Add ui test of struct cycle

tests/ui/struct_cycle.rs

@@ -0,0 +1,30 @@
+#[cxx::bridge]
+mod ffi {
+    struct Node0 {
+        i: i32,
+    }
+
+    struct Node1 {
+        node2: Node2,
+        vec: Vec<Node3>,
+    }
+
+    struct Node2 {
+        node4: Node4,
+    }
+
+    struct Node3 {
+        node1: Node1,
+    }
+
+    struct Node4 {
+        node0: Node0,
+        node5: Node5,
+    }
+
+    struct Node5 {
+        node2: Node2,
+    }
+}
+
+fn main() {}

tests/ui/struct_cycle.stderr

@@ -0,0 +1,5 @@
+error: unsupported cyclic data structure
+  --> $DIR/struct_cycle.rs:26:9
+   |
+26 |         node2: Node2,
+   |         ^^^^^^^^^^^^