error: Undefined Behavior: trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
  --> $DIR/return_invalid_shr.rs:LL:CC
   |
LL |     ret
   |     ^^^
   |     |
   |     trying to retag from <TAG> for SharedReadOnly permission at ALLOC[0x4], but that tag does not exist in the borrow stack for this location
   |     this error occurs as part of retag at ALLOC[0x4..0x8]
   |
   = 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 SharedReadOnly retag at offsets [0x4..0x8]
  --> $DIR/return_invalid_shr.rs:LL:CC
   |
LL |     let ret = unsafe { &(*xraw).1 };
   |                        ^^^^^^^^^^
help: <TAG> was later invalidated at offsets [0x0..0x8] by a write access
  --> $DIR/return_invalid_shr.rs:LL:CC
   |
LL |     unsafe { *xraw = (42, 23) }; // unfreeze
   |              ^^^^^^^^^^^^^^^^
   = note: BACKTRACE:
   = note: inside `foo` at $DIR/return_invalid_shr.rs:LL:CC
note: inside `main`
  --> $DIR/return_invalid_shr.rs:LL:CC
   |
LL |     foo(&mut (1, 2));
   |     ^^^^^^^^^^^^^^^^

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

error: aborting due to previous error