stabilize `cfg_select!` by folkertdev · Pull Request #149783 · rust-lang/rust

folkertdev · 2025-12-08T19:38:21Z

tracking issue: #115585
closes #115585
reference PR:

cfg_select! macro reference#2103

Request for Stabilization

Summary

The cfg_select! macro picks the expansion corresponding to the first cfg condition that evaluates to true. It simplifies complex conditional expressions.

cfg_select! {
    unix => {
        fn foo() { /* unix specific functionality */ }
    }
    target_pointer_width = "32" => {
        fn foo() { /* non-unix, 32-bit functionality */ }
    }
    _ => {
        fn foo() { /* fallback implementation */ }
    }
}

let is_unix_str = cfg_select! {
    unix => "unix",
    _ => "not unix",
};
println!("{is_unix_str}");

Semantics

The expansion of a cfg_select! call is the right-hand side of the first cfg rule that evaluates to true.

This can be roughly expressed using this macro:

macro_rules! cfg_select {
    ({ $($tt:tt)* }) => {{
        $crate::cfg_select! { $($tt)* }
    }};
    (_ => { $($output:tt)* }) => {
        $($output)*
    };
    (
        $cfg:meta => $output:tt
        $($( $rest:tt )+)?
    ) => {
        #[cfg($cfg)]
        $crate::cfg_select! { _ => $output }
        $(
            #[cfg(not($cfg))]
            $crate::cfg_select! { $($rest)+ }
        )?
    }
}

The actual implementation uses a builtin macro so that cfg_select! can be used both in item and expression position.

Documentation

reference PR:

cfg_select! macro reference#2103

Tests

The cfg_select! macro is already used extensively in the rust compiler codebase. It has several dedicated tests:

tests/ui/check-cfg/cfg-select.rstests that warnings are emitted when an unexpected cfg condition is used.
tests/ui/macros/cfg_select.rs tests that cfg_select! has the expected expansion, and tests that the expected syntax is accepted.

History

Resolved questions

Unresolved questions

The style team has decided on how to format cfg_select!, but this formatting has not yet been implemented. See #144323.

r? @traviscross

Note

Concerns (0 active)

~~allowing-comma-after-closing-brace~~ resolved in this comment

Managed by @rustbot—see help for details.

ehuss · 2025-12-08T23:02:22Z

Happy to see this move forward!

Is there a detailed description of how this macro works? For example:

What is the grammar that it accepts?
How does it expand? For example, it seems like if the RHS uses braces, the braces are implicitly removed.
Am I correct that whether or not this fails to compile depends on the current cfg's for the current target/environment? That is, there is essentially an implicit _ => {compile_error!()} as the last rule?
Is it correct that this did not suffer the same fate as assert_matches because the name cfg_select is not in use by any known libraries? (Just curious, cfg_select has already been in the prelude for a while and nobody complained.)
Is there a reason the unreachable predicate warning isn't a lint? (The main reason I ask is because this prevents -D warnings from catching it.)
Am I correct that the RHS is not parsed unless it is the arm that is selected? That is, the right hand side can contain any token tree?

rustbot · 2025-12-08T23:06:24Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

The Miri subtree was changed

cc @rust-lang/miri

folkertdev · 2025-12-08T23:26:17Z

Is there a detailed description of how this macro works? For example:

Not yet, I think.

What is the grammar that it accepts?

In item positions the body should consist of zero or more

('_' | ConfugurationPredicate) => { TokenTree }

In expression position we additionally accept

('_' | ConfugurationPredicate) => Expr,

How does it expand? For example, it seems like if the RHS uses braces, the braces are implicitly removed.

That is correct, one level of braces is removed if braces are used.

Am I correct that whether or not this fails to compile depends on the current cfg's for the current target/environment? That is, there is essentially an implicit _ => {compile_error!()} as the last rule?

Yes if none of the conditions evaluate to true you'll get a compile error (note: earlier iterations did not have this default behavior)

Is it correct that this did not suffer the same fate as assert_matches because the name cfg_select is not in use by any known libraries? (Just curious, cfg_select has already been in the prelude for a while and nobody complained.)

