rustfmt: Format `cfg_select!` by ytmimi · Pull Request #154202 · rust-lang/rust

ytmimi · 2026-03-22T05:04:25Z

tracking issue: #115585

Implementing cfg_select! formatting here in rust-lang/rust so that the feature can get out to nightly quicker.
The previous PR (#144323) is a bit old at this point and I felt like I could simplify the implementation so I've opted to reimplement the formatting instead of building off the previous PR.

I've tried to break the PR up into logical commits and I think this would be best reviewed one commit at a time.

Previous PR:

add rustfmt support for cfg_select #144323

See also:

Formatting for cfg_select style-team#201

rustbot · 2026-03-22T05:04:29Z

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

rustbot · 2026-03-22T05:04:31Z

r? @jieyouxu

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ytmimi · 2026-03-22T05:08:45Z

+// trailing comments on the last line are a little buggy and always wrap back up
+cfg_select! {
+    windows => {
+        "windows"
+    }
+    unix => {
+        "unix"
+    }
+    _ => {
+        "none"
+    } // FIXME. Prevent wrapping back up to the next line
+}
+
+cfg_select! {
+    windows => "windows",
+    unix => "unix",
+    _ => "none", // FIXME. Prevent wrapping back up to the next line
+}


A pre-existing issue with trailing comments in rustfmt. I wanted to explicitly capture the current behavior in a test case.

ytmimi · 2026-03-22T05:10:36Z

+// Can't format cfg_select! at all with style_edition <= 2021.
+// Things can be formatted with style_edition >= 2024
+cfg_select! {
+    feature = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff" =>
+    {
+        // abc
+        println!();
+    }
+    feature = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff" =>
+    {
+        // abc
+    }
+    all(anything(
+        "some other long long long long long thing long long long long long long long long long long long",
+        feature = "debug-with-rustfmt-long-long-long-long-loooooooonnnnnnnnnnnnnnnggggggffffffffffffffff"
+    )) => {
+        let x = 7;
+    }
+}


Through some testing I found that in style_edition <= 2021 this example won't be formatted at all because the predicate can't be formatted within max_width, but with style_edition >= 2024 that's not an issue.

ytmimi · 2026-03-22T05:17:06Z

+// comments within the predicate are fine with style_edition=2024+
+cfg_select! {
+    any(
+        true, /* comment */
+        true, true, // true,
+        true,
+    ) => {}
+
+    not(
+        false // comment
+    ) => {}
+
+    any(
+        false // comment
+    ) => "any",
+}


Not the best place for a comment to begin with IMO, but I wanted to call out that comments within the predicate aren't correctly handled in style_edition <= 2021. This is how the current example is formatted:

cfg_select! { any( true, /* comment */ true, true, // true, true, ) => {} not(false // comment) => {} any(false // comment) => "any", }

Clearly that's wrong and produces invalid code, but fixing this seems like it's outisde the scope of this PR. since it only impacts style_edition <= 2021.

Maybe this is something we can address later?

I agree, I think we shouldn't try to bundle that in this PR. We probably should open an issue in rustfmt/r-l/r to track this as a known-bug though.

folkertdev

Extremely happy to see this!

View changes since this review

jieyouxu · 2026-03-23T06:10:57Z

(I'll gradually review this; need to build up some background ctx first.)

jieyouxu

Thanks! The impl broadly looks good to me, just some minor nits and a few test coverage discussions.
@rustbot author

View changes since this review

jieyouxu · 2026-03-30T02:48:31Z

+// comments within the predicate are fine with style_edition=2024+
+cfg_select! {
+    any(
+        true, /* comment */
+        true, true, // true,
+        true,
+    ) => {}
+
+    not(
+        false // comment
+    ) => {}
+
+    any(
+        false // comment
+    ) => "any",
+}


I agree, I think we shouldn't try to bundle that in this PR. We probably should open an issue in rustfmt/r-l/r to track this as a known-bug though.

jieyouxu · 2026-03-30T02:49:26Z

+// comments before and after the `=>` get dropped right now
+cfg_select! {
+    any(true, true, true, true,) => {}
+
+    not(false) => {}
+
+    any(false) => "any",
+}


Question: this should also a "known bug" right? Should we also include a follow-up issue to track this?

