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

makai410 · 2026-04-17T16:41:46Z

View all comments

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.

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

rust-bors · 2026-06-10T05:42:07Z

☀️ Try build successful (CI)
Build commit: 5b7107a (5b7107a5c175fa4886818ef2b0c5ae6a5d9ade7b, parent: d56483a91d6cf5041351a3208b8d08f98f0c8b56)

rust-timer · 2026-06-10T07:01:55Z

Finished benchmarking commit (5b7107a): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.2%]	9
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.2%, 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)	5.0%	[3.0%, 7.1%]	2
Regressions ❌ (secondary)	1.4%	[1.0%, 2.2%]	3
Improvements ✅ (primary)	-3.4%	[-3.4%, -3.4%]	1
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	2.2%	[-3.4%, 7.1%]	3

Cycles

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

Binary size

Results (primary -0.1%, secondary 1.3%)

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)	1.3%	[0.3%, 1.8%]	12
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	11
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	11

Bootstrap: 525.47s -> 516.414s (-1.72%)
Artifact size: 400.80 MiB -> 401.32 MiB (0.13%)

panstromek · 2026-06-10T08:26:07Z

This looks legit. wg-grammar has those two big C-like enums with derive debug.

I'm suspicious of the numbers in bootstrap, that might be a noise spike or return from noise spike (we see these on wall-time numbers sometimes).

nnethercote · 2026-06-12T10:19:03Z

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

@rustbot blocked #148423

@makai410 Has the blocking problem been resolved?

makai410 · 2026-06-12T10:41:58Z

Given that it's a lang issue and we basically have no way to work around in this PR...
@rustbot blocked #148423

@makai410 Has the blocking problem been resolved?

I added bounds check back in the new commits, so in the worst scenario like #148423, it could panic or output an incorrect result but won't trigger UB, at least. So it depends on whether that's acceptable.

makai410 · 2026-06-12T14:13:46Z

I think the perf result looks ideal, not affecting most common cases and being able to improve cases when it actually matters.

One more optimization we can do here is to use u16 or even u8 instead of usize to store the offsets when possible, which I'd like to do as a follow-up PR.

nnethercote

Sorry for the slow response.

Please squash the three commits together, because they aren't worth keeping separate. Plus a minor nit below, otherwise this is looking good.

View changes since this review

nnethercote · 2026-06-23T04:43:18Z

+    fmt: Box<ast::Expr>,
+) -> Option<(ThinVec<ast::Stmt>, Box<ast::Expr>)> {
+    let variant_count = def.variants.len();
+    if variant_count < 39 {


Give this constant a name, and add a comment explaining how it was chosen.

Sorry for the late response. I'm trying to write a bench tool to decide this constant more seriously. For now this is just picked by my intuition, which is IMO not a very cool thing to say here :)

FWIW I think it's totally fine to just write down that it's picked by intuition and measured. The point is to give the next person who's gonna look into this enough information to decide how to follow up on this, it doesn't have to be cool.

lol I think it's too late :3 already got something going here: https://codeberg.org/makai410/debug-fmt-perf

just wait for me to write up a readme and get some data. looks like the threshold is probably around 10 based on my testing so far, but I still need to double-check that.

panstromek · 2026-07-23T10:24:16Z

I tried to inspect MIR for both versions to get an idea of how much code is generated by each: https://godbolt.org/z/7zv1jqcvT

In short, the match version generates a constant, a local and basic block with 5 statements for each variant.
This version generates 3 consts, and 2 mir bodies, with 1-3 basic blocks. Most of it fixed size, except for values of those constants. The overhead of the match version is quite big, so I think we can push this further, and try to apply it for even smaller enums. I'd try to measure 20 or 10 threshold on perf.

One thing that stands out is that each constant in the __OFFSETS array is individually heap allocated in MIR (if I look at the correct MIR type, which is RValue::Aggregate->Operand::Constant ). Using byte string literal where possible would probably help, because that one is stored as a single constant (I hope I read the code correctly, though :D).

