Enum Evictability 

pub enum Evictability {
    Always,
    Expensive,
    Never,
}

Eviction policy for a ValueType.

Orthogonal to ValueTypePersistence: a cell can be both persistable and non-evictable (e.g. DiskFileSystem, which holds session-scoped file watchers but whose serializable fields can still round-trip the cache).

Variants

Always

Cells of this type can be freely dropped from in-memory storage on the eviction sweep. The next reader either restores from disk (Persistable), recomputes from the producing task’s inputs (Skip / HashOnly), or — for HashOnly — short-circuits via the stored content_hash if the value is unchanged. Default when evict is omitted.

Expensive

Cells are evictable, but re-deriving them is non-trivial (e.g. WASM compile, spawning a Node process pool). Eviction policy may prefer evicting cheaper cells first. Maps to evict = "last".

Never

Cells must stay in memory across the session. Required for value types holding interior-mutable state (State<> cells, Arc<Mutex<_>> dedup histories) or session-scoped handles (file watchers, worker pools, plugin DSOs) — re-running the producing task would lose the accumulated state. Maps to evict = "never".