make `cfg_select` a builtin macro #143461

folkertdev · 2025-07-04T23:03:01Z

tracking issue: #115585

This parses mostly the same as the macro cfg_select version, except:

wrapping in double brackets is no longer supported (or needed): cfg_select {{ /* ... */ }} is now rejected.
in an expression context, the rhs is no longer wrapped in a block, so that this now works:

fn main() {
    println!(cfg_select! {
        unix => { "foo" }
        _ => { "bar" }
    });
}

a single wildcard rule is now supported: cfg_select { _ => 1 } now works

I've also added an error if none of the rules evaluate to true, and warnings for any arms that follow the _ wildcard rule.

cc @traviscross if I'm missing any feature that should/should not be included
r? @petrochenkov for the macro logic details

compiler/rustc_expand/src/mbe/macro_rules.rs

rustbot · 2025-07-05T06:07:41Z

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

This PR modifies tests/auxiliary/minicore.rs.

cc @jieyouxu

traviscross

The behavior, per the tests and the impl, looks correct to me.

compiler/rustc_builtin_macros/src/cfg_select.rs

folkertdev · 2025-07-05T12:55:34Z

I moved some things around (analogously to asm!) so that rustfmt can use the parsed representation of cfg_select!. Now that it is a builtin macro, I'm assuming formatting won't work and becomes a blocker for stabilization. I think the formatting rules are reasonably straightforward though.

Implementation-wise the one thing I'm not sure about is how to get rustfmt to format a TokenStream that represents the rhs of an arm. It gets expanded based on the context (can be an item, expr, stmt, etc.). But hopefully the rustfmt folks know how to deal with that.

traviscross · 2025-07-05T16:04:31Z

cc @rust-lang/rustfmt @rust-lang/style

compiler/rustc_parse/src/parser/cfg_select.rs

folkertdev · 2025-07-09T22:46:36Z

Given that formatting now seems straightforward, I think this can be merged without the precise formatting logic. It's probably better (also for review etc.) to add formatting separately in the rustfmt repo. I've played around with it a bit, but I think there is some benefit from refactoring the macro_rules! logic a bit for more code sharing.

So, this PR is complete from my perspective.

ytmimi · 2025-07-10T01:21:09Z

Given that formatting now seems straightforward, I think this can be merged without the precise formatting logic. It's probably better (also for review etc.) to add formatting separately in the rustfmt repo. I've played around with it a bit, but I think there is some benefit from refactoring the macro_rules! logic a bit for more code sharing.

Nice! I agree that a PR to the rustfmt repo would be ideal to add the formatting logic / refactoring for code sharing with macro_rules!

library/core/src/macros/mod.rs

compiler/rustc_parse/src/parser/mod.rs

compiler/rustc_expand/src/mbe/macro_rules.rs

compiler/rustc_parse/src/parser/cfg_select.rs

compiler/rustc_builtin_macros/src/cfg_select.rs

…o, r=petrochenkov make `cfg_select` a builtin macro tracking issue: rust-lang#115585 This parses mostly the same as the `macro cfg_select` version, except: 1. wrapping in double brackets is no longer supported (or needed): `cfg_select {{ /* ... */ }}` is now rejected. 2. in an expression context, the rhs is no longer wrapped in a block, so that this now works: ```rust fn main() { println!(cfg_select! { unix => { "foo" } _ => { "bar" } }); } ``` 3. a single wildcard rule is now supported: `cfg_select { _ => 1 }` now works I've also added an error if none of the rules evaluate to true, and warnings for any arms that follow the `_` wildcard rule. cc `@traviscross` if I'm missing any feature that should/should not be included r? `@petrochenkov` for the macro logic details

