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

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

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

Rollup of 19 pull requests Successful merges: - #140956 (`impl PartialEq<{str,String}> for {Path,PathBuf}`) - #141744 (Stabilize `ip_from`) - #144804 (Don't warn on never to any `as` casts as unreachable) - #144983 (Rehome 37 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`) - #145025 (run spellcheck as a tidy extra check in ci) - #145041 (rework GAT borrowck limitation error) - #145243 (take attr style into account in diagnostics) - #145359 (Fix bug where `rustdoc-js` tester would not pick the right `search.js` file if there is more than one) - #145429 (Couple of codegen_fn_attrs improvements) - #145452 (Do not strip binaries in bootstrap everytime if they are unchanged) - #145486 (Fix `unicode_data.rs` mention message) - #145489 (library: Migrate from `cfg_if` to `cfg_select`) - #145493 (remove `should_render` in `PrintAttribute` derive) - #145500 (Port must_use to the new target checking) - #145505 (Simplify span caches) - #145511 (Rust build fails on OpenBSD after using file_lock feature) - #145516 (Weekly `cargo update`) - #145533 (Reorder `lto` options from most to least optimizing) - #145550 (Avoid using `()` in `derive(From)` output.) r? `@ghost` `@rustbot` modify labels: rollup

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.

bors · 2025-08-18T19:37:36Z

⌛ Testing commit c44c1bb with merge 9eb4a26...

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

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

Zalathar mentioned this pull request Aug 18, 2025

Rollup of 19 pull requests #145552

Closed

Zalathar mentioned this pull request Aug 18, 2025

Rollup of 18 pull requests #145566

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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

Open

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

Open

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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