Use `transmute_neo` in `assume_init` by scottmcm · Pull Request #158345 · rust-lang/rust

scottmcm · 2026-06-24T06:46:28Z

I noticed this basically open-coding a transmute via read, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok.

r? @RalfJung as the author of #143327

Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

rustbot · 2026-06-24T06:46:33Z

RalfJung is not on the review rotation at the moment.
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RalfJung · 2026-06-24T23:04:51Z

r=me with track_caller made miri-conditional to avoid unnecessary busywork for the optimizer.

scottmcm · 2026-06-25T04:53:33Z

@bors r=RalfJung

rust-bors · 2026-06-25T04:53:36Z

📌 Commit 4eb7436 has been approved by RalfJung

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.

@RalfJung

…lfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of rust-lang#143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

scottmcm · 2026-06-25T05:10:49Z

Demo of the MIR improvement, for anyone curious: https://github.com/rust-lang/rust/compare/d6f80bf8be35adba77a72786afeba046c809d5e4..6308be088a0a2b25f00ba4f803a39bc2094102cd#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855L87-R79

@RalfJung

…lfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of rust-lang#143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

@RalfJung

…lfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of rust-lang#143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

…uwer Rollup of 17 pull requests Successful merges: - #153697 (Add arg splat experiment initial tuple impl) - #158294 (Use .drectve for MSVC DLL exports) - #158253 (codegen_ssa: multiply scalable vec size by `vscale`) - #158308 (Fix bug when rustdoc "go to only result" setting is not working as expected") - #158345 (Use `transmute_neo` in `assume_init`) - #158369 (std: abort when `resume_unwind` is called inside the panic hook) - #158374 (disallow tail calling extern "rust-call" functions) - #158380 (Revert "rebuild LLVM when `bootstrap.toml` config changes") - #154398 (Add test for async Send with PhantomData<*mut ()> + unsafe impl Send + dyn Trait) - #157181 (autodiff: stop always needing an alloca) - #158278 (autodiff - typtree cleanups) - #158311 (doc(unstable-book): fix typo "earier" -> "earlier" in default-visibility flag) - #158318 (Make normalization in a test case resilient to dist compilation) - #158338 (Reorganize `tests/ui/issues` [14/N]) - #158343 (Include `Item::const_stability` info in rustdoc JSON.) - #158355 (Fixup the refactoring errors in #156246) - #158390 (Fix: auto trait, const trait bound) Failed merges: - #155535 (export symbols: support macos/windows(32/64))

@RalfJung

…lfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of rust-lang#143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

…uwer Rollup of 17 pull requests Successful merges: - #153697 (Add arg splat experiment initial tuple impl) - #155535 (export symbols: support macos/windows(32/64)) - #158253 (codegen_ssa: multiply scalable vec size by `vscale`) - #158308 (Fix bug when rustdoc "go to only result" setting is not working as expected") - #158345 (Use `transmute_neo` in `assume_init`) - #158369 (std: abort when `resume_unwind` is called inside the panic hook) - #158374 (disallow tail calling extern "rust-call" functions) - #158380 (Revert "rebuild LLVM when `bootstrap.toml` config changes") - #154398 (Add test for async Send with PhantomData<*mut ()> + unsafe impl Send + dyn Trait) - #157181 (autodiff: stop always needing an alloca) - #158278 (autodiff - typtree cleanups) - #158311 (doc(unstable-book): fix typo "earier" -> "earlier" in default-visibility flag) - #158318 (Make normalization in a test case resilient to dist compilation) - #158338 (Reorganize `tests/ui/issues` [14/N]) - #158343 (Include `Item::const_stability` info in rustdoc JSON.) - #158355 (Fixup the refactoring errors in #156246) - #158390 (Fix: auto trait, const trait bound)

@RalfJung

…lfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of rust-lang#143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

