Constant ACTIVE_RESOLUTIONS 

const ACTIVE_RESOLUTIONS: LocalKey<RefCell<Vec<*const ()>>>;

During import resolution, recursive imports can form cycles. This set stores the active resolution stack for the current thread. So it’s essentially a recursion stack.

The key is the interned address of a RefCell<NameResolution<'ra>> allocated in the Resolver Arenas (lifetime 'ra), it is thus stable and allows casting to a *const () for comparison. This is done because we can’t use lifetimes other than 'static in thread local storage.