`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

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.

…-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).

…uwer Rollup of 11 pull requests Successful merges: - #145933 (Expand `str_as_str` to more types) - #148849 (Set -Cpanic=abort in windows-msvc stack protector tests) - #149925 (`cfg_select!`: parse unused branches) - #150022 (Generate macro expansion for rust compiler crates docs) - #150024 (Support recursive delegation) - #150048 (std_detect: AArch64 Darwin: expose SME F16F16 and B16B16 features) - #150083 (tests/run-make-cargo/same-crate-name-and-macro-name: New regression test) - #150102 (Fixed ICE for EII with multiple defaults due to duplicate definition in nameres) - #150124 (unstable.rs: fix typos in comments (implementatble -> implementable)) - #150125 (Port `#[rustc_lint_opt_deny_field_access]` to attribute parser) - #150126 (Subtree sync for rustc_codegen_cranelift) Failed merges: - #150127 (Port `#[rustc_lint_untracked_query_information]` and `#[rustc_lint_diagnostics]` to using attribute parsers) r? `@ghost` `@rustbot` modify labels: rollup

…uwer Rollup of 12 pull requests Successful merges: - #145933 (Expand `str_as_str` to more types) - #148849 (Set -Cpanic=abort in windows-msvc stack protector tests) - #149925 (`cfg_select!`: parse unused branches) - #149952 (Suggest struct pattern when destructuring Range with .. syntax) - #150022 (Generate macro expansion for rust compiler crates docs) - #150024 (Support recursive delegation) - #150048 (std_detect: AArch64 Darwin: expose SME F16F16 and B16B16 features) - #150083 (tests/run-make-cargo/same-crate-name-and-macro-name: New regression test) - #150102 (Fixed ICE for EII with multiple defaults due to duplicate definition in nameres) - #150124 (unstable.rs: fix typos in comments (implementatble -> implementable)) - #150125 (Port `#[rustc_lint_opt_deny_field_access]` to attribute parser) - #150126 (Subtree sync for rustc_codegen_cranelift) Failed merges: - #150127 (Port `#[rustc_lint_untracked_query_information]` and `#[rustc_lint_diagnostics]` to using attribute parsers) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #149925 - folkertdev:cfg-select-parse-unused-branches, r=jdonszelmann `cfg_select!`: parse unused branches 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 added the F-cfg_select `#![feature(cfg_select)]` label Dec 12, 2025

rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Dec 12, 2025

traviscross added the I-lang-radar Items that are on lang's radar and will need eventual work or consideration. 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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Rollup of 11 pull requests #150132

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Rollup of 12 pull requests #150135

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cfg_select!: parse unused branches #149925

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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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