Implement `Debug` for C-like enums with a concatenated string by makai410 · Pull Request #155452 · rust-lang/rust

makai410 · 2026-04-17T16:41:46Z

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Fixes: #114106 #133945

Related to: #88793

Continuation of: #109615 #114190

r? @ghost (I want to see the perf first)

rustbot · 2026-04-17T16:41:49Z

Changes to the code generated for builtin derived traits.

cc @nnethercote

makai410 · 2026-04-17T23:35:03Z

@bors try @rust-timer queue

Implement `Debug` for C-like enums with a concatenated string

rust-bors · 2026-04-18T01:44:10Z

☀️ Try build successful (CI)
Build commit: 5a131e3 (5a131e34e34187b36455f6f6809f9cd14e7ab55f, parent: e9e32aca5a4ffd08cbc29547b039d64b92a2c03b)

rust-timer · 2026-04-18T02:25:02Z

Finished benchmarking commit (5a131e3): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.4%	[0.2%, 2.4%]	50
Regressions ❌ (secondary)	0.9%	[0.2%, 1.6%]	9
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	0.4%	[-0.4%, 2.4%]	51

Max RSS (memory usage)

Results (primary -3.6%, 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)	2.6%	[1.2%, 6.0%]	5
Improvements ✅ (primary)	-3.6%	[-5.4%, -2.5%]	4
Improvements ✅ (secondary)	-4.4%	[-6.3%, -1.6%]	3
All ❌✅ (primary)	-3.6%	[-5.4%, -2.5%]	4

Cycles

Results (primary 2.8%, secondary 2.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.8%	[2.8%, 2.8%]	1
Regressions ❌ (secondary)	2.1%	[2.1%, 2.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Binary size

Results (primary 0.4%, secondary 0.7%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.0%, 1.4%]	26
Regressions ❌ (secondary)	0.7%	[0.1%, 1.1%]	8
Improvements ✅ (primary)	-0.7%	[-1.1%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-1.1%, 1.4%]	28

Bootstrap: 492.421s -> 491.503s (-0.19%)
Artifact size: 394.25 MiB -> 394.27 MiB (0.01%)

makai410 · 2026-04-18T11:38:53Z

There was a leftover condition from an earlier PR that I pulled in without a proper review, such that it actually didn't apply the optimization to large enums <_<;

I also want to try skipping bounds checks. I suppose it could improve runtime performance, at least on my machine, based on some pretty rough benchmarks with an enum of 10,000 variants.

makai410 · 2026-04-18T13:01:56Z

@bors try @rust-timer queue

Implement `Debug` for C-like enums with a concatenated string

nnethercote · 2026-04-18T13:07:14Z

I'm happy to review this once it's ready.

@rustbot author

makai410 · 2026-04-18T14:31:19Z

+    let variant_names = def
+        .variants
+        .iter()
+        .map(|v| v.disr_expr.is_none().then_some(v.ident.name.as_str()))
+        .collect::<Option<ThinVec<_>>>()?;


Hmmm, I think I missed a valid case, considering:

enum Uwu { QwQ = 0, AwA = 1, }

which has explicit discriminants but is actually dense.

View changes since the review

Also, we can use an offset when some variants have negative discriminants while the overall variants remain dense.

I'll do a follow-up PR to implement these.

rust-bors · 2026-04-18T15:08:53Z

☀️ Try build successful (CI)
Build commit: 7348f26 (7348f264e27b4bbcab21ed426532865b98fc5f55, parent: 8da2d28cbd5a4e2b93e028e709afe09541671663)

rust-timer · 2026-04-18T15:49:32Z

Finished benchmarking commit (7348f26): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@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)	0.5%	[0.2%, 7.1%]	50
Regressions ❌ (secondary)	1.2%	[0.2%, 4.1%]	12
Improvements ✅ (primary)	-0.6%	[-0.6%, -0.6%]	2
Improvements ✅ (secondary)	-0.2%	[-0.5%, -0.0%]	13
All ❌✅ (primary)	0.5%	[-0.6%, 7.1%]	52

Max RSS (memory usage)

Results (primary 0.7%, secondary 0.6%)

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

	mean	range	count
Regressions ❌ (primary)	3.2%	[1.8%, 6.9%]	5
Regressions ❌ (secondary)	2.4%	[0.8%, 6.1%]	10
Improvements ✅ (primary)	-5.6%	[-7.2%, -3.9%]	2
Improvements ✅ (secondary)	-5.6%	[-6.3%, -4.8%]	3
All ❌✅ (primary)	0.7%	[-7.2%, 6.9%]	7

Cycles

Results (primary 4.4%, secondary 3.2%)

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

	mean	range	count
Regressions ❌ (primary)	4.4%	[4.4%, 4.4%]	1
Regressions ❌ (secondary)	3.2%	[1.8%, 5.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.4%	[4.4%, 4.4%]	1

Binary size

Results (primary 0.4%, secondary 1.0%)

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

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.1%, 1.5%]	20
Regressions ❌ (secondary)	1.0%	[0.1%, 2.1%]	20
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.0%]	8
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-0.5%, 1.5%]	28

Bootstrap: 493.313s -> 504.472s (2.26%)
Artifact size: 394.36 MiB -> 394.34 MiB (-0.01%)

theemathas · 2026-04-18T15:59:30Z

This will probably run into #148423. Furthermore, I believe this is likely to cause unsoundness in actual programs, due to the enumflags2 crate which I think has a macro that modifies the discriminant values of enums.

makai410 · 2026-04-19T02:03:35Z

Given that it's a lang issue and we basically have no way to work around in this PR...

@rustbot blocked #148423

makai410 · 2026-04-19T02:31:17Z

We still have compile time regression, I suppose that for small enums it might not be worth doing this kind of optimization, I don't think the runtime performance would vary a lot but the compile time has already increased a lot.

Implement Debug for C-like enums with a concatenated string

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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 Apr 17, 2026

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rust-bors bot pushed a commit that referenced this pull request Apr 17, 2026

Auto merge of #155452 - makai410:enum-debug-array, r=<try>

5a131e3

Implement `Debug` for C-like enums with a concatenated string

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 17, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Apr 18, 2026

nnethercote reviewed Apr 18, 2026

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Comment thread compiler/rustc_builtin_macros/src/deriving/debug.rs Outdated

skip bounds check in fieldless enum

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rust-bors bot pushed a commit that referenced this pull request Apr 18, 2026

Auto merge of #155452 - makai410:enum-debug-array, r=<try>

7348f26

Implement `Debug` for C-like enums with a concatenated string

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 18, 2026

nnethercote self-assigned this Apr 18, 2026

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 Apr 18, 2026

makai410 commented Apr 18, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 18, 2026

theemathas mentioned this pull request Apr 18, 2026

derive(PartialEq) on enums is unsound with user-defined attribute macros. #148423

Open

rustbot added S-blocked Status: Blocked on something else such as an RFC or other implementation work. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 19, 2026

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Overall result: ❌✅ regressions and improvements - please read:

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Overall result: ❌✅ regressions and improvements - please read:

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