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

folkertdev · 2025-07-10T14:58:44Z

@rustbot ready

compiler/rustc_builtin_macros/src/cfg_select.rs

folkertdev · 2025-07-10T16:54:21Z

@rustbot ready

petrochenkov · 2025-07-11T14:02:47Z

Thanks!
@bors r+

bors · 2025-07-11T14:02:50Z

📌 Commit f8e7813 has been approved by petrochenkov

It is now in the queue for this repository.

…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 9 pull requests Successful merges: - #143213 (de-duplicate condition scoping logic between AST→HIR lowering and `ScopeTree` construction) - #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) - #143704 (Be a bit more careful around exotic cycles in in the inliner) - #143774 (constify `From` and `Into`) - #143786 (Fix fallback for CI_JOB_NAME) - #143796 (Fix ICE for parsed attributes with longer path not handled by CheckAttribute) - #143798 (Remove format short command trait) r? `@ghost` `@rustbot` modify labels: rollup

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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make cfg_select a builtin macro

f8e7813

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matthiaskrgr mentioned this pull request Jul 12, 2025

Rollup of 9 pull requests #143840

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make cfg_select a builtin macro #143461

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make cfg_select a builtin macro #143461

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make `cfg_select` a builtin macro #143461

make `cfg_select` a builtin macro #143461

folkertdev commented Jul 4, 2025 •

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