pub fn write_barrier_slot(_old: u64, _new: u64)Expand description
Write barrier for GC-tracked heap writes (raw u64 bits).
Called when a heap pointer in an existing slot is overwritten.
old is the NaN-boxed bits being replaced; new is the incoming bits.
Without gc feature: no-op (compiles away entirely).
With gc feature: enqueues the old reference into the SATB buffer
and marks the new reference gray if an incremental marking cycle is active.