detect cyclic bindings in attr_vars_of_term (#1666)

src/machine/attributed_variables.rs

@@ -147,6 +147,16 @@ impl MachineState {
 
                     let value = unmark_cell_bits!(value);
 
+                    if h != iter.focus() {
+                        let deref_value = heap_bound_store(iter.heap, heap_bound_deref(iter.heap, value));
+
+                        if deref_value.is_compound(iter.heap) {
+                            // a cyclic structure is bound to the attributed variable at h.
+                            // it mustn't be included in seen_vars.
+                            continue;
+                        }
+                    }
+
                     seen_vars.push(value);
                     seen_set.insert(h);