The ecosystem mostly uses cfg_if. But I'll defer to T-libs-api on whether this is exported from all the right places.

Is there a reason the unreachable predicate warning isn't a lint? (The main reason I ask is because this prevents -D warnings from catching it.)

I don't think there is a reason. We could make it a lint (or add it to an existing one?)

Am I correct that the RHS is not parsed unless it is the arm that is selected? That is, the right hand side can contain any token tree?

Yes, the rhs can be any token tree.

traviscross · 2025-12-09T00:13:32Z

Is there a reason the unreachable predicate warning isn't a lint? (The main reason I ask is because this prevents -D warnings from catching it.)

I don't think there is a reason. We could make it a lint (or add it to an existing one?)

For my part, it seems worth doing this. Then we can stabilize this lint along with this FCP. Perhaps a new lint named unreachable_cfgs would be correct. The name is by analogy to unexpected_cfgs and unreachable_patterns.

(In the alternate, if we did want "predicate" in the name, I'd suggest unreachable_cfg_predicates.)

library/core/src/macros/mod.rs

bors · 2025-12-09T13:07:59Z

☔ The latest upstream changes (presumably #149798) made this pull request unmergeable. Please resolve the merge conflicts.

JonathanBrouwer · 2026-02-22T18:06:46Z

@bors r-
#152967 (comment)

rust-bors · 2026-02-22T18:06:52Z

Commit eb2d00a has been unapproved.

This PR was contained in a rollup (#152967), which was also unapproved.

compiler/rustc_lint_defs/src/builtin.rs

folkertdev · 2026-02-22T19:04:39Z

@rustbot ready

(assuming CI is happy)

JonathanBrouwer · 2026-02-22T19:12:58Z

@bors try jobs=x86_64-gnu-llvm-21-3

stabilize `cfg_select!` try-job: x86_64-gnu-llvm-21-3

rust-bors · 2026-02-22T21:18:24Z

☀️ Try build successful (CI)
Build commit: 76f787d (76f787d0531d2c5dc810f0b770b1e5be5427e70f, parent: 1500f0f47f5fe8ddcd6528f6c6c031b210b4eac5)

JonathanBrouwer · 2026-02-22T21:19:14Z

@bors r+

rust-bors · 2026-02-22T21:19:16Z

📌 Commit 14d29f9 has been approved by JonathanBrouwer

It is now in the queue for this repository.

@traviscross

…=JonathanBrouwer stabilize `cfg_select!` *[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/149783)* tracking issue: rust-lang#115585 closes rust-lang#115585 reference PR: - rust-lang/reference#2103 # Request for Stabilization ## Summary The `cfg_select!` macro picks the expansion corresponding to the first `cfg` condition that evaluates to `true`. It simplifies complex conditional expressions. ```rust cfg_select! { unix => { fn foo() { /* unix specific functionality */ } } target_pointer_width = "32" => { fn foo() { /* non-unix, 32-bit functionality */ } } _ => { fn foo() { /* fallback implementation */ } } } let is_unix_str = cfg_select! { unix => "unix", _ => "not unix", }; println!("{is_unix_str}"); ``` ## Semantics The expansion of a `cfg_select!` call is the right-hand side of the first `cfg` rule that evaluates to true. This can be roughly expressed using this macro: ```rust macro_rules! cfg_select { ({ $($tt:tt)* }) => {{ $crate::cfg_select! { $($tt)* } }}; (_ => { $($output:tt)* }) => { $($output)* }; ( $cfg:meta => $output:tt $($( $rest:tt )+)? ) => { #[cfg($cfg)] $crate::cfg_select! { _ => $output } $( #[cfg(not($cfg))] $crate::cfg_select! { $($rest)+ } )? } } ``` The actual implementation uses a builtin macro so that `cfg_select!` can be used both in item and expression position. ## Documentation reference PR: - rust-lang/reference#2103 ## Tests The `cfg_select!` macro is already used extensively in the rust compiler codebase. It has several dedicated tests: - [`tests/ui/check-cfg/cfg-select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/check-cfg/cfg-select.rs)tests that warnings are emitted when an unexpected `cfg` condition is used. - [`tests/ui/macros/cfg_select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/macros/cfg_select.rs) tests that `cfg_select!` has the expected expansion, and tests that the expected syntax is accepted. ## History - rust-lang#115416 - rust-lang#117162 - rust-lang#133720 - rust-lang#135625 - rust-lang#137198 - rust-lang#138993 - rust-lang#138996 - rust-lang#143461 - rust-lang#143941 - rust-lang#145233 - rust-lang#148712 - rust-lang#149380 - rust-lang#149925 # Resolved questions # Unresolved questions The style team has decided on how to format `cfg_select!`, but this formatting has not yet been implemented. See rust-lang#144323. r? @traviscross   > [!NOTE] > # Concerns (0 active) > > - ~~[allowing-comma-after-closing-brace](rust-lang#149783 (comment) resolved in [this comment](rust-lang#149783 (comment)) > > *Managed by `@rustbot`—see [help](https://forge.rust-lang.org/triagebot/concern.html) for details.*

