Constant CLASS_CAPACITY_BYTES 

const CLASS_CAPACITY_BYTES: usize = _; // 1_099_511_627_776usize

Available on Unix and crate feature pool only.

Virtual reservation per size class. Purely virtual: physical memory materializes only for slots in use, and slots are scoped to residency, so this must exceed the largest plausible resident set per class, the budget plus in-flight slack, not the backlog. It is deliberately enormous (address space costs nothing, and touched pages are bounded by peak residency) so that no realistic budget, on any machine size, reaches the heap-fallback path.