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 around a pointer for transmute purposes only if

It is a struct without any repr flags except for repr(transparent) and repr(C).
It is a 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.

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 · 2026-04-17T07:01:18Z

@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

rust-rfcbot · 2026-05-13T15:05:33Z

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

rust-rfcbot · 2026-05-23T15:15:01Z

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.

RalfJung · 2026-05-24T06:55:06Z

@nikomatsakis this is just waiting for review now. :)
@rustbot ready

nikomatsakis · 2026-06-02T13:45:09Z

+                    // We can ignore the seed and some particular flags that can never affect the
+                    // layout of newtypes / NPO types, but we have to check everything else.
+                    // If you are adding a new field to `ReprOptions`, make sure to extend the check
+                    // below so that we bail out if it is not at its default value!


I guess it's ok, reading it more closely.

nikomatsakis · 2026-06-02T13:45:28Z

@bors r+

rust-bors · 2026-06-02T13:45:31Z

📌 Commit 4a09f12 has been approved by nikomatsakis

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 5. This pull request will be tested once the tree is reopened.

…uwer Rollup of 6 pull requests Successful merges: - #155418 (transmute: fix check for whether newtypes have equal size) - #156603 (Clarify E0381 diagnostics for branch conditions) - #156643 (Document run-make external dependencies) - #157009 (Avoid `unreachable_code` on required return values) - #157308 (make typing mode exhaustive again...) - #157312 (disallow most attrs on eiis)

Rollup merge of #155418 - RalfJung:transmute-newtypes, r=nikomatsakis transmute: fix check for whether newtypes have equal size 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 around a pointer for `transmute` purposes only if - It is a struct without any `repr` flags except for `repr(transparent)` and `repr(C)`. - It is a 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`. 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).

…uwer Rollup of 6 pull requests Successful merges: - rust-lang/rust#155418 (transmute: fix check for whether newtypes have equal size) - rust-lang/rust#156603 (Clarify E0381 diagnostics for branch conditions) - rust-lang/rust#156643 (Document run-make external dependencies) - rust-lang/rust#157009 (Avoid `unreachable_code` on required return values) - rust-lang/rust#157308 (make typing mode exhaustive again...) - rust-lang/rust#157312 (disallow most attrs on eiis)

This MR contains the following updates: | Package | Type | Update | Change | |---|---|---|---| | [rust](https://github.com/rust-lang/rust) | tools | minor | `1.97.1` → `1.98.0` | MR created with the help of [el-capitano/tools/renovate-bot](https://gitlab.com/el-capitano/tools/renovate-bot). **Proposed changes to behavior should be submitted there as MRs.** --- ### Release Notes <details> <summary>rust-lang/rust (rust)</summary> ### [`v1.98.0`](https://github.com/rust-lang/rust/blob/HEAD/RELEASES.md#Version-1980-2026-08-20) [Compare Source](rust-lang/rust@1.97.1...1.98.0) \========================== <a id="1.98.0-Language"></a> ## Language - [Allow shortening lifetime of `&mut` when unsize-coercing, even in an invariant position.](rust-lang/rust#149219) For example, you can now coerce a `Cell<&'long mut i32>` to a `Cell<&'short mut dyn Send>`. Such shortenings were already previously allowed when coercing a `&mut` to a `&`, or coercing a `&` to a `&`. - [Add deny-by-default `invalid_runtime_symbol_definitions` lint and warn-by-default `suspicious_runtime_symbol_definitions` lint](rust-lang/rust#155521) - The lints currently specifically targets `core` runtime symbols like `memcmp`, `memset`, `strlen`, ... and is planned to be expanded in the next few releases. - [Add warn-by-default `c_void_returns` lint to check `core::ffi::c_void` as a return type](rust-lang/rust#156379) <a id="1.98.0-Platform-Support"></a> ## Platform Support - [Add `powerpc64-unknown-linux-gnuelfv2` as Tier 3](rust-lang/rust#144220) - [Add `aarch64-unknown-linux-pauthtest` as Tier 3 target](rust-lang/rust#155722) - [Promote `thumbv7a-none-eabi` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7a-none-eabihf` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7r-none-eabi` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv7r-none-eabihf` to Tier 2](rust-lang/rust#155763) - [Promote `thumbv8r-none-eabihf` to Tier 2](rust-lang/rust#155763) Refer to Rust's [platform support page][platform-support-doc] for more information on Rust's tiered platform support. 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[`core::fmt::NumBuffer`](https://doc.rust-lang.org/stable/core/fmt/struct.NumBuffer.html) - [`<{integer}>::format_into`](https://doc.rust-lang.org/stable/core/primitive.usize.html#method.format_into) - [`Send/Sync for std::process::CommandArgs`](https://doc.rust-lang.org/stable/std/process/struct.CommandArgs.html#impl-Send-for-CommandArgs%3C'a%3E) - [`{fN}::algebraic_add`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_add) - [`{fN}::algebraic_sub`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_sub) - [`{fN}::algebraic_mul`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_mul) - [`{fN}::algebraic_div`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_div) - [`{fN}::algebraic_rem`](https://doc.rust-lang.org/stable/core/primitive.f32.html#method.algebraic_rem) - [`NonZero<{integer}>::from_str_radix`](https://doc.rust-lang.org/stable/core/num/struct.NonZero.html#method.from_str_radix-4) - [`String::from_utf16le`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16le) - [`String::from_utf16le_lossy`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16le_lossy) - [`String::from_utf16be`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16be) - [`String::from_utf16be_lossy`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.from_utf16be_lossy) - [`[T]::strip_circumfix`](https://doc.rust-lang.org/stable/core/primitive.slice.html#method.strip_circumfix) - [`str::strip_circumfix`](https://doc.rust-lang.org/stable/core/primitive.str.html#method.strip_circumfix) - [`Atomic<T>::from_mut`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.from_mut) - [`Atomic<T>::get_mut_slice`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.get_mut_slice) - [`Atomic<T>::from_mut_slice`](https://doc.rust-lang.org/stable/core/sync/atomic/struct.Atomic.html#method.from_mut_slice) - [`std::range::legacy`](https://doc.rust-lang.org/stable/std/range/legacy/index.html) <a id="1.98.0-Compatibility-Notes"></a> ## Compatibility Notes - [If fully elided, lifetime bounds of trait object types may now resolve differently or even get rejected in very specific niche scenarios](rust-lang/rust#129543) - [Error in more cases of ambiguous imports](rust-lang/rust#145108) - [Switch the destructors implementation for thread locals on Windows to use Fiber Local Storage (FLS)](rust-lang/rust#148799) - [Convert some cases of the `ambiguous_glob_imports` lint into a hard error](rust-lang/rust#149195) - [Where-bounds of the form `Type = Type` and `Type == Type` are no longer syntactically allowed](rust-lang/rust#153513) - [Ensure Send/Sync is not implemented for std::env::Vars{,Os}](rust-lang/rust#155153) - [Fix that in some attributes, arguments were not properly rejected](rust-lang/rust#155193) - [`repr(transparent)` is now more strict about which fields have "trivial" layout and hence can be ignored: `repr(C)` types, types with private fields, and `#[non_exhaustive]` types are no longer considered "trivial"](rust-lang/rust#155299) - 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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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Auto merge of #155418 - RalfJung:transmute-newtypes, r=<try>

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Tracking issue for release notes of #155418: transmute: fix check for whether newtypes have equal size #156852

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Rollup of 6 pull requests #157320

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Draft release notes for 1.98.0 #160700

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