test for no-validation-only failure

Author: RalfJung

tests/run-pass/without-validation.rs

@@ -0,0 +1,24 @@
+// When we notice something breaks only without validation, we add a test here.
+// compile-flags: -Zmiri-disable-validation
+use std::cell::*;
+
+fn refcell_unsize() {
+    let cell: RefCell<[i32; 3]> = RefCell::new([1, 2, 3]);
+    {
+        let mut cellref: RefMut<'_, [i32; 3]> = cell.borrow_mut();
+        cellref[0] = 4;
+        let mut coerced: RefMut<'_, [i32]> = cellref;
+        coerced[2] = 5;
+    }
+    {
+        let comp: &mut [i32] = &mut [4, 2, 5];
+        let cellref: Ref<'_, [i32; 3]> = cell.borrow();
+        assert_eq!(&*cellref, comp);
+        let coerced: Ref<'_, [i32]> = cellref;
+        assert_eq!(&*coerced, comp);
+    }
+}
+
+fn main() {
+    refcell_unsize();
+}