Rollup of 14 pull requests Successful merges: - #143301 (`tests/ui`: A New Order [26/N]) - #143461 (make `cfg_select` a builtin macro) - #143519 (Check assoc consts and tys later like assoc fns) - #143554 (slice: Mark `rotate_left`, `rotate_right` unstably const) - #143634 (interpret/allocation: expose init + write_wildcards on a range) - #143679 (Preserve the .debug_gdb_scripts section) - #143685 (Resolve: merge `source_bindings` and `target_bindings` into `bindings`) - #143704 (Be a bit more careful around exotic cycles in in the inliner) - #143734 (Refactor resolve resolution bindings) - #143774 (constify `From` and `Into`) - #143785 (Add --compile-time-deps argument for x check) - #143786 (Fix fallback for CI_JOB_NAME) - #143825 (clippy: fix test filtering when TESTNAME is empty) - #143826 (Fix command trace) r? `@ghost` `@rustbot` modify labels: rollup

…o, r=petrochenkov make `cfg_select` a builtin macro tracking issue: rust-lang#115585 This parses mostly the same as the `macro cfg_select` version, except: 1. wrapping in double brackets is no longer supported (or needed): `cfg_select {{ /* ... */ }}` is now rejected. 2. in an expression context, the rhs is no longer wrapped in a block, so that this now works: ```rust fn main() { println!(cfg_select! { unix => { "foo" } _ => { "bar" } }); } ``` 3. a single wildcard rule is now supported: `cfg_select { _ => 1 }` now works I've also added an error if none of the rules evaluate to true, and warnings for any arms that follow the `_` wildcard rule. cc ``@traviscross`` if I'm missing any feature that should/should not be included r? ``@petrochenkov`` for the macro logic details

Rollup of 13 pull requests Successful merges: - #143301 (`tests/ui`: A New Order [26/N]) - #143461 (make `cfg_select` a builtin macro) - #143519 (Check assoc consts and tys later like assoc fns) - #143554 (slice: Mark `rotate_left`, `rotate_right` unstably const) - #143634 (interpret/allocation: expose init + write_wildcards on a range) - #143679 (Preserve the .debug_gdb_scripts section) - #143685 (Resolve: merge `source_bindings` and `target_bindings` into `bindings`) - #143734 (Refactor resolve resolution bindings) - #143774 (constify `From` and `Into`) - #143785 (Add --compile-time-deps argument for x check) - #143786 (Fix fallback for CI_JOB_NAME) - #143825 (clippy: fix test filtering when TESTNAME is empty) - #143826 (Fix command trace) r? `@ghost` `@rustbot` modify labels: rollup

matthiaskrgr · 2025-07-13T12:29:26Z

@bors r- rollup=iffy
#143876 (comment)

matthiaskrgr · 2025-07-13T12:30:24Z

could also be related to #143519 ? 🤔

folkertdev · 2025-07-13T12:33:57Z

No it's definitely this PR, just finicky targets and a feature that is inherently tied to conditional compilation.

folkertdev · 2025-07-13T12:35:33Z

@bors2 try jobs=x86_64-msvc-2,test-various

rust-bors · 2025-07-13T12:35:36Z

⌛ Trying commit 3689b80 with merge 0080f64…

To cancel the try build, run the command @bors2 try cancel.

make `cfg_select` a builtin macro tracking issue: #115585 This parses mostly the same as the `macro cfg_select` version, except: 1. wrapping in double brackets is no longer supported (or needed): `cfg_select {{ /* ... */ }}` is now rejected. 2. in an expression context, the rhs is no longer wrapped in a block, so that this now works: ```rust fn main() { println!(cfg_select! { unix => { "foo" } _ => { "bar" } }); } ``` 3. a single wildcard rule is now supported: `cfg_select { _ => 1 }` now works I've also added an error if none of the rules evaluate to true, and warnings for any arms that follow the `_` wildcard rule. cc `@traviscross` if I'm missing any feature that should/should not be included r? `@petrochenkov` for the macro logic details try-job: x86_64-msvc-2 try-job: test-various

rust-bors · 2025-07-13T14:41:14Z

☀️ Try build successful (CI)
Build commit: 0080f64 (0080f6457f88ce3d932fb25a15255696a94a32d9, parent: 0fb279be1d96464c69209053ef0f97ad63088cda)

folkertdev · 2025-07-13T14:42:02Z

@bors r=petrochenkov

bors · 2025-07-13T14:42:04Z

📌 Commit 3689b80 has been approved by petrochenkov

It is now in the queue for this repository.

