Add accurate targeting for imperfect derives diagnostic in `E0277` by raushan728 · Pull Request #159363 · rust-lang/rust

raushan728 · 2026-07-16T06:46:45Z

This PR implements accurate targeting for the E0277 "imperfect derives" suggestion so it only triggers when all fields of the ADT satisfy the trait independently of the failing generic parameter. False positives (like struct S<T>(T)) are now successfully filtered out, while true cases (like PhantomData<T>) still correctly emit the help note.

r? @estebank
cc @mejrs @khyperia

khyperia · 2026-07-20T07:37:01Z

neat! I haven't looked through the PR at all, just wanted to suggest having a test with associated types as well, not just PhantomData. You can find an example of what that looks like in my comment here - feel free to rework it to be cleaner or whatever, but also copypasting it directly would work as well, up to you. #158764 (comment)

Perhaps also Rc with Clone, and also some coinductive shenanigans too (the List type from Niko's blog post we were talking about in the previous PR)

raushan728 · 2026-07-20T15:58:34Z

Thanks! Will update shortly.

raushan728 · 2026-07-20T16:26:56Z

just wanted to suggest having a test with associated types as well, not just PhantomData.

Done! please take a look.

raushan728 · 2026-08-08T02:43:31Z

Hi @mejrs @khyperia @estebank this PR has been open for a while and I haven't received any review yet. If you could help me out a bit, we can get this landed sooner 🙂

estebank

My apologies, the logic is a bit subtle and that makes it harder to follow. Would you mind adding more comments explaining what each section is accomplishing?

I believe the changes to be directionally correct, but suspect there might be helper methods to construct the obligation with fewer moving parts.

I am currently traveling, so my response rate is lower than it otherwise would be.

View changes since this review

raushan728 · 2026-08-08T16:41:33Z

My apologies, the logic is a bit subtle and that makes it harder to follow. Would you mind adding more comments explaining what each section is accomplishing?

Added

I believe the changes to be directionally correct, but suspect there might be helper methods to construct the obligation with fewer moving parts.

I looked but didn't find an existing helper for this exact "swap self_ty, keep the rest of the trait args" pattern, so I kept the explicit construction. Open to suggestions if you know of something more idiomatic.

I am currently traveling, so my response rate is lower than it otherwise would be.

No worries, thanks for taking the time to review!

raushan728 · 2026-08-27T18:17:22Z

Hi, @estebank @khyperia @mejrs please help me here so we can move forward 🙃

mejrs · 2026-08-27T21:08:00Z

+   |
+LL | #[derive(Clone)]
+   |          ----- in this derive macro expansion
+LL | struct List<T> {


Doesn't this span need to point at T, not the name of the type? (it looks like this wasn't something you did, but was already off)

View changes since the review

Confirmed, this is existing span issue. Feel free to file a separate issue for it if you think it's worth tracking.

mejrs · 2026-08-27T21:09:02Z

+LL |     require_clone(S::<X>(PhantomData));
+   |     -------------        ^^^^^^^^^^^ the trait `Clone` is not implemented for `S<X>`


this span also looks a bit off 🤔

View changes since the review

rustbot · 2026-08-27T21:15:50Z

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

rustbot · 2026-08-29T05:25:17Z

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.

Filters out false positives for the 'imperfect derives' E0277 note by checking if ADT fields satisfy the trait independently of the failing generic parameter.

mejrs

Thanks!

r? me @bors r+ rollup

View changes since this review

rust-bors · 2026-08-29T21:21:13Z

📌 Commit e735229 has been approved by mejrs

It is now in the queue for this repository.

@estebank

Add accurate targeting for imperfect derives diagnostic in `E0277` Resolves #157117 Follow-up #158764 This PR implements accurate targeting for the E0277 "imperfect derives" suggestion so it only triggers when all fields of the ADT satisfy the trait independently of the failing generic parameter. False positives (like `struct S<T>(T)`) are now successfully filtered out, while true cases (like `PhantomData<T>`) still correctly emit the help note. r? @estebank cc @mejrs @khyperia

@estebank

…17, r=mejrs Add accurate targeting for imperfect derives diagnostic in `E0277` Resolves rust-lang#157117 Follow-up rust-lang#158764 This PR implements accurate targeting for the E0277 "imperfect derives" suggestion so it only triggers when all fields of the ADT satisfy the trait independently of the failing generic parameter. False positives (like `struct S<T>(T)`) are now successfully filtered out, while true cases (like `PhantomData<T>`) still correctly emit the help note. r? @estebank cc @mejrs @khyperia

jhpratt · 2026-08-30T01:49:55Z

@bors yield to rollup

rust-bors · 2026-08-30T01:50:05Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/33286524752

The next pull request likely to be tested is #161997.

