mark anonymous variables in compile_is_call (#2257)

src/codegen.rs

@@ -860,6 +860,12 @@ impl<'b> CodeGenerator<'b> {
                     self.marker.mark_safe_var_unconditionally(var_num);
                     compile_expr!(self, &terms[1], term_loc, code)
                 } else {
+                    self.marker.mark_anon_var::<QueryInstruction>(
+                        Level::Shallow,
+                        term_loc,
+                        code,
+                    );
+
                     if let Term::Var(ref vr, ref var) = &terms[1] {
                         let var_num = var.to_var_num().unwrap();