error: Undefined Behavior: attempting a read access using <TAG> at ALLOC[$HEX], but that tag does not exist in the borrow stack for this location
  --> $DIR/shared_rw_borrows_are_weak2.rs:LL:CC
   |
LL |         let _val = *y;
   |                    ^^
   |                    |
   |                    attempting a read access using <TAG> at ALLOC[$HEX], but that tag does not exist in the borrow stack for this location
   |                    this error occurs as part of an access at ALLOC[$HEX..$HEX]
   |
   = 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 [$HEX..$HEX]
  --> $DIR/shared_rw_borrows_are_weak2.rs:LL:CC
   |
LL |         let y: &i32 = mem::transmute(&*x.borrow()); // launder lifetime
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: <TAG> was later invalidated at offsets [$HEX..$HEX] by a Unique retag
  --> $DIR/shared_rw_borrows_are_weak2.rs:LL:CC
   |
LL |         shr_rw.replace(1);
   |         ^^^^^^^^^^^^^^^^^
   = note: BACKTRACE:
   = note: inside `main` at $DIR/shared_rw_borrows_are_weak2.rs:LL:CC

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

error: aborting due to previous error