Module shared 

Cross-isolate shared mutable state: SharedValue, SharedAtom (Phase B3).

The isolate model is share-nothing for GC-heap data. But some state genuinely needs to be visible across isolates — global configuration, shared counters, published results. Phase B3 adds an explicit, honest escape hatch: shared-atom.

Two-tier mutable references (per the ADR)

Primitive	Backing	Send?	Use case
atom	GcPtr<Atom> (Mutex<Value>)	!Send	isolate-local, fast
shared-atom	Arc<SharedAtom> (ArcSwap<SharedValue>)	✓	cross-isolate, lock-free CAS

A value stored in a shared-atom must be promotable — it must be representable as a SharedValue. The promotion cost is paid once on publish; reads (deref) are an atomic load.

SharedValue

Covers only the “plain data” subset of Value:

• Scalars (stored inline, no allocation)
• Strings (Arc<str>, immutable, refcounted)
• Keywords / symbols (StaticGcPtr<T>, interned, program-lifetime)
• Large byte buffers (Arc<[u8]>, the BEAM off-heap-binary trick)

Closures, native resources, and isolate-bound GC objects are not promotable. This restriction is enforced at publish time via promote.

Structs

PromoteError

Returned when a Value cannot be promoted to SharedValue.

SharedAtom

A cross-isolate mutable reference backed by a lock-free ArcSwap.

Enums

SharedValue

A Send + Sync value representation for cross-isolate sharing.

Functions

demote

Demote a SharedValue back into an isolate-local Value.

promote

Promote a Value to a SharedValue for cross-isolate publishing.