Reject outer attributes on cfg_select branches

compiler/rustc_attr_parsing/src/attributes/cfg_select.rs

@@ -2,7 +2,7 @@ use rustc_ast::token::Token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::{AttrStyle, NodeId, token};
 use rustc_data_structures::fx::FxHashMap;
-use rustc_errors::Diagnostic;
+use rustc_errors::{Diagnostic, MultiSpan};
 use rustc_feature::{AttributeTemplate, Features};
 use rustc_hir::attrs::CfgEntry;
 use rustc_hir::{AttrPath, Target};
@@ -76,8 +76,11 @@ pub fn parse_cfg_select(
     lint_node_id: NodeId,
 ) -> Result<CfgSelectBranches, ErrorGuaranteed> {
     let mut branches = CfgSelectBranches::default();
+    let mut branch_attr_error: Option<ErrorGuaranteed> = None;
 
     while p.token != token::Eof {
+        reject_branch_outer_attrs(p, &mut branch_attr_error)?;
+
         if p.eat_keyword(exp!(Underscore)) {
             let underscore = p.prev_token;
             p.expect(exp!(FatArrow)).map_err(|e| e.emit())?;
@@ -131,6 +134,10 @@ pub fn parse_cfg_select(
         }
     }
 
+    if let Some(guar) = branch_attr_error {
+        return Err(guar);
+    }
+
     let it = branches
         .reachable
         .iter()
@@ -143,6 +150,27 @@ pub fn parse_cfg_select(
     Ok(branches)
 }
 
+fn reject_branch_outer_attrs(
+    p: &mut Parser<'_>,
+    branch_attr_error: &mut Option<ErrorGuaranteed>,
+) -> Result<(), ErrorGuaranteed> {
+    let Some(spans) = p.parse_cfg_select_branch_outer_attrs().map_err(|e| e.emit())? else {
+        return Ok(());
+    };
+
+    for (spans, msg) in [
+        (spans.doc_comments, "doc comments are not allowed on `cfg_select` branches"),
+        (spans.attrs, "attributes are not allowed on `cfg_select` branches"),
+    ] {
+        if !spans.is_empty() {
+            branch_attr_error
+                .get_or_insert(p.dcx().struct_span_err(MultiSpan::from_spans(spans), msg).emit());
+        }
+    }
+
+    Ok(())
+}
+
 fn lint_unreachable(
     p: &mut Parser<'_>,
     predicates: impl Iterator<Item = CfgSelectPredicate>,

compiler/rustc_parse/src/parser/cfg_select.rs

@@ -1,11 +1,18 @@
-use rustc_ast::token;
 use rustc_ast::tokenstream::{TokenStream, TokenTree};
 use rustc_ast::util::classify;
+use rustc_ast::{AttrKind, token};
 use rustc_errors::PResult;
+use rustc_span::Span;
 
 use crate::exp;
 use crate::parser::{AttrWrapper, ForceCollect, Parser, Restrictions, Trailing, UsePreAttrPos};
 
+#[derive(Default)]
+pub struct CfgSelectBranchAttrSpans {
+    pub attrs: Vec<Span>,
+    pub doc_comments: Vec<Span>,
+}
+
 impl<'a> Parser<'a> {
     /// Parses a `TokenTree` consisting either of `{ /* ... */ }` optionally followed by a comma
     /// (and strip the braces and the optional comma) or an expression followed by a comma
@@ -36,4 +43,33 @@ impl<'a> Parser<'a> {
         }
         Ok(TokenStream::from_ast(&expr))
     }
+
+    /// Parses outer attributes before a `cfg_select!` branch for recovery.
+    pub fn parse_cfg_select_branch_outer_attrs(
+        &mut self,
+    ) -> PResult<'a, Option<CfgSelectBranchAttrSpans>> {
+        let attrs = self.parse_outer_attributes()?;
+        if attrs.is_empty() {
+            return Ok(None);
+        }
+
+        let mut spans = CfgSelectBranchAttrSpans::default();
+        for attr in attrs.take_for_recovery(self.psess) {
+            match attr.kind {
+                AttrKind::Normal(..) => spans.attrs.push(attr.span),
+                // `parse_outer_attributes` already emitted E0753 for inner doc comments before
+                // recovering them as outer doc-comment attributes.
+                AttrKind::DocComment(comment_kind, _)
+                    if self.span_to_snippet(attr.span).ok().is_some_and(
+                        |snippet| match comment_kind {
+                            token::CommentKind::Line => snippet.starts_with("//!"),
+                            token::CommentKind::Block => snippet.starts_with("/*!"),
+                        },
+                    ) => {}
+                AttrKind::DocComment(..) => spans.doc_comments.push(attr.span),
+            }
+        }
+
+        Ok(Some(spans))
+    }
 }

