Add default to `TypeFoldable::fold_with` by camsteffen · Pull Request #159584 · rust-lang/rust

camsteffen · 2026-07-20T01:53:31Z

TypeFoldable::fold_with delegates to TypeFoldable::try_fold_with with an infallible error type.

fmease

Sorry for the wait! Could you rebase, then I'll issue a perf run? Otherwise looks good, amazing reduction in LOC!

Edit: I'll just rebase for you.

fmease · 2026-08-23T14:44:37Z

    /// infallible folders. Do not override this method, to ensure coherence
    /// with `try_super_fold_with`.
-    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
+    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {


Did you test #[inline] on it locally?

I didn't test locally. Added #[inline] here and also to the new struct methods. Wanna try another perf run?

fmease · 2026-08-23T14:47:44Z

    /// infallible folders. Do not override this method, to ensure coherence
    /// with `try_super_fold_with`.
-    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
+    fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {


_(similarly)

fmease · 2026-08-23T22:01:23Z

@bors try @rust-timer queue

Add default to `TypeFoldable::fold_with`

rust-bors · 2026-08-23T23:01:06Z

☀️ Try build successful (CI)
Build commit: 72fafa6 (72fafa63ba64b0a54986aa53d3b2508074285e19)
Base parent: fb6531d (fb6531d550e0075b9eb9a51464f404805eec87d9)

rust-timer · 2026-08-23T23:44:00Z

Finished benchmarking commit (72fafa6): 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 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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	0.3%	[0.2%, 0.6%]	32
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.4%, 0.6%]	4

Max RSS (memory usage)

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

Cycles

Results (primary 7.2%, secondary 8.1%)

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

	mean	range	count
Regressions ❌ (primary)	7.2%	[5.9%, 8.4%]	2
Regressions ❌ (secondary)	12.0%	[2.0%, 18.4%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	7.2%	[5.9%, 8.4%]	2

Binary size

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

Bootstrap: 469.882s -> 474.787s (1.04%)
Artifact size: 400.35 MiB -> 400.33 MiB (-0.01%)

camsteffen · 2026-08-25T22:21:32Z

@@ -94,7 +94,19 @@ pub trait TypeFoldable<I: Interner>: TypeVisitable<I> + Clone {
    ///
    /// Same as [`TypeFoldable::try_fold_with`], but not fallible. Make sure to keep
    /// the behavior in sync across functions.


I should probably delete this sentence.

View changes since the review

fmease · 2026-08-25T22:56:48Z

    /// the behavior in sync across functions.
-    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
+    #[inline]
+    #[cfg(feature = "nightly")] // for `final`


That's not sufficient unfortunately. final like all recent unstable syntax is gated pre-expansion, meaning this will still lead to a feature gate error if feature = "nightly" is not set.

Since we're now building+testing (a subset of?) rust-analyzer in CI (which uses the stable version of this crate), this should also manifest itself as a CI failure soon IINM.

In https://github.com/rust-lang/rust/pull/159584/changes#r3838787968 I did indeed mean "identity macro" (cf. first part of #154045's description).

View changes since the review

camsteffen · 2026-09-29T18:30:57Z

Sorry, more rebase churn... ready now

fmease · 2026-10-01T11:48:16Z

@bors try @rust-timer queue

Add default to `TypeFoldable::fold_with`

rust-bors · 2026-10-01T12:51:12Z

☀️ Try build successful (CI)
Build commit: 60a91bc (60a91bc8f25aa9abcff2b214dd5a1db990d76ca0)
Base parent: 837b18a (837b18aae097503892d7aab6c7ddce332a991a21)

rust-timer · 2026-10-01T14:14:45Z

Finished benchmarking commit (60a91bc): 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 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)	0.7%	[0.2%, 1.2%]	11
Improvements ✅ (primary)	-0.3%	[-0.6%, -0.1%]	10
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	38
All ❌✅ (primary)	-0.3%	[-0.6%, -0.1%]	10

Max RSS (memory usage)

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)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.9%	[-3.9%, -3.9%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.2%, 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
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-2.4%	[-2.7%, -2.1%]	3
All ❌✅ (primary)	-2.2%	[-2.2%, -2.2%]	1

Binary size

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

Bootstrap: 490.365s -> 493.578s (0.66%)
Artifact size: 406.38 MiB -> 406.51 MiB (0.03%)

camsteffen · 2026-10-01T15:53:14Z

I've added one more tweak and that's all I can think to do. I think this is ready to merge. Maybe run rust-timer queue benchmarks=wf-projection-stress-65510 if we want to test the tweak.

fmease · 2026-10-01T16:00:02Z

@bors try
@rust-timer queue benchmarks=wf-projection-stress-65510 priority=1

Add default to `TypeFoldable::fold_with`

rust-bors · 2026-10-01T17:01:30Z

☀️ Try build successful (CI)
Build commit: c24ec80 (c24ec809923cfa90bf58d3a8d2915e6ba5158809)
Base parent: 837b18a (837b18aae097503892d7aab6c7ddce332a991a21)

rust-timer · 2026-10-01T17:28:14Z

Finished benchmarking commit (c24ec80): comparison URL.

Overall result: ❌ regressions - 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 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)	1.0%	[0.9%, 1.2%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.2%, -0.2%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

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

Binary size

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

Bootstrap: missing data
Artifact size: 406.38 MiB -> 408.51 MiB (0.52%)

camsteffen · 2026-10-01T17:38:18Z

No change.

camsteffen · 2026-10-04T23:11:12Z

@fmease Do you think this is okay to merge? I think the last full perf run will be representative. The regression is basically isolated to wf-projection-stress-65510, a secondary test.

rust-bors · 2026-10-13T15:25:57Z

☔ The latest upstream changes (presumably #164256) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 20, 2026

rustbot assigned dingxiangfei2009 Jul 20, 2026

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jul 20, 2026

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camsteffen mentioned this pull request Jul 20, 2026

Use query for Variant InhabitedPredicate #159541

Merged