`cfg_select!`: parse unused branches #149925

folkertdev · 2025-12-12T17:34:16Z

tracking issue: #115585

Emit parse errors that are found in the branches that are not selected, like e.g. how #[cfg(false)] { 1 ++ 2 } still emits a parse error even though the expression is never used by the program. Parsing the unused branches was requested by T-lang #149783 (comment).

folkertdev · 2025-12-12T19:28:07Z

compiler/rustc_builtin_macros/src/cfg_select.rs

+fn tts_to_mac_result<'cx, 'sess>(
+    ecx: &'cx mut ExtCtxt<'sess>,
+    site_span: Span,
+    tts: TokenStream,
+    span: Span,
+) -> Box<dyn MacResult + 'cx> {
+    match ExpandResult::from_tts(ecx, tts, site_span, span, Ident::with_dummy_span(sym::cfg_select))
+    {
+        ExpandResult::Ready(x) => x,
+        _ => unreachable!("from_tts always returns Ready"),
+    }
+}


This works around ParserAnyMacro not being exported from rustc_expand. Maybe it should be exported though?

I'm trying to understand how much prettier that'd become. I think it'd be fine to export it from rustc_expand, but I wonder if it changes much. Would you not need the macro calls below anymore? I think you still do right? Because then I don't think this is so bad.

You'd still need the macros. The ParserAnyMacro variant also has an additional argument, which is nicely hidden now. So yeah I think this is the best we can do right now.

okay, that sounds reasonable!

compiler/rustc_builtin_macros/src/cfg_select.rs

folkertdev · 2025-12-13T17:59:17Z

compiler/rustc_builtin_macros/src/cfg_select.rs

+impl<'cx, 'sess> MacResult for CfgSelectResult<'cx, 'sess> {
+    forward_to_parser_any_macro!(make_expr, Box<Expr>);
+    forward_to_parser_any_macro!(make_stmts, SmallVec<[ast::Stmt; 1]>);
+    forward_to_parser_any_macro!(make_items, SmallVec<[Box<ast::Item>; 1]>);

-    branches.wildcard.map(|(_, tt, span)| (tt, span))
+    forward_to_parser_any_macro!(make_impl_items, SmallVec<[Box<ast::AssocItem>; 1]>);
+    forward_to_parser_any_macro!(make_trait_impl_items, SmallVec<[Box<ast::AssocItem>; 1]>);
+    forward_to_parser_any_macro!(make_trait_items, SmallVec<[Box<ast::AssocItem>; 1]>);
+    forward_to_parser_any_macro!(make_foreign_items, SmallVec<[Box<ast::ForeignItem>; 1]>);
 }


I've started a discussion at #t-lang > where can `cfg_select!` go to decide what we want to support.

expr/stmt/item is the bare minimum I think, but we already had some use of (i believe) impl items in the stdlib test suite, so including the other item flavors seems natural. We can see about e.g. patterns or where-clauses etc.

rustbot · 2025-12-13T17:59:43Z

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

rustbot · 2025-12-13T17:59:45Z

r? @chenyukang

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

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jdonszelmann · 2025-12-13T18:04:24Z

r? jdonszelmann

(Reviewed similar PRS recently, don't mind taking a look :)

compiler/rustc_attr_parsing/src/attributes/cfg_select.rs

tests/ui/macros/cfg_select.rs

jdonszelmann · 2025-12-15T10:17:29Z

@rustbot author

rustbot · 2025-12-15T10:17:33Z

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

folkertdev · 2025-12-15T12:42:30Z

@rustbot ready

jdonszelmann · 2025-12-17T13:05:22Z

@rustbot author
Let me know on the question about exporting from rustc_expand. If it becomes a lot prettier here I've nothing against making it pub. But otherwise r=me

folkertdev · 2025-12-17T13:08:16Z

@bors r=jdonszelmann

bors · 2025-12-17T13:08:19Z

📌 Commit d5ca05a has been approved by jdonszelmann

It is now in the queue for this repository.

…-branches, r=jdonszelmann `cfg_select!`: parse unused branches tracking issue: rust-lang#115585 Emit parse errors that are found in the branches that are not selected, like e.g. how `#[cfg(false)] { 1 ++ 2 }` still emits a parse error even though the expression is never used by the program. Parsing the unused branches was requested by T-lang rust-lang#149783 (comment).

matthiaskrgr · 2025-12-17T17:45:55Z

@bors r-
#150104 (comment)

matthiaskrgr · 2025-12-17T17:49:27Z

maybe this conflicts with #148937 ? 🤔

folkertdev · 2025-12-17T18:02:12Z

Looks like we should just also allow expansion in type position. I'll add that.

rustbot · 2025-12-17T20:06:44Z

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.

folkertdev · 2025-12-17T21:44:52Z

After some discussion with @joshtriplett I added support for types and patterns. I haven't been able to trigger the remainder of the MacResult methods from user code, I always run into parse errors before expansion even runs.

@rustbot ready

jdonszelmann · 2025-12-18T11:40:09Z

@bors r+

bors · 2025-12-18T11:40:11Z

📌 Commit 82ee8b2 has been approved by jdonszelmann

It is now in the queue for this repository.

bors · 2025-12-18T11:40:12Z

🌲 The tree is currently closed for pull requests below priority 1000. This pull request will be tested once the tree is reopened.

folkertdev added the F-cfg_select `#![feature(cfg_select)]` label Dec 12, 2025

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stabilize cfg_select! #149783

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Rollup of 5 pull requests #150104

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cfg_select!: emit parse errors in unused branches

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`cfg_select!`: parse unused branches #149925

`cfg_select!`: parse unused branches #149925

folkertdev commented Dec 12, 2025 •

edited by jdonszelmann

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