tests/ui/macros/cfg_select.rs

@@ -202,3 +202,52 @@ cfg_select! {
     //~^ ERROR expected one of `(`, `::`, `=>`, or `=`, found `!`
     //~| WARN unexpected `cfg` condition name
 }
+
+// Regression test for https://github.com/rust-lang/rust/issues/155701.
+cfg_select! {
+    /// doc comment
+    //~^ ERROR doc comments are not allowed on `cfg_select` branches
+    debug_assertions => {}
+    /// doc comment
+    //~^ ERROR doc comments are not allowed on `cfg_select` branches
+    _ => {}
+}
+
+cfg_select! {
+    #[cfg(false)]
+    //~^ ERROR attributes are not allowed on `cfg_select` branches
+    debug_assertions => {}
+    _ => {}
+}
+
+cfg_select! {
+    #![cfg(false)]
+    //~^ ERROR an inner attribute is not permitted in this context
+    debug_assertions => {}
+    _ => {}
+}
+
+cfg_select! {
+    //! inner doc comment
+    //~^ ERROR expected outer doc comment
+    debug_assertions => {}
+    _ => {}
+}
+
+cfg_select! {
+    debug_assertions => {}
+    /// line1
+    //~^ ERROR doc comments are not allowed on `cfg_select` branches
+    // line2
+    /// line3
+    _ => {}
+}
+
+cfg_select! {
+    /// outer doc comment
+    //~^ ERROR doc comments are not allowed on `cfg_select` branches
+    //! inner doc comment
+    //~^ ERROR expected outer doc comment
+    debug_assertions => {}
+    _ => {}
+}

tests/ui/macros/cfg_select.stderr

@@ -55,6 +55,74 @@ error: expected one of `(`, `::`, `=>`, or `=`, found `!`
 LL |     cfg!() => {}
    |        ^ expected one of `(`, `::`, `=>`, or `=`
 
+error: doc comments are not allowed on `cfg_select` branches
+  --> $DIR/cfg_select.rs:208:5
+   |
+LL |     /// doc comment
+   |     ^^^^^^^^^^^^^^^
+
+error: doc comments are not allowed on `cfg_select` branches
+  --> $DIR/cfg_select.rs:211:5
+   |
+LL |     /// doc comment
+   |     ^^^^^^^^^^^^^^^
+
+error: attributes are not allowed on `cfg_select` branches
+  --> $DIR/cfg_select.rs:217:5
+   |
+LL |     #[cfg(false)]
+   |     ^^^^^^^^^^^^^
+
+error: an inner attribute is not permitted in this context
+  --> $DIR/cfg_select.rs:224:5
+   |
+LL |     #![cfg(false)]
+   |     ^^^^^^^^^^^^^^
+   |
+   = note: inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files
+   = note: outer attributes, like `#[test]`, annotate the item following them
+
+error[E0753]: expected outer doc comment
+  --> $DIR/cfg_select.rs:231:5
+   |
+LL |     //! inner doc comment
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: inner doc comments like this (starting with `//!` or `/*!`) can only appear before items
+help: you might have meant to write a regular comment
+   |
+LL -     //! inner doc comment
+LL +     // inner doc comment
+   |
+
+error: doc comments are not allowed on `cfg_select` branches
+  --> $DIR/cfg_select.rs:239:5
+   |
+LL |     /// line1
+   |     ^^^^^^^^^
+...
+LL |     /// line3
+   |     ^^^^^^^^^
+
+error[E0753]: expected outer doc comment
+  --> $DIR/cfg_select.rs:249:5
+   |
+LL |     //! inner doc comment
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: inner doc comments like this (starting with `//!` or `/*!`) can only appear before items
+help: you might have meant to write a regular comment
+   |
+LL -     //! inner doc comment
+LL +     // inner doc comment
+   |
+
+error: doc comments are not allowed on `cfg_select` branches
+  --> $DIR/cfg_select.rs:247:5
+   |
+LL |     /// outer doc comment
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
 warning: unreachable configuration predicate
   --> $DIR/cfg_select.rs:136:5
    |
@@ -115,7 +183,7 @@ LL |     cfg!() => {}
    = help: to expect this configuration use `--check-cfg=cfg(cfg)`
    = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
 
-error: aborting due to 9 previous errors; 7 warnings emitted
+error: aborting due to 17 previous errors; 7 warnings emitted
 
-Some errors have detailed explanations: E0537, E0539.
+Some errors have detailed explanations: E0537, E0539, E0753.
 For more information about an error, try `rustc --explain E0537`.