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)

rustbot · 2026-08-23T22:01:08Z

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.

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-log-analyzer · 2026-08-25T23:05:56Z

The job x86_64-gnu-gcc failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

---- [run-make] tests/run-make-cargo/rustc-crates-on-stable stdout ----

error: rmake recipe failed to complete
status: exit status: 1
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake_out" && env -u RUSTFLAGS -u __RUSTC_DEBUG_ASSERTIONS_ENABLED -u __STD_DEBUG_ASSERTIONS_ENABLED -u __STD_REMAP_DEBUGINFO_ENABLED AR="ar" BUILD_ROOT="/checkout/obj/build/x86_64-unknown-linux-gnu" CARGO="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools-bin/cargo" CC="cc" CC_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64 -w" CXX="c++" CXX_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64 -w" HOST_RUSTC_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/acf3e91b66e4d2a5/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin" LLVM_COMPONENTS="aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils abi aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgputargetmca amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cas cfguard cgdata codegen codegentypes core coroutines coverage csky cskyasmparser cskycodegen cskydesc cskydisassembler cskyinfo debuginfobtf debuginfocodeview debuginfodwarf debuginfodwarflowlevel debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dtlto dwarfcfichecker dwarflinker dwarflinkerclassic dwarflinkerparallel dwp engine executionengine extensions filecheck frontendatomic frontenddirective frontenddriver frontendhlsl frontendoffloading frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo hipstdpar instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink libdriver lineeditor linker loongarch loongarchasmparser loongarchcodegen loongarchdesc loongarchdisassembler loongarchinfo lto m68k m68kasmparser m68kcodegen m68kdesc m68kdisassembler m68kinfo mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts objcopy object objectyaml option orcdebugging orcjit orcshared orctargetprocess passes plugins powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld sandboxir scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support supportlsp symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser telemetry textapi textapibinaryreader transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xray xtensa xtensaasmparser xtensacodegen xtensadesc xtensadisassembler xtensainfo" LLVM_FILECHECK="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin/FileCheck" PYTHON="/usr/bin/python3" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTDOC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" SOURCE_ROOT="/checkout" TARGET="x86_64-unknown-linux-gnu" TARGET_EXE_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" __BOOTSTRAP_JOBS="4" __RMAKE_VERBOSE_SUBPROCESS_OUTPUT="1" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake"
stdout: none
--- stderr -------------------------------
command failed at line 41
LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make-cargo/rustc-crates-on-stable/rmake_out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib:/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/build/run_make_support/acf3e91b66e4d2a5/out:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTC_STAGE="0" RUSTFLAGS="-D warnings -Zallow-features=" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2-tools-bin/cargo" "build" "--manifest-path" "/checkout/Cargo.toml" "--no-default-features" "--target-dir" "target" "-p" "rustc_type_ir" "-p" "rustc_next_trait_solver" "-p" "rustc_pattern_analysis" "-p" "rustc_lexer" "-p" "rustc_abi" "-p" "rustc_parse_format" "-p" "rustc_hashes"
output status: `exit status: 101`
=== STDOUT ===



=== STDERR ===
   Compiling unicode-ident v1.0.24
---
   Compiling rustc_type_ir v0.0.0 (/checkout/compiler/rustc_type_ir)
error[E0658]: `final` on trait functions is experimental
##[error]  --> compiler/rustc_type_ir/src/fold.rs:99:5
   |
99 |     final fn fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
   |     ^^^^^
   |
   = note: see issue #131179 <https://github.com/rust-lang/rust/issues/131179> for more information
   = help: add `#![feature(final_associated_functions)]` to the crate attributes to enable
   = note: this compiler was built on 2026-08-25; consider upgrading it if it is out of date

error[E0658]: `final` on trait functions is experimental
##[error]   --> compiler/rustc_type_ir/src/fold.rs:130:5
    |
130 |     final fn super_fold_with<F: TypeFolder<I>>(self, folder: &mut F) -> Self {
    |     ^^^^^
    |
    = note: see issue #131179 <https://github.com/rust-lang/rust/issues/131179> for more information
    = help: add `#![feature(final_associated_functions)]` to the crate attributes to enable
    = note: this compiler was built on 2026-08-25; consider upgrading it if it is out of date

Important

For more information how to resolve CI failures of this job, visit this link.

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 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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camsteffen force-pushed the fold-with-default branch from 23af8de to a915f8c Compare July 22, 2026 12:39

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fmease assigned fmease and unassigned dingxiangfei2009 Aug 23, 2026

fmease approved these changes Aug 23, 2026

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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

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

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rust-bors Bot pushed a commit that referenced this pull request Aug 23, 2026

Auto merge of #159584 - camsteffen:fold-with-default, r=<try>

72fafa6

Add default to `TypeFoldable::fold_with`

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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 Aug 23, 2026

camsteffen force-pushed the fold-with-default branch from 12bd193 to 80eb019 Compare August 25, 2026 22:11

Add default to TypeFoldable::fold_with

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camsteffen force-pushed the fold-with-default branch from 80eb019 to 14eda03 Compare August 25, 2026 22:16

camsteffen commented Aug 25, 2026

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

Auto merge of #159584 - camsteffen:fold-with-default, r=<try>

ca257ee

Add default to `TypeFoldable::fold_with`

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 26, 2026

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rust-timer commented Aug 23, 2026

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

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

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