s/fail/panic/ in doc text
//! }
//! ```
//!
-//! Notice that if this code were implemented without a `LocalKey`, it would fail whenever
+//! Notice that if this code were implemented without a `LocalKey`, it would panic whenever
//! one thread stores an instance of the constructor and a different thread attempts to
//! access it with the call to [`to_inner()`](crate::handle::Root::to_inner).
//!
