Fix invalid-pred-error test.

tests/ui/attr-invariant/invalid-pred-error/expected

@@ -1,21 +1,19 @@
-error[E0425]: cannot find value `x` in this scope
-  --> invalid-pred-error.rs:10:19
+error[E0308]: mismatched types
    |
-10 | #[kani::invariant(x.is_safe() && y.is_safe())]
-   |                   ^
+13 | #[kani::invariant(x >= 0 && y >= 0)]
+   |                        ^ expected `&i32`, found integer
    |
-help: you might have meant to use the available field
+help: consider dereferencing the borrow
    |
-10 | #[kani::invariant(self.x.is_safe() && y.is_safe())]
-   |                   +++++
+13 | #[kani::invariant(*x >= 0 && y >= 0)]
+   |                   +
 
-error[E0425]: cannot find value `y` in this scope
-  --> invalid-pred-error.rs:10:34
+error[E0308]: mismatched types
    |
-10 | #[kani::invariant(x.is_safe() && y.is_safe())]
-   |                                  ^
+13 | #[kani::invariant(x >= 0 && y >= 0)]
+   |                                  ^ expected `&i32`, found integer
    |
-help: you might have meant to use the available field
+help: consider dereferencing the borrow
+   |
+13 | #[kani::invariant(x >= 0 && *y >= 0)]
    |
-10 | #[kani::invariant(x.is_safe() && self.y.is_safe())]
-   |                                  +++++

tests/ui/attr-invariant/invalid-pred-error/invalid-pred-error.rs

@@ -7,10 +7,10 @@
 extern crate kani;
 use kani::Invariant;
 
-// Note: The `x.is_safe() && y.is_safe()` requires `self` before each struct
-// field to be evaluated in the `is_safe` function body.
+// Note: The struct fields `x` and `y` are references in this context, we should
+// refer to `*x` and `*y` instead.
 #[derive(kani::Arbitrary)]
-#[kani::invariant(x.is_safe() && y.is_safe())]
+#[kani::invariant(x >= 0 && y >= 0)]
 struct Point {
     x: i32,
     y: i32,