Do not deduce parameter attributes during CTFE by eggyal · Pull Request #151842 · rust-lang/rust

eggyal · 2026-01-29T22:08:34Z

Ever since #103172, fn_abi_of_instance might look at a function's body to deduce certain codegen optimization attributes for its indirectly-passed parameters beyond what can be determined purely from its signature (namely today ArgAttribute::ReadOnly and ArgAttribute::CapturesNone). Since #130201, looking at a function's body in this way entails generating, for any coroutine-closures, additional by-move MIR bodies (which aren't represented in the HIR)—but this requires knowing the types of their context and consequently cycles can ensue if such bodies are generated before typeck is complete (such as during CTFE).

Since they have no bearing on the evaluation result, this patch breaks a subquery out from fn_abi_of_instance, fn_abi_of_instance_no_deduced_attrs, which returns the ABI before such parameter attributes are deduced; and that new subquery is used in CTFE instead (however, since parameter attributes are only deduced in optimized builds, as a performance optimization we avoid calling the original query unless we are performing such a build).

Fixes #151748
Fixes #152497

rustbot · 2026-01-29T22:08:37Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustbot · 2026-01-29T23:23:55Z

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

RalfJung · 2026-01-30T07:14:49Z

Note that ABI "adjustments" typically do more than just deducing parameter attributes. So if "unadjusted" just refers to the attributes, that's confusing naming.

Also, having the full signature might be relevant for the UB checking that const-eval and Miri are doing. So it's at least slightly concerning that we shouldn't be able to access some of that info any more. What exactly is the difference between the signatures we obtain this way?

RalfJung · 2026-01-30T07:33:25Z

Looking at #151748, it seems like the underlying issue is that fn_sig_of_instance looks at the body of the function. That is indeed somewhat sus -- at least all the UB-relevant information should be deducible without the body. Please extend the PR description to explain this; it currently takes a bit of digging to fiure out what problem you're actually solving.

If we end up resolving this by splitting the query, it is very important that the queries have good documentation explaining their relationship. We want to avoid someone changing this code in the future in a way that removes too much information from the "unadjusted" signature.

RalfJung · 2026-01-30T18:50:02Z

That name and PR description are much better, thank you!

RalfJung · 2026-01-30T18:50:13Z

@bors try
@rust-timer queue

…tfe, r=<try> Do not deduce parameter attributes during CTFE

compiler/rustc_middle/src/query/mod.rs

RalfJung · 2026-01-30T18:54:33Z

I gave some feedback on the comments. I have not looked at the actual code changes at all.

RalfJung · 2026-02-22T12:42:00Z

However it relies on downstream code never invoking the fn_abi_of_instance_query query directly

FWIW this is something we already do quite a bit; we usually call these queries something_raw.

eggyal · 2026-02-22T16:21:53Z

@rustbot ready

RalfJung

Yeah that's much better, thank you. :)

Now that I can see through the code better, I did notice two more things.

View changes since this review

compiler/rustc_ty_utils/src/abi.rs

compiler/rustc_middle/src/ty/mod.rs

rustbot · 2026-03-01T12:41:58Z

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.

eggyal · 2026-03-01T12:46:09Z

All good points—I've incorporated, and rebased.

@rustbot ready

compiler/rustc_ty_utils/src/abi.rs

RalfJung · 2026-03-04T16:03:11Z

One last comment nit. :) r=me with that resolved. Thanks a lot for working through so many rounds of review with me!
@bors delegate+

rust-bors · 2026-03-04T16:03:14Z

✌️ @eggyal, you can now approve this pull request!

If @RalfJung told you to "r=me" after making some further change, then please make that change and post @bors r=RalfJung.

`fn_abi_of_instance` can look at function bodies to deduce codegen optimization attributes (namely `ArgAttribute::ReadOnly` and `ArgAttribute::CapturesNone`) of indirectly-passed parameters. This can lead to cycles when performed during typeck, when such attributes are irrelevant. This patch breaks a subquery out from `fn_abi_of_instance`, `fn_abi_of_instance_no_deduced_attrs`, which returns the ABI before such parameter attributes are deduced; and that new subquery is used in CTFE instead.

As a performance optimization, we altogether avoid ever invoking the query that computes an Instance fn's fully optimized ABI in cases where the build will not actually deduce parameter attributes.

eggyal · 2026-03-04T16:41:04Z

I updated the comment to reflect your suggestion, but with a minor difference in wording. Hope that's okay?

@bors r=RalfJung

Thanks a lot for working through so many rounds of review with me!

Au contraire - thank you for putting up with my constant changes, and providing such excellent feedback. This PR is so much better as a result!

rust-bors · 2026-03-04T16:41:08Z

📌 Commit f460bb1 has been approved by RalfJung

It is now in the queue for this repository.

JonathanBrouwer · 2026-03-04T20:05:42Z

@bors p=1000
This might spuriously fail with the same disk space errors as in #153416, we'll see

rust-bors · 2026-03-04T20:14:56Z

💔 Test for 8f8f174 failed: CI. Failed job:

auto - aarch64-gnu-llvm-20-2 (web logs, enhanced plaintext logs)

eggyal · 2026-03-04T20:17:19Z

The runner has received a shutdown signal.

😆 sounds like GitHub's having a good day !

