error: Undefined Behavior: attempting a write access using <TAG> at ALLOC[0x0], but that tag only grants SharedReadOnly permission for this location
  --> $DIR/illegal_write1.rs:LL:CC
   |
LL |         unsafe { *x = 42 };
   |                  ^^^^^^^
   |                  |
   |                  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/illegal_write1.rs:LL:CC
   |
LL |         let x: *mut u32 = xref as *const _ as *mut _;
   |                           ^^^^
   = note: BACKTRACE:
   = note: inside `main` at $DIR/illegal_write1.rs:LL:CC

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

error: aborting due to previous error