@traviscross

…=JonathanBrouwer stabilize `cfg_select!` *[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/149783)* tracking issue: rust-lang#115585 closes rust-lang#115585 reference PR: - rust-lang/reference#2103 # Request for Stabilization ## Summary The `cfg_select!` macro picks the expansion corresponding to the first `cfg` condition that evaluates to `true`. It simplifies complex conditional expressions. ```rust cfg_select! { unix => { fn foo() { /* unix specific functionality */ } } target_pointer_width = "32" => { fn foo() { /* non-unix, 32-bit functionality */ } } _ => { fn foo() { /* fallback implementation */ } } } let is_unix_str = cfg_select! { unix => "unix", _ => "not unix", }; println!("{is_unix_str}"); ``` ## Semantics The expansion of a `cfg_select!` call is the right-hand side of the first `cfg` rule that evaluates to true. This can be roughly expressed using this macro: ```rust macro_rules! cfg_select { ({ $($tt:tt)* }) => {{ $crate::cfg_select! { $($tt)* } }}; (_ => { $($output:tt)* }) => { $($output)* }; ( $cfg:meta => $output:tt $($( $rest:tt )+)? ) => { #[cfg($cfg)] $crate::cfg_select! { _ => $output } $( #[cfg(not($cfg))] $crate::cfg_select! { $($rest)+ } )? } } ``` The actual implementation uses a builtin macro so that `cfg_select!` can be used both in item and expression position. ## Documentation reference PR: - rust-lang/reference#2103 ## Tests The `cfg_select!` macro is already used extensively in the rust compiler codebase. It has several dedicated tests: - [`tests/ui/check-cfg/cfg-select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/check-cfg/cfg-select.rs)tests that warnings are emitted when an unexpected `cfg` condition is used. - [`tests/ui/macros/cfg_select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/macros/cfg_select.rs) tests that `cfg_select!` has the expected expansion, and tests that the expected syntax is accepted. ## History - rust-lang#115416 - rust-lang#117162 - rust-lang#133720 - rust-lang#135625 - rust-lang#137198 - rust-lang#138993 - rust-lang#138996 - rust-lang#143461 - rust-lang#143941 - rust-lang#145233 - rust-lang#148712 - rust-lang#149380 - rust-lang#149925 # Resolved questions # Unresolved questions The style team has decided on how to format `cfg_select!`, but this formatting has not yet been implemented. See rust-lang#144323. r? @traviscross   > [!NOTE] > # Concerns (0 active) > > - ~~[allowing-comma-after-closing-brace](rust-lang#149783 (comment) resolved in [this comment](rust-lang#149783 (comment)) > > *Managed by `@rustbot`—see [help](https://forge.rust-lang.org/triagebot/concern.html) for details.*

