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transmute: fix check for whether newtypes have equal size#155418

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transmute: fix check for whether newtypes have equal size#155418
RalfJung wants to merge 1 commit intorust-lang:mainfrom
RalfJung:transmute-newtypes

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@RalfJung RalfJung commented Apr 17, 2026 •
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Fixes #155412
Fixes #88290

I manually checked that the playground example linked in #88290 now emits all three expected errors (and we have equivalent cases in the tests, either preexisting or added by this PR, so it didn't seem worth adding that example as well).

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r? @JohnTitor

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@RalfJung RalfJung marked this pull request as draft April 17, 2026 06:59
@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 17, 2026
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RalfJung commented Apr 17, 2026
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Comment thread compiler/rustc_middle/src/ty/layout.rs Outdated
if int.is_some()
|| align.is_some()
|| pack.is_some()
|| flags.difference(ReprFlags::IS_TRANSPARENT) != Default::default()
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@RalfJung RalfJung Apr 17, 2026 •
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FWIW repr(C) is probably sometimes okay, but this code also handles enums with null pointer optimization and there repr(C) is definitely not okay.

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RalfJung commented Apr 17, 2026

@bors try

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Auto merge of #155418 - RalfJung:transmute-newtypes, r=<try>
c9eed2c 
transmute: fix check for whether newtypes have equal size
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RalfJung commented Apr 17, 2026 via email

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theemathas commented Apr 17, 2026 •
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@RalfJung IIRC, the target that failed has -Zrandomize-layout turned on in CI.

Edit: Yup. #t-compiler/help > `Span` is 96 bits?

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rust-bors bot commented Apr 17, 2026

☀️ Try build successful (CI)
Build commit: c9eed2c (c9eed2c4c04c13e48841f9c446608a7322ad16d5, parent: 0204aca0663ec534b773f78639b9ebe20b3da001)

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RalfJung commented Apr 17, 2026

Ah! There's a "randomize layout" flag.

That should not affect crater though so we can go ahead and
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craterbot commented Apr 17, 2026

👌 Experiment pr-155418 created and queued.
🤖 Automatically detected try build c9eed2c
🔍 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 17, 2026
@RalfJung RalfJung force-pushed the transmute-newtypes branch from f5db719 to 9d897cb Compare April 17, 2026 09:54
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RalfJung commented Apr 17, 2026

Actually, there's another flag we should ignore, and it's set on Box, which could affect crater.

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rust-bors bot pushed a commit that referenced this pull request Apr 17, 2026
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Auto merge of #155418 - RalfJung:transmute-newtypes, r=<try>
ad93f57 
transmute: fix check for whether newtypes have equal size
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craterbot commented Apr 17, 2026

🗑️ Experiment pr-155418 deleted!

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@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Apr 17, 2026
@RalfJung RalfJung force-pushed the transmute-newtypes branch from 9d897cb to c634619 Compare April 17, 2026 10:31
@RalfJung RalfJung marked this pull request as ready for review April 17, 2026 10:31
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JohnTitor commented Apr 17, 2026

@rustbot reroll

@rustbot rustbot assigned nikomatsakis and unassigned JohnTitor Apr 17, 2026
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rust-bors bot commented Apr 17, 2026

☀️ Try build successful (CI)
Build commit: ad93f57 (ad93f57d532978d6739002a21bc66b45ed22f998, parent: 1b8f2e46e14b08208a53585570edd9206374aae8)

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RalfJung commented Apr 17, 2026

@craterbot check

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craterbot commented Apr 17, 2026

👌 Experiment pr-155418 created and queued.
🤖 Automatically detected try build ad93f57
⚠️ Try build based on commit 9d897cb, but latest commit is c634619. Did you forget to make a new try build?
🔍 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 Apr 17, 2026
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transmute size check is wrong for overaligned newtype Transmute special-case doesn't take into consideration alignment or enum repr.

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