d3ab81b833
Stable Rust emits stack probes inline (subq/movq/jne loop) rather than calling __rust_probestack, so there's no transparent hook for stack- frame preemption. Override of __rust_probestack links cleanly but never runs. Falling back to an explicit check!() that users drop into hot compute loops. check!() decrements the same ALLOC_COUNT counter as the heap path, so both event sources fire timeslice checks at the same rate. Documents the prep-to-park invariant on maybe_preempt — library code that registers a wakeup and then parks must keep that window alloc-free and check-free, or a preemption-driven yield in the middle would lose the wakeup.