Add side-effect test

tests/ui/derive-invariant/helper-side-effect/expected

@@ -0,0 +1,10 @@
+error[E0596]: cannot borrow `*x` as mutable, as it is behind a `&` reference
+  --> safety_constraint_helper_side_effect/safety_constraint_helper_side_effect.rs:12:28
+   |
+|     #[safety_constraint({*(x.as_mut()) = 0; true})]
+   |                            ^ `x` is a `&` reference, so the data it refers to cannot be borrowed as mutable
+   |
+help: consider specifying this binding's type
+   |
+|     x: &mut std::boxed::Box<i32>: Box<i32>,
+   |      +++++++++++++++++++++++++++

tests/ui/derive-invariant/helper-side-effect/helper-side-effect.rs

@@ -0,0 +1,22 @@
+// Copyright Kani Contributors
+// SPDX-License-Identifier: Apache-2.0 OR MIT
+
+//! Check that side effect expressions in the `#[safety_constraint(...)]`
+//! attribute helpers are not allowed.
+
+extern crate kani;
+use kani::Invariant;
+
+#[derive(kani::Arbitrary)]
+#[derive(kani::Invariant)]
+struct PositivePoint {
+    #[safety_constraint({*(x.as_mut()) = 0; true})]
+    x: Box<i32>,
+    y: i32,
+}
+
+#[kani::proof]
+fn check_invariant_helper_ok() {
+    let pos_point: PositivePoint = kani::any();
+    assert!(pos_point.is_safe());
+}