eat close paren if capture_cfg

compiler/rustc_parse/src/parser/item.rs

@@ -2494,11 +2494,19 @@ impl<'a> Parser<'a> {
         // Parse the arguments, starting out with `self` being allowed...
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_diff_marker();
+            let starts_with_paren = p.check_noexpect(&token::OpenDelim(Delimiter::Parenthesis));
             let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {
-                e.emit();
+                if p.capture_cfg {
+                    e.cancel();
+                } else {
+                    e.emit();
+                }
                 let lo = p.prev_token.span;
                 // Skip every token until next possible arg or end.
                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(Delimiter::Parenthesis)]);
+                if p.capture_cfg && starts_with_paren {
+                    p.eat_noexpect(&token::CloseDelim(Delimiter::Parenthesis));
+                }
                 // Create a placeholder argument for proper arg count (issue #34264).
                 Ok(dummy_arg(Ident::new(kw::Empty, lo.to(p.prev_token.span))))
             });

tests/ui/parser/issue-116781.rs

@@ -0,0 +1,8 @@
+#[derive(Debug)]
+struct Foo {
+    #[cfg(all())]
+    field: fn(($),), //~ ERROR expected pattern, found `$`
+    //~^ ERROR expected identifier, found `)`
+}
+
+fn main() {}

tests/ui/parser/issue-116781.stderr

@@ -0,0 +1,17 @@
+error: expected pattern, found `$`
+  --> $DIR/issue-116781.rs:4:16
+   |
+LL |     field: fn(($),),
+   |                ^ expected pattern
+
+error: expected identifier, found `)`
+  --> $DIR/issue-116781.rs:4:19
+   |
+LL | struct Foo {
+   |        --- while parsing this struct
+LL |     #[cfg(all())]
+LL |     field: fn(($),),
+   |                   ^ expected identifier
+
+error: aborting due to 2 previous errors
+