…uwer Rollup of 16 pull requests Successful merges: - #153697 (Add arg splat experiment initial tuple impl) - #155535 (export symbols: support macos/windows(32/64)) - #158253 (codegen_ssa: multiply scalable vec size by `vscale`) - #158308 (Fix bug when rustdoc "go to only result" setting is not working as expected") - #158345 (Use `transmute_neo` in `assume_init`) - #158369 (std: abort when `resume_unwind` is called inside the panic hook) - #158374 (disallow tail calling extern "rust-call" functions) - #158380 (Revert "rebuild LLVM when `bootstrap.toml` config changes") - #154398 (Add test for async Send with PhantomData<*mut ()> + unsafe impl Send + dyn Trait) - #157181 (autodiff: stop always needing an alloca) - #158278 (autodiff - typtree cleanups) - #158311 (doc(unstable-book): fix typo "earier" -> "earlier" in default-visibility flag) - #158318 (Make normalization in a test case resilient to dist compilation) - #158338 (Reorganize `tests/ui/issues` [14/N]) - #158343 (Include `Item::const_stability` info in rustdoc JSON.) - #158390 (Fix: auto trait, const trait bound) Failed merges: - #158355 (Fixup the refactoring errors in #156246)

…uwer Rollup of 15 pull requests Successful merges: - #155535 (export symbols: support macos/windows(32/64)) - #158253 (codegen_ssa: multiply scalable vec size by `vscale`) - #158308 (Fix bug when rustdoc "go to only result" setting is not working as expected") - #158345 (Use `transmute_neo` in `assume_init`) - #158369 (std: abort when `resume_unwind` is called inside the panic hook) - #158374 (disallow tail calling extern "rust-call" functions) - #158380 (Revert "rebuild LLVM when `bootstrap.toml` config changes") - #154398 (Add test for async Send with PhantomData<*mut ()> + unsafe impl Send + dyn Trait) - #157181 (autodiff: stop always needing an alloca) - #158278 (autodiff - typtree cleanups) - #158311 (doc(unstable-book): fix typo "earier" -> "earlier" in default-visibility flag) - #158318 (Make normalization in a test case resilient to dist compilation) - #158338 (Reorganize `tests/ui/issues` [14/N]) - #158343 (Include `Item::const_stability` info in rustdoc JSON.) - #158390 (Fix: auto trait, const trait bound)

@RalfJung

Rollup merge of #158345 - scottmcm:assume-init-no-read, r=RalfJung Use `transmute_neo` in `assume_init` I noticed this basically open-coding a transmute via `read`, but now that we have a transmute that doesn't fail there on the typeck size check I figured that we might as well use it here. The Miri errors are preserved (at least locally, but CI will double-check) so hopefully this is ok. r? @RalfJung as the author of #143327 Context is that I saw the raw-ref-cast-and-read in https://github.com/rust-lang/rust/pull/158341/changes#diff-ee00b0dbbebae9c10bf879a79377c87a7a68208a7e213d6ca792a6ef5f216855R87-R90 and thought it could be simpler 🙂

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 24, 2026

rustbot assigned RalfJung Jun 24, 2026

rustbot added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Jun 24, 2026

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scottmcm force-pushed the assume-init-no-read branch from fc37ba8 to 89ee1ba Compare June 24, 2026 15:59

scottmcm commented Jun 24, 2026

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Comment thread library/core/src/mem/mod.rs Outdated

scottmcm 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 Jun 25, 2026

Use transmute_neo in assume_init

4eb7436

scottmcm force-pushed the assume-init-no-read branch from 89ee1ba to 4eb7436 Compare June 25, 2026 01:21

rust-bors Bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jun 25, 2026

JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 18 pull requests #158386

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scottmcm mentioned this pull request Jun 25, 2026

[only tests] Add tests for current read/write unaligned behaviour #158341

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JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 17 pull requests #158389

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JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 17 pull requests #158391

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JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 17 pull requests #158395

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JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 16 pull requests #158413

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JonathanBrouwer mentioned this pull request Jun 25, 2026

Rollup of 15 pull requests #158416

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rust-bors Bot merged commit 512c162 into rust-lang:main Jun 25, 2026
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rustbot added this to the 1.98.0 milestone Jun 25, 2026

scottmcm deleted the assume-init-no-read branch June 26, 2026 03:02

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Use `transmute_neo` in `assume_init`#158345

Use `transmute_neo` in `assume_init`#158345
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