transmute: fix check for whether newtypes have equal size by RalfJung · Pull Request #155418 · rust-lang/rust

RalfJung · 2026-04-17T06:59:35Z

The transmute check has some logic to be able to tell whether two types that involve generics will always have the same size. That logic has a bug: it ignores the fact that repr(align) can affect the size of a type. This PR fixes that bug by making the check bail out for most repr flags. That rejects some previously accepted code, hence the @rust-lang/lang nomination.

The new logic says that a type is a transparent newtype wrapper for transmute purposes only if

It is a struct or single-variant enum without any repr flags except for repr(transparent).
Or it is a two-variant enum without any repr flags except for repr(transparent) that matches the NPO rules. The inner type must be non_zero: true (which holds for references and NonNull), and the wrapper type is then marked as non_zero: false.

The new check is more conservative than it has to be. We do end up rejecting code like this:

#[repr(C)]
pub struct Wrapper<T>(T);

pub fn foo<T: ?Sized>(x: *const T) -> Wrapper<*const T> {
  unsafe { std::mem::transmute::<*const T, Wrapper<*const T>>(x) }
}

We could refine the logic to understand that repr(C) is fine here, but we have to be careful since repr(C) has to be taken into account by the null-pointer-optimization logic in the transmute check. It seemed easier to just bail out on repr(C) entirely, and crater found no regression.

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).

rustbot · 2026-04-17T06:59:40Z

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RalfJung · 2026-04-17T07:00:39Z

+                    if int.is_some()
+                        || align.is_some()
+                        || pack.is_some()
+                        || flags.difference(ReprFlags::IS_TRANSPARENT) != Default::default()


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.

View changes since the review

RalfJung · 2026-04-17T07:01:18Z

@bors try

transmute: fix check for whether newtypes have equal size

RalfJung · 2026-04-17T07:52:16Z

That's strange, I thought I had fixed this and it worked locally...

theemathas · 2026-04-17T08:00:25Z

@RalfJung IIRC, the target that failed has -Zrandomize-layout turned on in CI.

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

rust-bors · 2026-04-17T09:08:55Z

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

RalfJung · 2026-04-17T09:52:10Z

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

That should not affect crater though so we can go ahead and
@craterbot check

craterbot · 2026-04-17T09:52:41Z

👌 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

RalfJung · 2026-04-17T09:55:48Z

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

@craterbot abort
@bors try

transmute: fix check for whether newtypes have equal size

craterbot · 2026-04-17T09:55:59Z

🗑️ Experiment pr-155418 deleted!

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

JohnTitor · 2026-04-17T12:02:46Z

@rustbot reroll

craterbot · 2026-05-02T01:31:58Z

🚧 Experiment pr-155418 is now running

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

craterbot · 2026-05-04T03:06:07Z

🎉 Experiment pr-155418 is completed!
📊 1 regressed and 3 fixed (896654 total)
📊 5288 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-155418/retry-regressed-list.txt ↩

RalfJung · 2026-05-04T07:27:36Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-155418/retry-regressed-list.txt p=1

craterbot · 2026-05-04T07:27:43Z

👌 Experiment pr-155418-1 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

RalfJung · 2026-05-04T07:29:41Z

Yeah that's entirely spurious. :) Nominating for team discussing.

RalfJung · 2026-05-04T07:56:34Z

We seem to have absolutely no test for the NPO part of the transmute logic...

EDIT: except for a test recently added by @oli-obk , that I took the liberty to merge into this.

rustbot · 2026-05-04T08:04:10Z

This PR was rebased onto a different main 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.

craterbot · 2026-05-06T00:24:38Z

🚧 Experiment pr-155418-1 is now running

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

craterbot · 2026-05-06T06:19:01Z

🎉 Experiment pr-155418-1 is completed!
📊 0 regressed and 0 fixed (4983 total)
📊 945 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-155418-1/retry-regressed-list.txt ↩

rustbot assigned JohnTitor Apr 17, 2026

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RalfJung marked this pull request as draft April 17, 2026 06:59

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RalfJung mentioned this pull request Apr 17, 2026

transmute size check is wrong for overaligned newtype #155412

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Auto merge of #155418 - RalfJung:transmute-newtypes, r=<try>

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transmute: fix check for whether newtypes have equal size

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Auto merge of #155418 - RalfJung:transmute-newtypes, r=<try>

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transmute: fix check for whether newtypes have equal size

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RalfJung mentioned this pull request Apr 19, 2026

[ICE]: "improper_ctypes: Option nonnull optimization not applied?" #155479

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repr(transparent): don't consider most length-0 arrays trivial #155984

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transmute: fix check for whether newtypes have equal size

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