Rollup of 9 pull requests Successful merges: - #152229 (Remove deterministic picking from query cycle handling) - #152970 (extend unpin noalias tests to cover mutable references) - #149783 (stabilize `cfg_select!`) - #151744 (fix refining_impl_trait suggestion with return_type_notation) - #152366 (Add try_shrink_to and try_shrink_to_fit to Vec) - #152640 (Add direct link to rustc-dev-guide in README) - #152963 (Revert "Stabilize `str_as_str`") - #152984 (Remove redundant call to `check_codegen_attributes_extra` in Inliner) - #152987 (Use `HashStable` derive in more places)

@traviscross

Rollup merge of #149783 - folkertdev:stabilize-cfg-select, r=JonathanBrouwer stabilize `cfg_select!` *[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/149783)* tracking issue: #115585 closes #115585 reference PR: - rust-lang/reference#2103 # Request for Stabilization ## Summary The `cfg_select!` macro picks the expansion corresponding to the first `cfg` condition that evaluates to `true`. It simplifies complex conditional expressions. ```rust cfg_select! { unix => { fn foo() { /* unix specific functionality */ } } target_pointer_width = "32" => { fn foo() { /* non-unix, 32-bit functionality */ } } _ => { fn foo() { /* fallback implementation */ } } } let is_unix_str = cfg_select! { unix => "unix", _ => "not unix", }; println!("{is_unix_str}"); ``` ## Semantics The expansion of a `cfg_select!` call is the right-hand side of the first `cfg` rule that evaluates to true. This can be roughly expressed using this macro: ```rust macro_rules! cfg_select { ({ $($tt:tt)* }) => {{ $crate::cfg_select! { $($tt)* } }}; (_ => { $($output:tt)* }) => { $($output)* }; ( $cfg:meta => $output:tt $($( $rest:tt )+)? ) => { #[cfg($cfg)] $crate::cfg_select! { _ => $output } $( #[cfg(not($cfg))] $crate::cfg_select! { $($rest)+ } )? } } ``` The actual implementation uses a builtin macro so that `cfg_select!` can be used both in item and expression position. ## Documentation reference PR: - rust-lang/reference#2103 ## Tests The `cfg_select!` macro is already used extensively in the rust compiler codebase. It has several dedicated tests: - [`tests/ui/check-cfg/cfg-select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/check-cfg/cfg-select.rs)tests that warnings are emitted when an unexpected `cfg` condition is used. - [`tests/ui/macros/cfg_select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/macros/cfg_select.rs) tests that `cfg_select!` has the expected expansion, and tests that the expected syntax is accepted. ## History - #115416 - #117162 - #133720 - #135625 - #137198 - #138993 - #138996 - #143461 - #143941 - #145233 - #148712 - #149380 - #149925 # Resolved questions # Unresolved questions The style team has decided on how to format `cfg_select!`, but this formatting has not yet been implemented. See #144323. r? @traviscross   > [!NOTE] > # Concerns (0 active) > > - ~~[allowing-comma-after-closing-brace](#149783 (comment) resolved in [this comment](#149783 (comment)) > > *Managed by `@rustbot`—see [help](https://forge.rust-lang.org/triagebot/concern.html) for details.*

