library: Migrate from `cfg_if` to `cfg_select` #145489

joshtriplett · 2025-08-16T10:55:08Z

Migrate the standard library from using the external cfg_if crate to using the now-built-in cfg_select macro.

This does not yet eliminate the dependency from library/std/Cargo.toml, because while the standard library itself no longer uses cfg_if, it also incorporates the backtrace crate, which does.

Migration assisted by the following vim command (after selecting the full cfg_if! invocation):

'<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e

This is imperfect, but substantially accelerated the process. This prompts for confirmation on the } else { since that can also appear inside one of the arms. This also requires manual intervention to handle any multi-line conditions.

rustbot · 2025-08-16T10:55:13Z

r? @Mark-Simulacrum

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They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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Migrate the standard library from using the external `cfg_if` crate to using the now-built-in `cfg_select` macro. This does not yet eliminate the dependency from `library/std/Cargo.toml`, because while the standard library itself no longer uses `cfg_if`, it also incorporates the `backtrace` crate, which does. Migration assisted by the following vim command (after selecting the full `cfg_if!` invocation): ``` '<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e ``` This is imperfect, but substantially accelerated the process. This prompts for confirmation on the `} else {` since that can also appear inside one of the arms. This also requires manual intervention to handle any multi-line conditions.

joshtriplett · 2025-08-16T22:03:03Z

r? @Amanieu

Amanieu

r=me

library/unwind/src/libunwind.rs

The previous code inside `cfg_if!` wasn't indented, so the conversion to `cfg_select!` left it not indented. Indent it.

joshtriplett · 2025-08-17T06:30:20Z

@bors r=Amanieu p=1
(Prone to conflicts.)

bors · 2025-08-17T06:30:23Z

📌 Commit bc9725c has been approved by Amanieu

It is now in the queue for this repository.

library: Migrate from `cfg_if` to `cfg_select` Migrate the standard library from using the external `cfg_if` crate to using the now-built-in `cfg_select` macro. This does not yet eliminate the dependency from `library/std/Cargo.toml`, because while the standard library itself no longer uses `cfg_if`, it also incorporates the `backtrace` crate, which does. Migration assisted by the following vim command (after selecting the full `cfg_if!` invocation): ``` '<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e ``` This is imperfect, but substantially accelerated the process. This prompts for confirmation on the `} else {` since that can also appear inside one of the arms. This also requires manual intervention to handle any multi-line conditions.

Rollup of 10 pull requests Successful merges: - #144476 (rustdoc-search: search backend with partitioned suffix tree) - #144838 (Fix outdated doc comment) - #145206 (Port `#[custom_mir(..)]` to the new attribute system) - #145208 (Implement declarative (`macro_rules!`) derive macros (RFC 3698)) - #145420 (cg_llvm: Use LLVM-C bindings for `LLVMSetTailCallKind`, `LLVMGetTypeKind`) - #145451 (Add static glibc to the nix dev shell) - #145460 (Speedup `copy_src_dirs` in bootstrap) - #145476 (Fix typo in doc for library/std/src/fs.rs#set_permissions) - #145485 (Fix deprecation attributes on foreign statics) - #145489 (library: Migrate from `cfg_if` to `cfg_select`) r? `@ghost` `@rustbot` modify labels: rollup

Zalathar · 2025-08-17T09:07:09Z

Failed in rollup: #145521 (comment)

@bors r-
@bors try jobs=x86_64-gnu-llvm-19-3

bors · 2025-08-18T23:19:02Z

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing 9eb4a26 to master...

github-actions · 2025-08-18T23:21:50Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing aaf87a6 (parent) -> 9eb4a26 (this PR)

Test differences

Show 85 test diffs

85 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 9eb4a2652031ed5ba97c29ef21c79db1645f7883 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-aarch64-apple: 8343.8s -> 6230.8s (-25.3%)
x86_64-mingw-1: 10076.4s -> 12352.6s (22.6%)
dist-apple-various: 6812.6s -> 5320.5s (-21.9%)
dist-aarch64-msvc: 5308.7s -> 6124.9s (15.4%)
dist-x86_64-apple: 12572.4s -> 11116.5s (-11.6%)
i686-msvc-1: 10349.9s -> 11528.9s (11.4%)
x86_64-msvc-2: 7045.2s -> 7803.7s (10.8%)
x86_64-apple-1: 8238.2s -> 9093.2s (10.4%)
i686-msvc-2: 7836.4s -> 8580.8s (9.5%)
x86_64-msvc-1: 9157.6s -> 9955.7s (8.7%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2025-08-19T00:29:29Z

Finished benchmarking commit (9eb4a26): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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)	0.3%	[0.2%, 0.4%]	9
Improvements ✅ (primary)	-1.2%	[-1.2%, -1.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-1.2%, -1.2%]	1

Max RSS (memory usage)

Results (primary 4.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	4.4%	[4.4%, 4.4%]	1

Cycles

This benchmark run did not return any 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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 472.035s -> 469.299s (-0.58%)
Artifact size: 377.77 MiB -> 377.68 MiB (-0.02%)

library: Migrate from `cfg_if` to `cfg_select` Migrate the standard library from using the external `cfg_if` crate to using the now-built-in `cfg_select` macro. This does not yet eliminate the dependency from `library/std/Cargo.toml`, because while the standard library itself no longer uses `cfg_if`, it also incorporates the `backtrace` crate, which does. Migration assisted by the following vim command (after selecting the full `cfg_if!` invocation): ``` '<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e ``` This is imperfect, but substantially accelerated the process. This prompts for confirmation on the `} else {` since that can also appear inside one of the arms. This also requires manual intervention to handle any multi-line conditions.