JonathanBrouwer · 2026-03-04T20:18:41Z

Lmao lets try again then
@bors retry

rust-log-analyzer · 2026-03-04T20:52:15Z

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

/dev/sda15       98M  6.4M   92M   7% /boot/efi
tmpfs           1.6G   16K  1.6G   1% /run/user/1001
================================================================================

Sufficient disk space available (120307164KB >= 52428800KB). Skipping cleanup.
##[group]Run src/ci/scripts/setup-environment.sh
src/ci/scripts/setup-environment.sh
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
---
set -ex

# Run a subset of tests. Used to run tests in parallel in multiple jobs.

# When this job partition is run as part of PR CI, skip tidy to allow revealing more failures. The
# dedicated `tidy` job failing won't block other PR CI jobs from completing, and so tidy failures
# shouldn't inhibit revealing other failures in PR CI jobs.
if [ "$PR_CI_JOB" == "1" ]; then
  echo "PR_CI_JOB set; skipping tidy"
  SKIP_TIDY="--skip tidy"
fi

../x.py --stage 2 test \
  ${SKIP_TIDY:+$SKIP_TIDY} \
  --skip compiler \
  --skip src
#!/bin/bash

set -ex

# Run a subset of tests. Used to run tests in parallel in multiple jobs.

# When this job partition is run as part of PR CI, skip tidy to allow revealing more failures. The
# dedicated `tidy` job failing won't block other PR CI jobs from completing, and so tidy failures
# shouldn't inhibit revealing other failures in PR CI jobs.
if [ "$PR_CI_JOB" == "1" ]; then
  echo "PR_CI_JOB set; skipping tidy"
  SKIP_TIDY="--skip tidy"
fi

../x.py --stage 2 test \
  ${SKIP_TIDY:+$SKIP_TIDY} \
  --skip tests \
  --skip coverage-map \
  --skip coverage-run \
  --skip library \
  --skip tidyselftest

RalfJung · 2026-03-04T20:55:08Z

@bors retry The runner has received a shutdown signal

rust-bors · 2026-03-04T20:55:12Z

❗ You can only retry pull requests that are approved and have a previously failed auto build.

Hint: There is currently a pending auto build on this PR. To cancel it, run @bors cancel.

RalfJung · 2026-03-04T20:55:41Z

Oh that was probably just a slow log analyzer?

JonathanBrouwer · 2026-03-04T21:29:14Z

See #t-infra > free disk space step takes 8-10 minutes @ 💬 for discussion around disk space failures

rust-bors · 2026-03-05T00:14:21Z

☀️ Test successful - CI
Approved by: RalfJung
Duration: 3h 55m 22s
Pushing b55e20a to main...

github-actions · 2026-03-05T00:17:40Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing b90dc1e (parent) -> b55e20a (this PR)

Test differences

Show 82 test diffs

Stage 1

[ui] tests/ui/coroutine/avoid-query-cycle-in-ctfe.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/coroutine/avoid-query-cycle-in-ctfe.rs: [missing] -> pass (J0)

Additionally, 80 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard b55e20ad905a336395ca80863488d0827712d067 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-android: 28m 28s -> 20m 37s (-27.6%)
aarch64-apple: 3h 15m -> 3h 48m (+17.3%)
dist-apple-various: 1h 48m -> 2h 5m (+15.9%)
dist-x86_64-msvc: 1h 51m -> 2h 4m (+11.7%)
dist-ohos-armv7: 1h 12m -> 1h 20m (+11.2%)
dist-aarch64-llvm-mingw: 1h 31m -> 1h 41m (+10.6%)
aarch64-gnu-debug: 1h 8m -> 1h 14m (+9.8%)
x86_64-msvc-2: 2h 30m -> 2h 15m (-9.5%)
dist-ohos-aarch64: 1h 16m -> 1h 23m (+9.4%)
x86_64-msvc-ext3: 1h 42m -> 1h 51m (+8.8%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-03-05T00:55:09Z

Finished benchmarking commit (b55e20a): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -0.2%, secondary -1.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.0%	[2.0%, 2.0%]	1
Regressions ❌ (secondary)	5.7%	[5.7%, 5.7%]	1
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-5.5%	[-7.5%, -3.6%]	2
All ❌✅ (primary)	-0.2%	[-2.4%, 2.0%]	2

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 479.576s -> 478.218s (-0.28%)
Artifact size: 397.00 MiB -> 394.94 MiB (-0.52%)

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 Jan 29, 2026

rustbot assigned jdonszelmann Jan 29, 2026

eggyal force-pushed the skip-deducing-parameter-attrs-during-ctfe branch from 5650820 to ea095f5 Compare January 29, 2026 22:26

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eggyal force-pushed the skip-deducing-parameter-attrs-during-ctfe branch from ea095f5 to de839fc Compare January 30, 2026 06:50

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eggyal force-pushed the skip-deducing-parameter-attrs-during-ctfe branch from de839fc to 6a5ec89 Compare January 30, 2026 08:19

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eggyal force-pushed the skip-deducing-parameter-attrs-during-ctfe branch from 6a5ec89 to 132a633 Compare January 30, 2026 08:27

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rust-bors bot pushed a commit that referenced this pull request Jan 30, 2026

Auto merge of #151842 - eggyal:skip-deducing-parameter-attrs-during-c…

cbab83a

…tfe, r=<try> Do not deduce parameter attributes during CTFE

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 30, 2026

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eggyal added 2 commits March 4, 2026 16:38

Only invoke query that deduces attrs in full, opt builds

f460bb1

As a performance optimization, we altogether avoid ever invoking the query that computes an Instance fn's fully optimized ABI in cases where the build will not actually deduce parameter attributes.

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This was referenced Mar 5, 2026

Move coroutine upvars into locals for better memory economy #135527

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Chore/move more types to ir #152706

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