restore snapshot when parse_param_general

compiler/rustc_parse/src/parser/item.rs

@@ -2494,9 +2494,11 @@ pub(super) fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, ThinV
         // Parse the arguments, starting out with `self` being allowed...
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
             p.recover_diff_marker();
+            let snapshot = p.create_snapshot_for_diagnostic();
             let param = p.parse_param_general(req_name, first_param).or_else(|mut e| {
                 e.emit();
                 let lo = p.prev_token.span;
+                p.restore_snapshot(snapshot);
                 // Skip every token until next possible arg or end.
                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(Delimiter::Parenthesis)]);
                 // Create a placeholder argument for proper arg count (issue #34264).

tests/ui/parser/issue-116781.rs

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

tests/ui/parser/issue-116781.stderr

@@ -0,0 +1,16 @@
+error: expected pattern, found `$`
+  --> $DIR/issue-116781.rs:4:16
+   |
+LL |     field: fn(($),),
+   |                ^ expected pattern
+
+error: expected pattern, found `$`
+  --> $DIR/issue-116781.rs:4:16
+   |
+LL |     field: fn(($),),
+   |                ^ expected pattern
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 2 previous errors
+