bors · 2025-07-13T18:34:17Z

⌛ Testing commit 3689b80 with merge e9182f1...

bors · 2025-07-13T21:36:23Z

☀️ Test successful - checks-actions
Approved by: petrochenkov
Pushing e9182f1 to master...

github-actions · 2025-07-13T21:39:14Z

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 56835d7 (parent) -> e9182f1 (this PR)

Test differences

Show 80 test diffs

Stage 1

errors::verify_builtin_macros_cfg_select_no_matches_84: [missing] -> pass (J0)
errors::verify_builtin_macros_cfg_select_unreachable_85: [missing] -> pass (J0)
[ui] tests/ui/macros/cfg_select.rs: [missing] -> pass (J1)

Stage 2

[ui] tests/ui/macros/cfg_select.rs: [missing] -> pass (J2)

Additionally, 76 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard e9182f195b8505c87c4bd055b9f6e114ccda0981 --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

x86_64-apple-2: 4845.3s -> 3156.9s (-34.8%)
pr-check-2: 2608.4s -> 2211.0s (-15.2%)
pr-check-1: 1753.5s -> 1516.1s (-13.5%)
aarch64-apple: 4637.4s -> 4095.4s (-11.7%)
dist-apple-various: 6961.7s -> 6167.3s (-11.4%)
x86_64-gnu-llvm-19-1: 3694.0s -> 3305.9s (-10.5%)
x86_64-apple-1: 6096.8s -> 6728.5s (10.4%)
tidy: 74.3s -> 66.8s (-10.2%)
x86_64-gnu-tools: 3568.6s -> 3220.7s (-9.7%)
i686-gnu-2: 6038.0s -> 5526.6s (-8.5%)

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.

folkertdev · 2025-07-13T21:48:04Z

@ytmimi now that this has been merged, we can think about the formatting. I have a working implementation locally.

However, the rustfmt repo is using a rust toolchain that is 4 months old. Is there some sort of policy for updating that? (I imagine that might be kind of tricky to do for me, I'm not really familiar with the code base).

rust-timer · 2025-07-13T23:35:04Z

Finished benchmarking commit (e9182f1): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.2%, -0.1%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 2.6%, secondary 4.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	4.2%	[4.2%, 4.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Cycles

Results (secondary -5.2%)

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)	-5.2%	[-7.5%, -2.9%]	2
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 465.532s -> 464.056s (-0.32%)
Artifact size: 374.73 MiB -> 374.70 MiB (-0.01%)

rustbot added A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jul 4, 2025

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folkertdev force-pushed the cfg-select-builtin-macro branch from 53b3b88 to 5d550b1 Compare July 7, 2025 18:46

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folkertdev force-pushed the cfg-select-builtin-macro branch from 01f41cc to b4afa4a Compare July 10, 2025 14:57

bors added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Jul 12, 2025

fmease mentioned this pull request Jul 13, 2025

Rollup of 14 pull requests #143866

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Rollup of 13 pull requests #143876

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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 Jul 13, 2025

make cfg_select a builtin macro

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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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 13, 2025

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bors merged commit e9182f1 into rust-lang:master Jul 13, 2025
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rustbot added this to the 1.90.0 milestone Jul 13, 2025

make cfg_select a builtin macro #143461

make cfg_select a builtin macro #143461

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Test differences

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Overall result: ✅ improvements - no action needed

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make `cfg_select` a builtin macro #143461

make `cfg_select` a builtin macro #143461

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