error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
  --> $DIR/shr_frozen_violation1.rs:LL:CC
   |
LL |         *(x as *const i32 as *mut i32) = 7;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |         |
   |         attempting a write access using <TAG> at ALLOC[0x0], but that tag only grants SharedReadOnly permission 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 SharedReadOnly retag at offsets [0x0..0x4]
  --> $DIR/shr_frozen_violation1.rs:LL:CC
   |
LL |         *(x as *const i32 as *mut i32) = 7;
   |           ^
   = note: BACKTRACE:
   = note: inside `unknown_code` at $DIR/shr_frozen_violation1.rs:LL:CC
note: inside `foo`
  --> $DIR/shr_frozen_violation1.rs:LL:CC
   |
LL |     unknown_code(&*x);
   |     ^^^^^^^^^^^^^^^^^
note: inside `main`
  --> $DIR/shr_frozen_violation1.rs:LL:CC
   |
LL |     println!("{}", foo(&mut 0));
   |                    ^^^^^^^^^^^

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

error: aborting due to previous error