Guarantee `repr(C)` union field offset #2128

joshlf · 2025-12-30T00:49:58Z

AFAICT, this behavior is widely assumed, and is the intended behavior. repr(C) unions are designed to match the behavior of C unions, and per the C standard:

The size of a union is sufficient to contain the
largest of its members. The value of at most one of the
members can be stored in a union object at any time. A
pointer to a union object, suitably converted, points
to each of its members (or if a member is a bit- field,
then to the unit in which it resides), and vice versa.

Makes progress on rust-lang/unsafe-code-guidelines#595

joshlf · 2025-12-30T00:53:48Z

r? t-lang

(Is that valid r? syntax?)

rustbot · 2025-12-30T00:53:53Z

Failed to set assignee to syntax: invalid assignee

Note: Only org members with at least the repository "read" role, users with write permissions, or people who have commented on the PR may be assigned.

RalfJung · 2025-12-30T10:44:38Z

I'm surprised we don't already document this.^^

scottmcm

TBH I think this is so settled that we can just merge it without even an FCP, but cc @rust-lang/lang in case anyone has objections to that.

(It's hard to even argue that it's useful to allow differently even for repr(Rust), so for repr(C) I see absolutely no reason we'd even consider something different.)

traviscross · 2025-12-30T20:12:12Z

Agreed with @scottmcm it seems well settled, and I'm tempted to merge it too, but from a Reference maintainer point-of-view, it's a bit more straightforward to follow the program and lang FCP new (-ly-documented) language guarantees of this kind.

@rfcbot fcp merge lang

(I'll check @scottmcm's box.)

rust-rfcbot · 2025-12-30T20:12:15Z

Team member @traviscross 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.

tmandry · 2026-01-21T19:00:47Z

If this weren't true we'd be in trouble :)

@rfcbot reviewed

rust-rfcbot · 2026-01-21T19:00:56Z

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

Makes progress on rust-lang/unsafe-code-guidelines#595

rustbot · 2026-01-27T22:35:01Z

This PR was rebased onto a different master 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.

traviscross · 2026-01-28T07:12:42Z

cc @rust-lang/fls

rust-rfcbot · 2026-01-31T19:01:58Z

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.

ehuss

Thanks!

Update books ## rust-lang/book 1 commits in 39aeceaa3aeab845bc4517e7a44e48727d3b9dbe..05d114287b7d6f6c9253d5242540f00fbd6172ab 2026-02-03 15:19:04 UTC to 2026-02-03 15:19:04 UTC - Temporarily remove the link to `Drop::drop` (rust-lang/book#4576) ## rust-lang/nomicon 1 commits in 050c002a360fa45b701ea34feed7a860dc8a41bf..b8f254a991b8b7e8f704527f0d4f343a4697dfa9 2026-01-29 12:15:01 UTC to 2026-01-29 12:15:01 UTC - Fix deprecation warning for compare_and_swap in atomics.md (rust-lang/nomicon#519) ## rust-lang/reference 12 commits in 990819b86c22bbf538c0526f0287670f3dc1a67a..addd0602c819b6526b9cc97653b0fadca395528c 2026-02-04 14:35:59 UTC to 2026-01-26 18:02:14 UTC - const-eval.const-expr.field: make paragraph more clear (rust-lang/reference#2157) - make more clear what the link target is (rust-lang/reference#2156) - Update two URLs (rust-lang/reference#2154) - Add a chapter on divergence (rust-lang/reference#2067) - Guarantee `repr(C)` union field offset (rust-lang/reference#2128) - Reference updates for forbidding object lifetime changing pointer casts (rust-lang/reference#1951) - Clarify only arrays undergo unsized coercion during dispatch (rust-lang/reference#2139) - Split the textual chapter into separate char and str chapters (rust-lang/reference#2145) - Document ppc inline asm support (rust-lang/reference#2056) - Unwrap items, expressions, patterns, and types (rust-lang/reference#2141) - undefined behavior: add missing plural in `undefined.misaligned.ptr` (rust-lang/reference#2146) - inline-assembly: add a space to the `asm.abi-clobbers.many` example (rust-lang/reference#2144)

Rollup merge of #152397 - rustbot:docs-update, r=ehuss Update books ## rust-lang/book 1 commits in 39aeceaa3aeab845bc4517e7a44e48727d3b9dbe..05d114287b7d6f6c9253d5242540f00fbd6172ab 2026-02-03 15:19:04 UTC to 2026-02-03 15:19:04 UTC - Temporarily remove the link to `Drop::drop` (rust-lang/book#4576) ## rust-lang/nomicon 1 commits in 050c002a360fa45b701ea34feed7a860dc8a41bf..b8f254a991b8b7e8f704527f0d4f343a4697dfa9 2026-01-29 12:15:01 UTC to 2026-01-29 12:15:01 UTC - Fix deprecation warning for compare_and_swap in atomics.md (rust-lang/nomicon#519) ## rust-lang/reference 12 commits in 990819b86c22bbf538c0526f0287670f3dc1a67a..addd0602c819b6526b9cc97653b0fadca395528c 2026-02-04 14:35:59 UTC to 2026-01-26 18:02:14 UTC - const-eval.const-expr.field: make paragraph more clear (rust-lang/reference#2157) - make more clear what the link target is (rust-lang/reference#2156) - Update two URLs (rust-lang/reference#2154) - Add a chapter on divergence (rust-lang/reference#2067) - Guarantee `repr(C)` union field offset (rust-lang/reference#2128) - Reference updates for forbidding object lifetime changing pointer casts (rust-lang/reference#1951) - Clarify only arrays undergo unsized coercion during dispatch (rust-lang/reference#2139) - Split the textual chapter into separate char and str chapters (rust-lang/reference#2145) - Document ppc inline asm support (rust-lang/reference#2056) - Unwrap items, expressions, patterns, and types (rust-lang/reference#2141) - undefined behavior: add missing plural in `undefined.misaligned.ptr` (rust-lang/reference#2146) - inline-assembly: add a space to the `asm.abi-clobbers.many` example (rust-lang/reference#2144)

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joshlf mentioned this pull request Dec 30, 2025

Can we guarantee that repr(rust) union fields live at offset 0? rust-lang/unsafe-code-guidelines#595

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Guarantee repr(C) union field offset

57aea01

Makes progress on rust-lang/unsafe-code-guidelines#595

ehuss force-pushed the patch-8 branch from ffc7623 to 57aea01 Compare January 27, 2026 22:34

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Merged via the queue into rust-lang:master with commit bb41777 Feb 1, 2026
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Update books rust-lang/rust#152397

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RalfJung mentioned this pull request Feb 12, 2026

Fields must fit in a struct, even for repr(Rust) #2166

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Guarantee repr(C) union field offset #2128

Guarantee repr(C) union field offset #2128

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Guarantee `repr(C)` union field offset #2128

Guarantee `repr(C)` union field offset #2128

joshlf commented Dec 30, 2025 •

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traviscross commented Dec 30, 2025 •

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rust-rfcbot commented Dec 30, 2025 •

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