Show packed field alignment in mir_transform_unaligned_packed_ref #147743

21aslade · 2025-10-15T20:53:06Z

I left the expected padding for the field out of the error message so the message would be the same on all platforms. It also isn't always possible to know the expected alignment, so this makes the message simpler.

rustbot · 2025-10-15T20:53:09Z

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustbot · 2025-10-15T20:53:12Z

r? @madsmtm

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compiler/rustc_mir_transform/src/errors.rs

madsmtm

Thanks, and welcome to working on the compiler! Have a few comments, see below. Remember to squash once you're done.

Also, I see that you originally implemented the full message as discussed by the referenced issue, but then decided to do something simpler - could you edit your top-level comment to discuss why this wasn't done in the end? (see the review policy for why to put stuff in the PR message).

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compiler/rustc_mir_transform/messages.ftl

compiler/rustc_const_eval/src/util/alignment.rs

tests/ui/binding/issue-53114-safety-checks.stderr

21aslade · 2025-10-20T16:35:58Z

Should be good now. The message still says "only x-byte aligned" because the alignment reported here should always be the minimum alignment, not the maximum.
@rustbot ready

madsmtm

Looks good to me now, but since Ralf had input, I'll let him have the final say.
r? RalfJung

Feel free to r=me as well if you're happy with the message.

View changes since this review

compiler/rustc_mir_transform/messages.ftl

compiler/rustc_const_eval/src/util/alignment.rs

RalfJung · 2025-10-30T20:15:21Z

tests/ui/binding/issue-53114-safety-checks.stderr

   |             ^^^^
   |
-   = note: packed structs are only aligned by one byte, and many modern architectures penalize unaligned field accesses
+   = note: this struct is at most 1-byte aligned, but the type of this field may require higher alignment


Should we have a special case that drops the "at most" when the alignment is 1? It can't be less than 1, after all...

compiler/rustc_mir_transform/messages.ftl

RalfJung · 2025-10-30T20:16:22Z

@rustbot author

rustbot · 2025-10-30T20:16:27Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2025-11-07T20:58:07Z

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.

rust-log-analyzer · 2025-11-07T21:04:55Z

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spellchecking files
building external tool typos from package [email protected]
finished building tool typos
npm WARN deprecated [email protected]: < 24.10.2 is no longer supported
npm ERR! code 127
npm ERR! git dep preparation failed
npm ERR! command /node/bin/node /node/lib/node_modules/npm/bin/npm-cli.js install --force --cache=/home/user/.npm --prefer-offline=false --prefer-online=false --offline=false --no-progress --no-save --no-audit --include=dev --include=peer --include=optional --no-package-lock-only --no-dry-run
npm ERR! npm WARN using --force Recommended protections disabled.
npm ERR! npm WARN deprecated [email protected]: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm ERR! npm WARN deprecated [email protected]: Glob versions prior to v9 are no longer supported
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npm ERR! npm WARN deprecated [email protected]: This package is no longer supported. Please use @npmcli/package-json instead.
npm ERR! npm WARN deprecated [email protected]: This package is no longer supported.
npm ERR! npm ERR! code 127
npm ERR! npm ERR! path /home/user/.npm/_cacache/tmp/git-cloneXXXXXXg9iAQl/node_modules/rollup
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c patch-package
npm ERR! npm ERR! sh: 1: patch-package: not found
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T21_04_16_242Z-debug-0.log

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T21_04_09_017Z-debug-0.log
npm install did not exit successfully
tidy [extra_checks]: IO error: npm install returned exit code exit status: 127
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:59
  local time: Fri Nov  7 21:04:34 UTC 2025
  network time: Fri, 07 Nov 2025 21:04:34 GMT
##[error]Process completed with exit code 1.

21aslade · 2025-11-07T21:05:02Z

@rustbot ready

RalfJung · 2025-11-08T11:55:12Z

CI is not happy.
@rustbot author

RalfJung · 2025-11-08T11:58:46Z

Ah, that was the npm network issue.
@rustbot ready
@bors r+

bors · 2025-11-08T11:58:49Z

📌 Commit 566a86b has been approved by RalfJung

It is now in the queue for this repository.

