error: Undefined Behavior: trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
  --> $DIR/box-cell-alias.rs:LL:CC
   |
LL |     unsafe { (*ptr).set(20); }
   |              ^^^^^^^^^^^^^^
   |              |
   |              trying to reborrow <TAG> for SharedReadWrite permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   |              this error occurs as part of a reborrow at ALLOC[0x0..0x1]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <TAG> was created by a retag at offsets [0x0..0x1]
  --> $DIR/box-cell-alias.rs:LL:CC
   |
LL |     let ptr: *const Cell<u8> = &*val;
   |                                ^^^^^
help: <TAG> was later invalidated at offsets [0x0..0x1]
  --> $DIR/box-cell-alias.rs:LL:CC
   |
LL |     let res = helper(val, ptr);
   |                      ^^^
   = note: inside `helper` at $DIR/box-cell-alias.rs:LL:CC
note: inside `main` at $DIR/box-cell-alias.rs:LL:CC
  --> $DIR/box-cell-alias.rs:LL:CC
   |
LL |     let res = helper(val, ptr);
   |               ^^^^^^^^^^^^^^^^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error