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Add default to TypeFoldable::fold_with - #159584

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Add default to TypeFoldable::fold_with#159584
camsteffen wants to merge 1 commit into
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camsteffen:fold-with-default

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@camsteffen camsteffen commented Jul 20, 2026

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

@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jul 20, 2026
@rustbot 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 camsteffen mentioned this pull request Jul 20, 2026
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@fmease fmease assigned fmease and unassigned dingxiangfei2009 Aug 23, 2026
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fmease approved these changes Aug 23, 2026
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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.

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Comment thread compiler/rustc_type_ir/src/fold.rs
/// 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 {

@fmease fmease Aug 23, 2026

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Did you test #[inline] on it locally?

Comment thread compiler/rustc_type_ir/src/fold.rs
/// 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 {

@fmease fmease Aug 23, 2026 •
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(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.)

Comment thread compiler/rustc_type_ir/src/fold.rs
/// 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 {

@fmease fmease Aug 23, 2026 •
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(similarly)

@fmease fmease 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 Aug 23, 2026
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fmease force-pushed the fold-with-default branch from a915f8c to 12bd193 Compare August 23, 2026 22:01
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rustbot commented Aug 23, 2026

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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.

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fmease commented Aug 23, 2026

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@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 23, 2026
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rust-bors Bot pushed a commit that referenced this pull request Aug 23, 2026
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Auto merge of #159584 - camsteffen:fold-with-default, r=<try>
72fafa6 
Add default to `TypeFoldable::fold_with`
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☀️ Try build successful (CI)
Build commit: 72fafa6 (72fafa63ba64b0a54986aa53d3b2508074285e19)
Base parent: fb6531d (fb6531d550e0075b9eb9a51464f404805eec87d9)

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rust-timer commented Aug 23, 2026

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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%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Aug 23, 2026
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