- Add `debug_c_like_enum_write_str` to avoid duplicating similar code

rustbot · 2026-07-25T15:59:07Z

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.

hanna-kruppe · 2026-07-25T16:02:10Z

@bors try @rust-timer queue

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

rust-bors · 2026-07-25T18:07:56Z

☀️ Try build successful (CI)
Build commit: b5d9a4c (b5d9a4ca70a16905acc6881bb36b6fc5aae504f2)
Base parent: da86f4d (da86f4d0726be475afbbffe40cb2f65741c51ad3)

rust-timer · 2026-07-25T18:48:23Z

Finished benchmarking commit (b5d9a4c): comparison URL.

Overall result: ✅ improvements - no action needed

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.

@bors rollup=never 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)	-0.7%	[-0.7%, -0.7%]	1
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.2%]	12
All ❌✅ (primary)	-0.7%	[-0.7%, -0.7%]	1

Max RSS (memory usage)

Results (primary 4.2%, secondary 5.8%)

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

	mean	range	count
Regressions ❌ (primary)	4.2%	[4.2%, 4.2%]	1
Regressions ❌ (secondary)	5.8%	[5.8%, 5.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.2%	[4.2%, 4.2%]	1

Cycles

Results (primary 2.8%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.8%	[2.8%, 2.8%]	1

Binary size

Results (primary 0.1%, secondary 1.2%)

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

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.5%, 0.9%]	8
Regressions ❌ (secondary)	1.2%	[0.4%, 1.8%]	14
Improvements ✅ (primary)	-0.2%	[-0.5%, -0.1%]	14
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.5%, 0.9%]	22

Bootstrap: 489.287s -> 489.11s (-0.04%)
Artifact size: 387.73 MiB -> 387.61 MiB (-0.03%)

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

This comment has been minimized.

Sign in to view

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

This comment has been minimized.

Sign in to view

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

View reviewed changes

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

This comment has been minimized.

Sign in to view

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

View reviewed changes

This comment has been minimized.

Sign in to view

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

This comment has been minimized.

Sign in to view

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 10, 2026

This comment has been minimized.

Sign in to view

rust-bors Bot pushed a commit that referenced this pull request Jun 10, 2026

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

5b7107a

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

This comment has been minimized.

Sign in to view

rustbot removed perf-regression Performance regression. S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 10, 2026

nnethercote reviewed Jun 23, 2026

View reviewed changes

nnethercote added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-blocked Status: Blocked on something else such as an RFC or other implementation work. labels Jun 23, 2026

This comment has been minimized.

Sign in to view

Implement Debug for C-like enums with a concatenated string

5d12b9b

- Add `debug_c_like_enum_write_str` to avoid duplicating similar code

makai410 force-pushed the enum-debug-array branch from d9870e7 to 5d12b9b Compare July 25, 2026 15:59

This comment has been minimized.

Sign in to view

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 25, 2026

This comment has been minimized.

Sign in to view

rust-bors Bot pushed a commit that referenced this pull request Jul 25, 2026

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

b5d9a4c

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

This comment has been minimized.

Sign in to view

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jul 25, 2026

Uh oh!

Uh oh!

Conversation

makai410 commented Apr 17, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Apr 17, 2026

Uh oh!

makai410 commented Apr 17, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors Bot commented Apr 18, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Apr 18, 2026

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

Uh oh!

Uh oh!

makai410 commented Apr 18, 2026

Uh oh!

makai410 commented Apr 18, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

nnethercote commented Apr 18, 2026

Uh oh!

makai410 Apr 18, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

rust-bors Bot commented Apr 18, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Apr 18, 2026

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

Uh oh!

theemathas commented Apr 18, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors Bot commented Jun 10, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Jun 10, 2026

Overall result: ✅ improvements - no action needed

Uh oh!

panstromek commented Jun 10, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

nnethercote commented Jun 12, 2026

Uh oh!

makai410 commented Jun 12, 2026

Uh oh!

makai410 commented Jun 12, 2026

Uh oh!

nnethercote left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

This comment has been minimized.

panstromek commented Jul 23, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jul 25, 2026

Uh oh!

hanna-kruppe commented Jul 25, 2026

Uh oh!

This comment has been minimized.

This comment has been minimized.

makai410 commented Apr 17, 2026 •

edited by rustbot

Loading

makai410 Apr 18, 2026 •

edited by rustbot

Loading

panstromek commented Jun 10, 2026 •

edited

Loading

nnethercote left a comment •

edited by rustbot

Loading

panstromek commented Jul 23, 2026 •

edited

Loading