z

The span for this is slightly wrong, I'd expect it to only cover u.f.0. I don't know how this is happening, since the span is right above.

I would guess this has to do with the special handling of multiple tuple field accesses in a row. Perhaps 0.0 is a float token.

I didn't know there was such special handling.^^ So this would be a problem elsewhere, in the code that sets up the span for these places?

petrochenkov · 2020-08-16T09:34:58Z

r? @matthewjasper

matthewjasper

This looks good.

RalfJung · 2020-08-31T11:03:40Z

Rebased to resolve conflict (but it was a clean rebase); @matthewjasper this is ready for another round of review.

matthewjasper · 2020-09-04T18:55:54Z

@bors r+

bors · 2020-09-04T18:55:56Z

📌 Commit 66b340f has been approved by matthewjasper

@matthewjasper

…asper do not apply DerefMut on union field This implements the part of [RFC 2514](https://github.com/rust-lang/rfcs/blob/master/text/2514-union-initialization-and-drop.md) about `DerefMut`. Unlike described in the RFC, we only apply this warning specifically when doing `DerefMut` of a `ManuallyDrop` field; that is really the case we are worried about here. @matthewjasper suggested I patch `convert_place_derefs_to_mutable` and `convert_place_op_to_mutable` for this, but I could not find anything to do in `convert_place_op_to_mutable` and this is sufficient to make the test pass. However, maybe there are some other cases this misses? I have no familiarity with this code. This is a breaking change *in theory*, if someone used `ManuallyDrop<T>` in a union field and relied on automatic `DerefMut`. But on stable this means `T: Copy`, so the `ManuallyDrop` is rather pointless. Cc rust-lang#55149

@matthewjasper

…asper do not apply DerefMut on union field This implements the part of [RFC 2514](https://github.com/rust-lang/rfcs/blob/master/text/2514-union-initialization-and-drop.md) about `DerefMut`. Unlike described in the RFC, we only apply this warning specifically when doing `DerefMut` of a `ManuallyDrop` field; that is really the case we are worried about here. @matthewjasper suggested I patch `convert_place_derefs_to_mutable` and `convert_place_op_to_mutable` for this, but I could not find anything to do in `convert_place_op_to_mutable` and this is sufficient to make the test pass. However, maybe there are some other cases this misses? I have no familiarity with this code. This is a breaking change *in theory*, if someone used `ManuallyDrop<T>` in a union field and relied on automatic `DerefMut`. But on stable this means `T: Copy`, so the `ManuallyDrop` is rather pointless. Cc rust-lang#55149

bors · 2020-09-05T09:32:28Z

⌛ Testing commit 66b340f with merge ea590e7b64fbc953a5d33618e5af7c32915f465e...

bors · 2020-09-05T09:35:14Z

💔 Test failed - checks-actions

rust-log-analyzer · 2020-09-05T09:35:39Z

Your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @rust-lang/infra. (Feature Requests)

RalfJung · 2020-09-05T09:36:38Z

Clang installation failure.
@bors retry

bors · 2020-09-05T11:47:07Z

⌛ Testing commit 66b340f with merge 81a769f...

bors · 2020-09-05T13:55:21Z

☀️ Test successful - checks-actions, checks-azure
Approved by: matthewjasper
Pushing 81a769f to master...

rust-highfive assigned petrochenkov Aug 16, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 16, 2020

RalfJung commented Aug 16, 2020
View reviewed changes

RalfJung force-pushed the union-no-deref branch from 736a46c to 85cb4a2Compare August 16, 2020 09:19

rust-highfive assigned matthewjasper and unassigned petrochenkov Aug 16, 2020

matthewjasper reviewed Aug 30, 2020
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Comment threadsrc/test/ui/union/union-deref.rs Outdated

Centril mentioned this pull request Aug 30, 2020
Tracking issue for RFC 2514, "Union initialization and Drop" #55149
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RalfJung added 4 commits August 31, 2020 10:14

do not apply DerefMut on union field
ec0924f

also detect DerefMut in nested union fields
44defae

only emit error for ManuallyDrop derefs
97974e3

test more ways of mutably accessing a place
66b340f

RalfJung force-pushed the union-no-deref branch from dabedc3 to 66b340fCompare August 31, 2020 08:15

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 Sep 4, 2020

Dylan-DPC-zz mentioned this pull request Sep 4, 2020
Rollup of 14 pull requests #76348
Closed

Dylan-DPC-zz mentioned this pull request Sep 4, 2020
Rollup of 13 pull requests #76350
Closed

bors removed the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Sep 5, 2020

bors added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 5, 2020

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 Sep 5, 2020

bors added the merged-by-bors This PR was explicitly merged by bors. label Sep 5, 2020

bors merged commit 81a769f into rust-lang:masterSep 5, 2020

rustbot added this to the 1.48.0 milestone Sep 5, 2020

RalfJung deleted the union-no-deref branch September 8, 2020 12:19

Alexendoo mentioned this pull request May 26, 2025
Do not suggest to use implicit DerefMut on ManuallyDrop reached through unions rust-lang/rust-clippy#14387
Merged

eggyal mentioned this pull request Jan 31, 2026
Do not automatically DerefMut ManuallyDrop through union references #151920
Closed

eggyal mentioned this pull request Feb 13, 2026
DerefMut auto-deref error for union fields sometimes doesn't trigger #141621
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