Constant LAST_PANIC_LOCATION 

const LAST_PANIC_LOCATION: LocalKey<Cell<Option<(String, u32)>>>;

Source location of the most recent panic on this thread, captured by the panic hook. Location borrows the PanicHookInfo, so we snapshot an owned (file, line).

Per-thread because the hook fires on the panicking thread: a worker-thread panic records here on the worker; a main-thread panic records here on main. take_last_panic_location reads-and-clears so a stale location can never leak onto a later, location-less message on the same (reused) thread.