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 · 2026-08-23T14:47:19Z

    /// the behavior in sync across functions.
-    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self;
+    #[inline]
+    fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {


_{(Ideally we would mark this final but unfortunately this crate needs to be able to compile on stable, too. We could make it conditional on feature="nightly" using an identity macro but at this point we should just give up I guess.)}

I went ahead and did this the naive way. Doesn't seem like too much copy-paste?

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

fmease · 2026-08-25T22:57:26Z

@bors try @rust-timer queue

Add default to `TypeFoldable::fold_with`

rust-bors · 2026-08-25T23:57:03Z

☀️ Try build successful (CI)
Build commit: ca257ee (ca257eeeac9753a2e07ff38cbe1e2e8204a7c4f4)
Base parent: 787af2b (787af2b8c80638c51a4fc8e44f84e6891f243ec7)

rust-timer · 2026-08-26T00:39:41Z

Finished benchmarking commit (ca257ee): 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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	0.6%	[0.2%, 1.2%]	12
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.2%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.3%, 0.4%]	3

Max RSS (memory usage)

Results (primary 3.3%, 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)	3.3%	[3.3%, 3.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	3.3%	[3.3%, 3.3%]	1

Cycles

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

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: 474.247s -> 477.756s (0.74%)
Artifact size: 402.66 MiB -> 402.68 MiB (0.00%)

rustbot · 2026-09-02T21:53:06Z

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.

rust-bors · 2026-09-03T16:45:43Z

☔ The latest upstream changes (presumably #162229) 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

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