Fix lookahead with None-delimited group

compiler/rustc_parse/src/parser/mod.rs

@@ -930,15 +930,38 @@ impl<'a> Parser<'a> {
         }
 
         let frame = &self.token_cursor.frame;
-        match frame.tree_cursor.look_ahead(dist - 1) {
-            Some(tree) => match tree {
-                TokenTree::Token(token) => looker(token),
-                TokenTree::Delimited(dspan, delim, _) => {
-                    looker(&Token::new(token::OpenDelim(*delim), dspan.open))
-                }
-            },
-            None => looker(&Token::new(token::CloseDelim(frame.delim), frame.span.close)),
+        if frame.delim != DelimToken::NoDelim {
+            let all_normal = (0..dist).all(|i| {
+                let token = frame.tree_cursor.look_ahead(i);
+                !matches!(token, Some(TokenTree::Delimited(_, DelimToken::NoDelim, _)))
+            });
+            if all_normal {
+                return match frame.tree_cursor.look_ahead(dist - 1) {
+                    Some(tree) => match tree {
+                        TokenTree::Token(token) => looker(token),
+                        TokenTree::Delimited(dspan, delim, _) => {
+                            looker(&Token::new(token::OpenDelim(*delim), dspan.open))
+                        }
+                    },
+                    None => looker(&Token::new(token::CloseDelim(frame.delim), frame.span.close)),
+                };
+            }
+        }
+
+        let mut cursor = self.token_cursor.clone();
+        let mut i = 0;
+        let mut token = Token::dummy();
+        while i < dist {
+            token = cursor.next().0;
+            if matches!(
+                token.kind,
+                token::OpenDelim(token::NoDelim) | token::CloseDelim(token::NoDelim)
+            ) {
+                continue;
+            }
+            i += 1;
         }
+        return looker(&token);
     }
 
     /// Returns whether any of the given keywords are `dist` tokens ahead of the current one.

src/test/ui/macros/none-delim-lookahead.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+macro_rules! make_struct {
+    ($name:ident) => {
+        #[derive(Debug)]
+        struct Foo {
+            #[cfg(not(FALSE))]
+            field: fn($name: bool)
+        }
+    }
+}
+
+make_struct!(param_name);
+
+fn main() {}