add test for int -> fn ptr cast

src/error.rs

@@ -61,7 +61,7 @@ impl<'tcx> Error for EvalError<'tcx> {
             EvalError::DanglingPointerDeref =>
                 "dangling pointer was dereferenced",
             EvalError::InvalidFunctionPointer =>
-                "tried to use an integer pointer as a function pointer",
+                "tried to use an integer pointer or a dangling pointer as a function pointer",
             EvalError::InvalidBool =>
                 "invalid boolean value read",
             EvalError::InvalidDiscriminant =>

tests/compile-fail/cast_box_int_to_fn_ptr.rs

@@ -0,0 +1,8 @@
+fn main() {
+    let b = Box::new(42);
+    let g = unsafe {
+        std::mem::transmute::<&usize, &fn(i32)>(&b)
+    };
+
+    (*g)(42) //~ ERROR tried to use an integer pointer or a dangling pointer as a function pointer
+}