Fix incorrect gating of nonterminals in key-value attributes

Fixes #85432

When processing a `#[derive]` or `#[cfg_eval]` attribute, we need to
re-parse our attribute target, which requires flattenting all
`Nonterminals`. However, this caused us to incorrectly gate on a
(flattented) nonterminal in a key-value attribute, which is supposed to
be allowed.

Since we already perform this gating during the initial parse, we
suppress it in `capture_cfg` mode.

compiler/rustc_parse/src/parser/mod.rs

@@ -1078,7 +1078,10 @@ impl<'a> Parser<'a> {
 
                     match &expr.kind {
                         // Not gated to support things like `doc = $expr` that work on stable.
-                        _ if is_interpolated_expr => {}
+                        // Do not gate in `capture_cfg` mode, since we flatten all nontemrinals
+                        // before parsing. `capture_cfg` mode is only used to reparse existing
+                        // tokens, so the gating will be performed by the initial parse
+                        _ if is_interpolated_expr || self.capture_cfg => {}
                         ExprKind::Lit(lit) if lit.kind.is_unsuffixed() => {}
                         _ => self.sess.gated_spans.gate(sym::extended_key_value_attributes, span),
                     }

src/test/ui/macros/issue-85432-ungated-attr-macro.rs

@@ -0,0 +1,30 @@
+// check-pass
+// Regression test for issue #85432
+// Ensures that we don't incorrectly gate nonterminals
+// in key-value macros when we need to reparse due to
+// the presence of `#[derive]`
+
+macro_rules! with_doc_comment {
+    ($comment:expr, $item:item) => {
+        #[doc = $comment]
+        $item
+    };
+}
+
+macro_rules! database_table_doc {
+    () => {
+        ""
+    };
+}
+
+with_doc_comment! {
+    database_table_doc!(),
+    #[derive(Debug)]
+    struct Image {
+        #[cfg(FALSE)]
+        _f: (),
+    }
+
+}
+
+fn main() {}