Rollup of 3 pull requests Successful merges: - #159363 (Add accurate targeting for imperfect derives diagnostic in `E0277`) - #161969 (Add regression test for TAIT in extern fn ICE with the new solver) - #161993 (add regression test for ICE unexpected region)

rust-bors · 2026-08-30T05:49:15Z

☀️ Test successful - CI
Approved by: mejrs
Duration: 3h 6m 25s
Pushing d0ea9cd to main...

github-actions · 2026-08-30T05:52:53Z

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 4545c83 (parent) -> d0ea9cd (this PR)

Test differences

Show 11 test diffs

Stage 1

[ui (polonius)] tests/ui/derives/imperfect-derive-advanced.rs: [missing] -> pass (J0)
[ui (polonius)] tests/ui/derives/imperfect-derive-copy.rs: [missing] -> pass (J0)
[ui (polonius)] tests/ui/derives/imperfect-derive-phantom.rs: [missing] -> pass (J0)
[ui] tests/ui/derives/imperfect-derive-advanced.rs: [missing] -> pass (J2)
[ui] tests/ui/derives/imperfect-derive-copy.rs: [missing] -> pass (J2)
[ui] tests/ui/derives/imperfect-derive-phantom.rs: [missing] -> pass (J2)

Stage 2

[ui] tests/ui/derives/imperfect-derive-advanced.rs: [missing] -> pass (J1)
[ui] tests/ui/derives/imperfect-derive-copy.rs: [missing] -> pass (J1)
[ui] tests/ui/derives/imperfect-derive-phantom.rs: [missing] -> pass (J1)

Additionally, 2 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 d0ea9cd00084a023259154a8631075bdbc1713ea --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

optional-x86_64-gnu-autodiff: 58m -> 33m 10s (-42.8%)
x86_64-gnu-stdlib-semver-check: 18m 13s -> 10m 26s (-42.7%)
x86_64-mingw-1: 2h 6m -> 2h 59m (+42.0%)
dist-arm-linux-musl: 1h 47m -> 1h 6m (-38.5%)
x86_64-gnu-miri: 1h 32m -> 1h 1m (-33.1%)
x86_64-gnu-nopt: 2h 20m -> 1h 34m (-32.7%)
dist-x86_64-solaris: 1h 43m -> 1h 11m (-31.1%)
x86_64-gnu-tools: 1h 9m -> 49m 38s (-28.5%)
pr-check-2: 42m 43s -> 30m 52s (-27.7%)
dist-s390x-linux: 1h 33m -> 1h 8m (-27.4%)

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 · 2026-08-30T06:48:50Z

Finished benchmarking commit (d0ea9cd): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

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This perf run didn't have relevant results for this metric.

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Results (secondary 3.8%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 14.8%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	14.8%	[14.8%, 14.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary -0.0%)

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

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 474.853s -> 474.031s (-0.17%)
Artifact size: 402.86 MiB -> 402.86 MiB (0.00%)

rustbot assigned estebank Jul 16, 2026

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 Jul 16, 2026

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raushan728 force-pushed the accurate-targeting-157117 branch from b7656d5 to a8756f1 Compare July 16, 2026 09:33

raushan728 force-pushed the accurate-targeting-157117 branch from a8756f1 to e4402f3 Compare July 20, 2026 16:23

estebank reviewed Aug 8, 2026

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Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs Outdated

Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs Outdated

Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

raushan728 force-pushed the accurate-targeting-157117 branch from e4402f3 to 8575e8b Compare August 8, 2026 16:34

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raushan728 requested a review from estebank August 8, 2026 16:41

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raushan728 force-pushed the accurate-targeting-157117 branch from 8575e8b to ab83e7d Compare August 29, 2026 05:25

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Fix accurate targeting for imperfect derives diagnostic

e735229

Filters out false positives for the 'imperfect derives' E0277 note by checking if ADT fields satisfy the trait independently of the failing generic parameter.

raushan728 force-pushed the accurate-targeting-157117 branch from ab83e7d to e735229 Compare August 29, 2026 05:52

raushan728 commented Aug 29, 2026

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Comment thread compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

raushan728 requested a review from mejrs August 29, 2026 06:09

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 Aug 29, 2026

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jhpratt mentioned this pull request Aug 30, 2026

Rollup of 3 pull requests #161997

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 30, 2026

rust-bors Bot merged commit d0ea9cd into rust-lang:main Aug 30, 2026
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rustbot added this to the 1.100.0 milestone Aug 30, 2026

raushan728 deleted the accurate-targeting-157117 branch August 31, 2026 12:45

		LL \| require_clone(S::<X>(PhantomData));
		\| ------------- ^^^^^^^^^^^ the trait `Clone` is not implemented for `S<X>`

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