Function begin_phase 

pub fn begin_phase()

Activates the arena and resets every thread’s slab. All allocations until the next end_phase() go to the arena; the previous phase’s data is overwritten in place.

Phases must not nest

Calling begin_phase() while another phase is already active panics. The arena is a flat lifetime — nested phases were previously tolerated via a depth counter, but the depth counter masked correctness bugs (panics orphaning the count, accidental double-begin recycling the outer phase’s slab on the next allocation). The contract is now: every begin_phase() is paired with one end_phase() (or use PhaseGuard / phase for panic-safe pairing), and no second begin_phase() is reachable from within an active phase.

Retention is unsafe

Allocations made during phase N that are still held when phase N+1 begins are silently overwritten by phase N+1’s first allocations at the same slab offset. Any of the following held across begin_phase() will be corrupted:

• Vec<T> with capacity ≥ min_arena_bytes() (push triggers realloc that copies from now-recycled source memory).
• Arc<T> / Rc<T> with payload ≥ min_arena_bytes() (refcount fields become arbitrary bytes — silent leak or use-after-free).
• HashMap, BTreeMap, etc. with bucket allocation ≥ min_arena_bytes() (lookup may infinite-loop on corrupted ctrl bytes).
• Box<dyn Trait> with backing data ≥ min_arena_bytes() (vtable dispatch survives but field reads return filler bytes).

To preserve data across phases, clone() it into a System-backed copy (e.g., wrap in Box::leak(Box::new(...)) while ARENA_ACTIVE is false, or copy into a Vec allocated outside any phase).