RalfJung · 2025-11-08T11:59:44Z

tests/ui/packed/packed-union-borrow-element.stderr

+LL |     let brw = unsafe { &bar.foo1.0 };
+   |                        ^^^^^^^^^^^
+   |
+   = note: this struct is 1-byte aligned, but the type of this field may require higher alignment


When there are multiple projections it is not clear which one it means... further improvements are possible here, e.g. by saying "the struct Name is 1-byte aligned". But that can be left to a future PR.

…Jung Show packed field alignment in mir_transform_unaligned_packed_ref Fixes rust-lang#147528 I left the expected padding for the field out of the error message so the message would be the same on all platforms. It also isn't always possible to know the expected alignment, so this makes the message simpler.

Rollup of 22 pull requests Successful merges: - #128666 (Add `overflow_checks` intrinsic) - #146305 (Add correct suggestion for multi-references for self type in method) - #147179 ([DebugInfo] Fix container types failing to find template args) - #147743 (Show packed field alignment in mir_transform_unaligned_packed_ref) - #148079 (Rename `downcast_[ref|mut]_unchecked` -> `downcast_unchecked_[ref|mut]`) - #148084 (Optimize path components iteration on platforms that don't have prefixes) - #148126 (Fix rust stdlib build failing for VxWorks) - #148204 (Modify contributor email entries in .mailmap) - #148279 (rustc_builtin_macros: rename bench parameter to avoid collisions with user-defined function names) - #148333 (constify result unwrap unchecked) - #148539 (Add Allocator proxy impls for Box, Rc, and Arc) - #148601 (`invalid_atomic_ordering`: also lint `update` & `try_update`) - #148612 (Add note for identifier with attempted hygiene violation) - #148613 (Switch hexagon targets to rust-lld) - #148619 (Enable std locking functions on AIX) - #148644 ([bootstrap] Make `--open` option work with `doc src/tools/error_index_generator`) - #148649 (don't completely reset `HeadUsages`) - #148673 (Remove a remnant of `dyn*` from the parser) - #148675 (Remove eslint-js from npm dependencies) - #148680 (Recover `[T: N]` as `[T; N]`) - #148688 (Remove unused argument `features` from `eval_config_entry`) - #148711 (Use the current lint note id when parsing `cfg!()`) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147743 - 21aslade:packed-diagnostic, r=RalfJung Show packed field alignment in mir_transform_unaligned_packed_ref Fixes #147528 I left the expected padding for the field out of the error message so the message would be the same on all platforms. It also isn't always possible to know the expected alignment, so this makes the message simpler.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 15, 2025

rustbot assigned madsmtm Oct 15, 2025

21aslade commented Oct 15, 2025

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madsmtm added A-diagnostics Area: Messages for errors, warnings, and lints A-repr-packed Area: the naughtiest repr labels Oct 16, 2025

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21aslade force-pushed the packed-diagnostic branch from e314e04 to 4945197 Compare October 20, 2025 14:42

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21aslade force-pushed the packed-diagnostic branch from 4945197 to 077fc4e Compare October 20, 2025 16:26

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 20, 2025

21aslade force-pushed the packed-diagnostic branch from 077fc4e to 5e6ea43 Compare October 20, 2025 17:08

madsmtm approved these changes Oct 24, 2025

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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 Oct 30, 2025

show packed alignment in mir_transform_unaligned_packed_ref

566a86b

21aslade force-pushed the packed-diagnostic branch from 5e6ea43 to 566a86b Compare November 7, 2025 20:58

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Nov 7, 2025

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bors 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-review Status: Awaiting review from the assignee but also interested parties. labels Nov 8, 2025

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matthiaskrgr mentioned this pull request Nov 8, 2025

Rollup of 10 pull requests #148693

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matthiaskrgr mentioned this pull request Nov 8, 2025

Rollup of 9 pull requests #148701

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Rollup of 22 pull requests #148721

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bors merged commit 068e784 into rust-lang:master Nov 9, 2025
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rustbot added this to the 1.93.0 milestone Nov 9, 2025

Show packed field alignment in mir_transform_unaligned_packed_ref #147743

Show packed field alignment in mir_transform_unaligned_packed_ref #147743

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