error: Undefined Behavior: trying to reborrow from <TAG> for Unique permission at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
  --> $DIR/load_invalid_mut.rs:LL:CC
   |
LL |     let _val = *xref_in_mem;
   |                ^^^^^^^^^^^^
   |                |
   |                trying to reborrow from <TAG> for Unique 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..0x4]
   |
   = 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 retag at offsets [0x0..0x4]
  --> $DIR/load_invalid_mut.rs:LL:CC
   |
LL |     let xref_in_mem = Box::new(xref);
   |                       ^^^^^^^^^^^^^^
help: <TAG> was later invalidated at offsets [0x0..0x4]
  --> $DIR/load_invalid_mut.rs:LL:CC
   |
LL |     let _val = unsafe { *xraw }; // invalidate xref
   |                         ^^^^^
   = note: backtrace:
   = note: inside `main` at $DIR/load_invalid_mut.rs:LL:CC

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

error: aborting due to previous error