@traviscross

…Brouwer stabilize `cfg_select!` *[View all comments](https://triagebot.infra.rust-lang.org/gh-comments/rust-lang/rust/pull/149783)* tracking issue: rust-lang/rust#115585 closes rust-lang/rust#115585 reference PR: - rust-lang/reference#2103 # Request for Stabilization ## Summary The `cfg_select!` macro picks the expansion corresponding to the first `cfg` condition that evaluates to `true`. It simplifies complex conditional expressions. ```rust cfg_select! { unix => { fn foo() { /* unix specific functionality */ } } target_pointer_width = "32" => { fn foo() { /* non-unix, 32-bit functionality */ } } _ => { fn foo() { /* fallback implementation */ } } } let is_unix_str = cfg_select! { unix => "unix", _ => "not unix", }; println!("{is_unix_str}"); ``` ## Semantics The expansion of a `cfg_select!` call is the right-hand side of the first `cfg` rule that evaluates to true. This can be roughly expressed using this macro: ```rust macro_rules! cfg_select { ({ $($tt:tt)* }) => {{ $crate::cfg_select! { $($tt)* } }}; (_ => { $($output:tt)* }) => { $($output)* }; ( $cfg:meta => $output:tt $($( $rest:tt )+)? ) => { #[cfg($cfg)] $crate::cfg_select! { _ => $output } $( #[cfg(not($cfg))] $crate::cfg_select! { $($rest)+ } )? } } ``` The actual implementation uses a builtin macro so that `cfg_select!` can be used both in item and expression position. ## Documentation reference PR: - rust-lang/reference#2103 ## Tests The `cfg_select!` macro is already used extensively in the rust compiler codebase. It has several dedicated tests: - [`tests/ui/check-cfg/cfg-select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/check-cfg/cfg-select.rs)tests that warnings are emitted when an unexpected `cfg` condition is used. - [`tests/ui/macros/cfg_select.rs`](https://github.com/rust-lang/rust/blob/main/tests/ui/macros/cfg_select.rs) tests that `cfg_select!` has the expected expansion, and tests that the expected syntax is accepted. ## History - rust-lang/rust#115416 - rust-lang/rust#117162 - rust-lang/rust#133720 - rust-lang/rust#135625 - rust-lang/rust#137198 - rust-lang/rust#138993 - rust-lang/rust#138996 - rust-lang/rust#143461 - rust-lang/rust#143941 - rust-lang/rust#145233 - rust-lang/rust#148712 - rust-lang/rust#149380 - rust-lang/rust#149925 # Resolved questions # Unresolved questions The style team has decided on how to format `cfg_select!`, but this formatting has not yet been implemented. See rust-lang/rust#144323. r? @traviscross   > [!NOTE] > # Concerns (0 active) > > - ~~[allowing-comma-after-closing-brace](rust-lang/rust#149783 (comment) resolved in [this comment](rust-lang/rust#149783 (comment)) > > *Managed by `@rustbot`—see [help](https://forge.rust-lang.org/triagebot/concern.html) for details.*

Rollup of 9 pull requests Successful merges: - rust-lang/rust#152229 (Remove deterministic picking from query cycle handling) - rust-lang/rust#152970 (extend unpin noalias tests to cover mutable references) - rust-lang/rust#149783 (stabilize `cfg_select!`) - rust-lang/rust#151744 (fix refining_impl_trait suggestion with return_type_notation) - rust-lang/rust#152366 (Add try_shrink_to and try_shrink_to_fit to Vec) - rust-lang/rust#152640 (Add direct link to rustc-dev-guide in README) - rust-lang/rust#152963 (Revert "Stabilize `str_as_str`") - rust-lang/rust#152984 (Remove redundant call to `check_codegen_attributes_extra` in Inliner) - rust-lang/rust#152987 (Use `HashStable` derive in more places)

folkertdev added the F-cfg_select `#![feature(cfg_select)]` label Dec 8, 2025

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folkertdev force-pushed the stabilize-cfg-select branch from c87e26a to 67d1d43 Compare December 8, 2025 21:07

folkertdev marked this pull request as ready for review December 8, 2025 23:06

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traviscross added T-lang Relevant to the language team T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang labels Dec 8, 2025

traviscross added the needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. label Dec 9, 2025

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Stabilize cfg_select

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Auto merge of #149783 - folkertdev:stabilize-cfg-select, r=<try>

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stabilize `cfg_select!` try-job: x86_64-gnu-llvm-21-3

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Rollup of 5 pull requests #152992

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Rollup of 5 pull requests #152993

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Rollup of 9 pull requests #153000

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polarathene mentioned this pull request Feb 26, 2026

Consider switching cfg-if to cfg_select! alexcrichton/dlmalloc-rs#64

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Request for Stabilization

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