Struct WheelEntry 

pub struct WheelEntry<T> { /* private fields */ }

Timer wheel entry stored inside a slab slot.

This type appears in the public type signature of TimerWheel as the slab’s item type, but is opaque — users never construct or access it directly.

Layout

DLL links and deadline are hot (touched every list walk / poll check). The refcount is accessed on schedule/cancel/fire. Level and slot index are set at insertion and read on cancel — they replace the recomputation that was unsound after time advances. The value is cold relative to the links — only read on fire or cancel.

UnsafeCell<Option<T>> provides interior mutability for .take() through shared references (slab gives &self access). Option<T> makes Drop safe: drop_in_place on None is a no-op. Single-threaded (!Send), so UnsafeCell is sound.