Yeah, I think we can create an issue for it. I'll handle that after the initial formatting lands

rustbot · 2026-03-30T03:04:15Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

jieyouxu · 2026-03-30T08:30:16Z

Backlinks / context for myself (and future travellers):

Metadata

Library feature: cfg_select

std docs: https://doc.rust-lang.org/nightly/std/macro.cfg_select.html

Tracking issue: Tracking issue for cfg_select (formerly cfg_match) #115585

Style team discussion: Formatting for cfg_select style-team#201

Reference update PR: cfg_select! macro reference#2103

History

Initial style team discussions

The arms must be wrapped in braces, so rustfmt will have to ensure to not remove those.

Follow-up: unbraced expressions are permitted

PR: #145233

@ytmimi

…es, r=ytmimi,jieyouxu rustfmt: Discover modules via `cfg_select!` This PR renames all occurrences of `cfg_match!` in rustfmt to `cfg_select!`. This makes the module file detection logic from rust-lang/rustfmt#6522 kick in for `cfg_select!` instead of `cfg_match!`, which no longer exists. This PR performs no other adjustments to the logic to be as small as possible. I am opening this PR in this repo since that is also the target for the more comprehensive rust-lang#154202, which covers more than merely detecting other files through `cfg_select!`. Closes rust-lang#158371. CC: @ytmimi, @CAD97

@ytmimi

Rollup merge of #158372 - mkroening:rustfmt-cfg_select-modules, r=ytmimi,jieyouxu rustfmt: Discover modules via `cfg_select!` This PR renames all occurrences of `cfg_match!` in rustfmt to `cfg_select!`. This makes the module file detection logic from rust-lang/rustfmt#6522 kick in for `cfg_select!` instead of `cfg_match!`, which no longer exists. This PR performs no other adjustments to the logic to be as small as possible. I am opening this PR in this repo since that is also the target for the more comprehensive #154202, which covers more than merely detecting other files through `cfg_select!`. Closes #158371. CC: @ytmimi, @CAD97

@ytmimi

…imi,jieyouxu rustfmt: Discover modules via `cfg_select!` This PR renames all occurrences of `cfg_match!` in rustfmt to `cfg_select!`. This makes the module file detection logic from rust-lang/rustfmt#6522 kick in for `cfg_select!` instead of `cfg_match!`, which no longer exists. This PR performs no other adjustments to the logic to be as small as possible. I am opening this PR in this repo since that is also the target for the more comprehensive rust-lang/rust#154202, which covers more than merely detecting other files through `cfg_select!`. Closes rust-lang/rust#158371. CC: @ytmimi, @CAD97

`cfg_select!` parsing needs to be implemented in rustfmt right now because there's no good way to call `rustc_attr_parsing::parse_cfg_select`.

The plan is to leverage `rewrite_match_body` to help with `cfg_select!` formatting.

rustbot · 2026-07-30T04:29:57Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

ytmimi · 2026-07-30T04:35:08Z

@jieyouxu Thanks for all the good feedback on this. Finally got around to making all the updates.

@rustbot ready

jieyouxu

I have two residual questions/nits, but the impl itself and tests look good

View changes since this review

jieyouxu

(Also)

View changes since this review

Apply feedback from PR review.

Per the PR review I'm making `context: &RewriteContext<'_>` the first argument. Also moved the `shape` and `span` to follow the `context`.

… arms

Because `inside_macro` is a `Rc<Cell<bool>>` cloning the entire context doesn't actually isolate the `inside_macro` state. However, I've added a `debug_assert!` to make sure that we only ever call `context.leave_macro` when we're on a code path that immediately returns from `rewrite_macro_inner` so that we don't unexpectedly impact default macro handling where we need to be more cautious about adding or removing tokens.

…andling

jieyouxu

Thanks @ytmimi!