rust-lang#145489 changed `std_detect` to no longer depend on `cfg-if`, which meant it no longer indirectly pulled in `rustc-std-workspace-core` via `cfg-if`. That caused it to no longer depend on `compiler-builtins`. Change `std_detect` to use `rustc-std-workspace-core` and `rustc-std-workspace-alloc`, to integrate with the rustc workspace. This also pulls in `compiler-builtins` via `rustc-std-workspace-core`. Closes: rust-lang#145594

…=tgross35 `std_detect`: Use `rustc-std-workspace-*` to pull in `compiler-builtins` rust-lang#145489 changed `std_detect` to no longer depend on `cfg-if`, which meant it no longer indirectly pulled in `rustc-std-workspace-core` via `cfg-if`. That caused it to no longer depend on `compiler-builtins`. Change `std_detect` to use `rustc-std-workspace-core` and `rustc-std-workspace-alloc`, to integrate with the rustc workspace. This also pulls in `compiler-builtins` via `rustc-std-workspace-core`. Closes: rust-lang#145594

Rollup merge of #145619 - joshtriplett:use-the-right-core, r=tgross35 `std_detect`: Use `rustc-std-workspace-*` to pull in `compiler-builtins` #145489 changed `std_detect` to no longer depend on `cfg-if`, which meant it no longer indirectly pulled in `rustc-std-workspace-core` via `cfg-if`. That caused it to no longer depend on `compiler-builtins`. Change `std_detect` to use `rustc-std-workspace-core` and `rustc-std-workspace-alloc`, to integrate with the rustc workspace. This also pulls in `compiler-builtins` via `rustc-std-workspace-core`. Closes: #145594

`std_detect`: Use `rustc-std-workspace-*` to pull in `compiler-builtins` rust-lang/rust#145489 changed `std_detect` to no longer depend on `cfg-if`, which meant it no longer indirectly pulled in `rustc-std-workspace-core` via `cfg-if`. That caused it to no longer depend on `compiler-builtins`. Change `std_detect` to use `rustc-std-workspace-core` and `rustc-std-workspace-alloc`, to integrate with the rustc workspace. This also pulls in `compiler-builtins` via `rustc-std-workspace-core`. Closes: rust-lang/rust#145594

…iplett Migrate `panic_unwind` to use `cfg_select!` This follows rust-lang#145489 with an additional place we can drop the `cfg-if` dependency.

Rollup merge of #145689 - cuviper:panic_unwind-cfg, r=joshtriplett Migrate `panic_unwind` to use `cfg_select!` This follows #145489 with an additional place we can drop the `cfg-if` dependency.

Migrate `panic_unwind` to use `cfg_select!` This follows rust-lang/rust#145489 with an additional place we can drop the `cfg-if` dependency.

panstromek · 2025-08-25T16:17:58Z

perf triage:

include-blob regersion is noise (bimodality, returning to the default state).

@rustbot label: +perf-regression-triaged

joshtriplett added the T-libs Relevant to the library team, which will review and decide on the PR/issue. label Aug 16, 2025

rustbot assigned Mark-Simulacrum Aug 16, 2025

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joshtriplett force-pushed the cfg-if-not branch from bc7d167 to edeadb0 Compare August 16, 2025 10:58

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joshtriplett force-pushed the cfg-if-not branch from edeadb0 to 1ae4a0c Compare August 16, 2025 12:28

jieyouxu added the F-cfg_select `#![feature(cfg_select)]` label Aug 16, 2025

rustbot assigned Amanieu and unassigned Mark-Simulacrum Aug 16, 2025

Amanieu reviewed Aug 16, 2025

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Indent some code inside cfg_select!

bc9725c

The previous code inside `cfg_if!` wasn't indented, so the conversion to `cfg_select!` left it not indented. Indent it.

a4lg mentioned this pull request Aug 17, 2025

std_detect: RISC-V: implement implication to "C" #145415

Merged

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 17, 2025

Zalathar mentioned this pull request Aug 17, 2025

Rollup of 10 pull requests #145521

Closed

bors 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-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Aug 17, 2025

bors added the merged-by-bors This PR was explicitly merged by bors. label Aug 18, 2025

bors merged commit 9eb4a26 into rust-lang:master Aug 18, 2025
11 checks passed

rustbot added this to the 1.91.0 milestone Aug 18, 2025

This was referenced Aug 18, 2025

dirfd: preliminary unix and windows implementations #139514

Open

Don't require eh_personality lang item on targets that have a personality #143893

Open

Allow the global allocator to use thread-local storage and std::thread::current() #144465

Open

rustbot added the perf-regression Performance regression. label Aug 19, 2025

This was referenced Aug 19, 2025

std: use a TAIT to define SplitPaths on UNIX #145242

Open

Fix LazyLock poison panic message #145307

Merged

std: make address resolution weirdness local to SGX #145327

Open

joshtriplett deleted the cfg-if-not branch August 19, 2025 07:58

ehuss mentioned this pull request Aug 19, 2025

error: can't find crate for compiler_builtins #145594

Closed

joshtriplett mentioned this pull request Aug 19, 2025

std_detect: Use rustc-std-workspace-* to pull in compiler-builtins #145619

Merged

lqd mentioned this pull request Aug 20, 2025

Regression: E0152 duplicate lang item 'sized' when using -Z build-std with Apple iOS targets #145629

Closed

cuviper mentioned this pull request Aug 20, 2025

Migrate panic_unwind to use cfg_select! #145689

Merged

rustbot added the perf-regression-triaged The performance regression has been triaged. label Aug 25, 2025

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Test differences

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Overall result: ❌✅ regressions and improvements - please read the text below

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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library: Migrate from `cfg_if` to `cfg_select` #145489

library: Migrate from `cfg_if` to `cfg_select` #145489