error: Undefined Behavior: attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
  --> $DIR/pointer_smuggling.rs:LL:CC
   |
LL |     let _x = unsafe { *PTR };
   |                       ^^^^
   |                       |
   |                       attempting a read access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
   |                       this error occurs as part of an access at ALLOC[0x0..0x1]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows 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 SharedReadWrite retag at offsets [0x0..0x1]
  --> $DIR/pointer_smuggling.rs:LL:CC
   |
LL |         PTR = x;
   |               ^
help: <TAG> was later invalidated at offsets [0x0..0x1] by a write access
  --> $DIR/pointer_smuggling.rs:LL:CC
   |
LL |     *val = 2; // this invalidates any raw ptrs `fun1` might have created.
   |     ^^^^^^^^
   = note: BACKTRACE:
   = note: inside `fun2` at $DIR/pointer_smuggling.rs:LL:CC
note: inside `main`
  --> $DIR/pointer_smuggling.rs:LL:CC
   |
LL |     fun2(); // if they now use a raw ptr they break our reference
   |     ^^^^^^

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

error: aborting due to previous error