@bors r+ rollup

View changes since this review

jieyouxu · 2026-07-31T04:48:54Z

+// Doesn't parse as expected so this is handled by the default macro handling
+cfg_select! (
+    A   +    B    +     C
+);
+cfg_select! [
+    A   +    B    +     C
+];
+// rustfmt doesn't format macros with brace delimiters
+cfg_select! {
+    A   +    B    +     C
+}


Remark: good call, I forgot about this coverage

rust-bors · 2026-07-31T04:49:26Z

📌 Commit ca398cd has been approved by jieyouxu

It is now in the queue for this repository.

rustfmt: Format `cfg_select!` tracking issue: rust-lang#115585 Implementing `cfg_select!` formatting here in `rust-lang/rust` so that the feature can get out to nightly quicker. The previous PR (rust-lang#144323) is a bit old at this point and I felt like I could simplify the implementation so I've opted to reimplement the formatting instead of building off the previous PR. I've tried to break the PR up into logical commits and I think this would be best reviewed one commit at a time. --- Previous PR: * rust-lang#144323 See also: * rust-lang/style-team#201

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue. labels Mar 22, 2026

rustbot assigned jieyouxu Mar 22, 2026

ytmimi commented Mar 22, 2026

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Comment thread src/tools/rustfmt/tests/target/cfg_select.rs Outdated

ytmimi commented Mar 22, 2026

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folkertdev mentioned this pull request Mar 22, 2026

add rustfmt support for cfg_select #144323

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folkertdev reviewed Mar 22, 2026

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Comment thread src/tools/rustfmt/tests/source/cfg_select.rs

jieyouxu reviewed Mar 30, 2026

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This was referenced Jun 24, 2026

rustfmt does not discover modules through cfg_select! #158371

Closed

rustfmt: Discover modules via cfg_select! #158372

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ytmimi added 6 commits July 30, 2026 00:25

feat: parse cfg_select! within rustfmt

8c8a3d7

`cfg_select!` parsing needs to be implemented in rustfmt right now because there's no good way to call `rustc_attr_parsing::parse_cfg_select`.

chore: make rewrite_match_body pub(crate) within rustfmt

65fb2bb

The plan is to leverage `rewrite_match_body` to help with `cfg_select!` formatting.

feat: implement cfg_select! formatting in rustfmt

ea47e34

test: Add more cfg_select! test cases based on the PR review feedback

a932a6d

docs: Add doc comments based on PR feedback

d18984c

feat: flatten cfg_select! arms if they're a single expression

d32c7e9

ytmimi force-pushed the format_cfg_select branch from c267330 to d32c7e9 Compare July 30, 2026 04:29

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 30, 2026

jieyouxu reviewed Jul 30, 2026

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Comment thread src/tools/rustfmt/src/parse/macros/cfg_select.rs Outdated

Comment thread src/tools/rustfmt/src/macros.rs

Comment thread src/tools/rustfmt/src/macros.rs

Comment thread src/tools/rustfmt/tests/target/cfg_select.rs

jieyouxu reviewed Jul 30, 2026

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Comment thread src/tools/rustfmt/src/macros.rs Outdated

Comment thread src/tools/rustfmt/src/parse/macros/cfg_select.rs Outdated

ytmimi added 5 commits July 30, 2026 10:03

fix: rename parse_cfg_select -> parse_cfg_select_arms

668f27f

Apply feedback from PR review.

refactor: reorder format_cfg_select arguments

069095c

Per the PR review I'm making `context: &RewriteContext<'_>` the first argument. Also moved the `shape` and `span` to follow the `context`.

fix: make sure we cancel diagnostic errors when parsing cfg_select!…

6d7cb41

… arms

test: Add test case where cfg_select! falls back to default macro h…

ca398cd

…andling

jieyouxu approved these changes Jul 31, 2026

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rust-bors Bot 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 Jul 31, 2026

jieyouxu added the F-cfg_select `#![feature(cfg_select)]` label Jul 31, 2026

JonathanBrouwer mentioned this pull request Jul 31, 2026

Rollup of 11 pull requests #160273

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JonathanBrouwer mentioned this pull request Jul 31, 2026

Rollup of 17 pull requests #160278

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Metadata

History

Initial style team discussions

Follow-up: unbraced expressions are permitted

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