More derive(Debug) cleanups for structs by panstromek · Pull Request #165140 · rust-lang/rust

panstromek · 2026-10-29T15:54:59Z

This removes duplication from the debug helpers and unifies tuples with structs.

More details in individual commits.

I think this is makes the code simpler, but it's a bit of a judgement call.

r? nnethercote

They have the same signature now, so we can unify them. The implementation for tuple is selected based on whether the concatenated string contains field names.

rustbot · 2026-10-29T15:55:03Z

Changes to the code generated for builtin derived traits.

cc @nnethercote

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

panstromek · 2026-10-29T15:55:21Z

This could maybe help std size and binary size.

@bors try @rust-timer queue

More derive(Debug) cleanups for structs

rust-bors · 2026-10-29T16:54:07Z

☀️ Try build successful (CI)
Build commit: 1f87bc3 (1f87bc31991d0e278a00ab88b78a17495a0f5ea3)
Base parent: 96a18bf (96a18bf5ffa2b24f9cf3d1c731a9eb121c1acad9)

rust-timer · 2026-10-29T20:45:47Z

Finished benchmarking commit (1f87bc3): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@bors rollup=iffy rustc-perf
@rustbot label: -S-waiting-on-perf -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)	-0.5%	[-0.5%, -0.5%]	4
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	4

Max RSS (memory usage)

Results (primary -0.9%, secondary -0.3%)

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)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-2.0%	[-2.6%, -1.4%]	2
All ❌✅ (primary)	-0.9%	[-0.9%, -0.9%]	1

Cycles

Results (primary 0.5%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.3%, 2.8%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.6%	[-3.6%, -3.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-3.6%, 2.8%]	6

Binary size

Results (primary -0.4%, secondary 0.3%)

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)	0.3%	[0.2%, 0.4%]	2
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	4

Bootstrap: 487.353s -> 485.918s (-0.29%)
Artifact size: 406.68 MiB -> 406.88 MiB (0.05%)

nnethercote

The shorter code is nice. It does make debug printing a little slower, as seen in the fmt-debug-derive runtime test. That's probably ok?

View changes since this review

nnethercote · 2026-10-30T05:38:05Z

@rustbot author

rustbot · 2026-10-30T05:38:10Z

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

panstromek · 2026-10-30T07:12:41Z

It does make debug printing a little slower, as seen in the fmt-debug-derive runtime test. That's probably ok?

I think it's still ok (the concatenation change made it 30% slower), but there's of course a question of how much can we make the debug impls slower to help other metrics. I still assume most of this code in the ecosystem is either dead or used extremely rarely and for a small number of things at once, so I place it on a similar level as a panic path. Not sure that's always the right assumption

Either way, I want try whether we can make it little better, I suspect the split(' ') iterator might not be very optimal for this case. I wanted to unify all the code paths into one place first.

@rustbot ready

nnethercote · 2026-10-30T07:45:51Z

@bors r+

rust-bors · 2026-10-30T07:45:54Z

📌 Commit ae82815 has been tentatively approved by nnethercote

It will be put into the queue for this repository once PR CI succeeds.

rust-bors · 2026-10-30T17:50:06Z

⚠️ A new commit a4331d431ca8238af94bd1adad03063aef41e69f was pushed.

This pull request was unapproved.

panstromek · 2026-10-30T17:52:38Z

Ok, I think I've handled all of those now - split(' ') cases and also checked indexing error handling (that's another one that @folkertdev pointed out). @Jules-Bertholet can you check it out?

Let's check perf as well.

@bors try @rust-timer queue

More derive(Debug) cleanups for structs

…impl for c like enums

rust-bors · 2026-10-30T19:07:35Z

☀️ Try build successful (CI)
Build commit: 2207ca9 (2207ca9689c9c24ff1113563b1aeb97dc5a0b4a4)
Base parent: 92b5b8c (92b5b8c87c61b20adc9001b41baf587634a74734)

panstromek · 2026-10-30T19:09:58Z

fixed clippy, should be good now (I don't expect perf run to show problems)

@rustbot ready

rust-timer · 2026-10-30T19:57:11Z

Finished benchmarking commit (2207ca9): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@bors rollup=iffy rustc-perf
@rustbot label: -S-waiting-on-perf -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)	-0.5%	[-0.5%, -0.5%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	4

Max RSS (memory usage)

Results (secondary 4.8%)

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)	4.8%	[4.8%, 4.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -1.7%)

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)	-1.7%	[-1.7%, -1.7%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.4%, secondary -0.1%)

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)	-0.4%	[-0.4%, -0.4%]	4
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	4

Bootstrap: 484.843s -> 486.345s (0.31%)
Artifact size: 406.61 MiB -> 406.71 MiB (0.02%)

nnethercote · 2026-11-02T05:38:06Z

@bors r+

rust-bors · 2026-11-02T05:38:08Z

📌 Commit 8cde1fb has been approved by nnethercote

It is now in the queue for this repository.

…uwer Rollup of 2 pull requests Successful merges: - #165140 (More derive(Debug) cleanups for structs) - #165160 (New solver opts for diesel (part 3))

Rollup merge of #165140 - panstromek:derive-debug-cleanups, r=nnethercote More derive(Debug) cleanups for structs This removes duplication from the debug helpers and unifies tuples with structs. More details in individual commits. I think this is makes the code simpler, but it's a bit of a judgement call. r? nnethercote

…uwer Rollup of 2 pull requests Successful merges: - rust-lang/rust#165140 (More derive(Debug) cleanups for structs) - rust-lang/rust#165160 (New solver opts for diesel (part 3))

rust-timer · 2026-11-02T22:17:50Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (d4ef51e): 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)	-0.4%	[-0.5%, -0.4%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.5%, -0.4%]	4

Max RSS (memory usage)

Results (secondary 2.0%)

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)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

Results (primary -0.4%)

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)	-0.4%	[-0.4%, -0.4%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	4

Artifact size: 406.62 MiB -> 406.61 MiB (-0.00%)

panstromek added 2 commits October 29, 2026 15:33

ref: implement debug_struct helpers in terms of each other

5b88658

ref: replace debug_tuple helpers with debug struct ones

ff7c30e

They have the same signature now, so we can unify them. The implementation for tuple is selected based on whether the concatenated string contains field names.

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. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Oct 29, 2026

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rust-bors Bot pushed a commit that referenced this pull request Oct 29, 2026

Auto merge of #165140 - panstromek:derive-debug-cleanups, r=<try>

1f87bc3

More derive(Debug) cleanups for structs

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nnethercote approved these changes Oct 30, 2026

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Comment thread library/core/src/fmt/mod.rs Resolved

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doc: improve comments for debug_struct_fields_finish

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panstromek mentioned this pull request Oct 30, 2026

Reland debug impl for fieldless enums with concatenated string without using discriminant_value #162535

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rust-bors Bot pushed a commit that referenced this pull request Oct 30, 2026

Auto merge of #165140 - panstromek:derive-debug-cleanups, r=<try>

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More derive(Debug) cleanups for structs

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Jules-Bertholet approved these changes Oct 30, 2026

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perf: avoid binary bloat from slice indexing error handling in debug …

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…impl for c like enums

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panstromek force-pushed the derive-debug-cleanups branch from a4331d4 to 8cde1fb Compare October 30, 2026 19:09

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JonathanBrouwer mentioned this pull request Nov 2, 2026

Rollup of 2 pull requests #165320

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rust-bors Bot merged commit 7da2c58 into rust-lang:main Nov 2, 2026
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panstromek mentioned this pull request Nov 2, 2026

[experiments] Faster debug impls #165179

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