error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x0], but that tag does not exist in the borrow stack for this location
  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
   |
LL |         *fieldref = 0;
   |         ^^^^^^^^^^^^^
   |         |
   |         attempting a write 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..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 Unique retag at offsets [0x0..0x4]
  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
   |
LL |         let fieldref = &mut *(&mut x.0 as *mut i32);
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: <TAG> was later invalidated at offsets [0x0..0x4] by a SharedReadWrite retag
  --> $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC
   |
LL |         let _xref = &mut x;
   |                     ^^^^^^
   = note: BACKTRACE:
   = note: inside `main` at $DIR/notunpin_dereferenceable_fakeread.rs:LL:CC

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

error: aborting due to previous error