Construct test so that it would fail for old code

src/tools/miri/tests/pass/tree_borrows/cell-inside-box.rs

@@ -8,12 +8,28 @@ use std::cell::UnsafeCell;
 
 pub fn main() {
     let cell = UnsafeCell::new(42);
-    let mut root = Box::new(cell);
+    let box1 = Box::new(cell);
 
-    let a = Box::as_mut_ptr(&mut root);
     unsafe {
-        name!(a);
-        let alloc_id = alloc_id!(a);
+        let ptr1: *mut UnsafeCell<i32> = Box::into_raw(box1);
+        name!(ptr1);
+
+        let mut box2 = Box::from_raw(ptr1);
+        // `ptr2` will be a descendant of `ptr1`.
+        let ptr2: *mut UnsafeCell<i32> = Box::as_mut_ptr(&mut box2);
+        name!(ptr2);
+
+        // We perform a write through `x`.
+        // Because `ptr1` is ReservedIM, a child write will make it transition to Active.
+        // Because `ptr2` is ReservedIM, a foreign write doesn't have any effect on it.
+        let x = (*ptr1).get();
+        *x = 1;
+
+        // We can still read from `ptr2`.
+        let val = *(*ptr2).get();
+        assert_eq!(val, 1);
+
+        let alloc_id = alloc_id!(ptr1);
         print_state!(alloc_id);
     }
 }

src/tools/miri/tests/pass/tree_borrows/cell-inside-box.stderr

@@ -2,5 +2,6 @@
 Warning: this tree is indicative only. Some tags may have been hidden.
 0..   4
 | Act |    └─┬──<TAG=root of the allocation>
-| ReIM|      └────<TAG=a>
+| Act |      └─┬──<TAG=ptr1>
+| ReIM|        └────<TAG=ptr2>
 ──────────────────────────────────────────────────