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/illegal_read3.rs:LL:CC
   |
LL |     let _val = *xref2;
   |                ^^^^^^
   |                |
   |                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..0x4]
   |
   = 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..0x4]
  --> $DIR/illegal_read3.rs:LL:CC
   |
LL |     let xref2 = &mut *xref1;
   |                 ^^^^^^^^^^^
help: <TAG> was later invalidated at offsets [0x0..0x4]
  --> $DIR/illegal_read3.rs:LL:CC
   |
LL |     let _val = unsafe { *xref1.to };
   |                         ^^^^^^^^^
   = note: inside `main` at $DIR/illegal_read3.rs:LL:CC

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

error: aborting due to previous error