Function last_with_default 

pub fn last_with_default(
    core: &Core,
    source: NodeId,
    default: HandleId,
) -> Result<FlowRegistration, OperatorFactoryError>

last_with_default(source, default) — buffers the latest DATA; emits Data(latest) then Complete on upstream COMPLETE. On empty stream (no DATA arrived), emits Data(default) then Complete.

Core takes one retain on default for the LastState’s lifetime; the caller should retain a share for themselves if they want to reference the handle elsewhere.

Errors

• OperatorFactoryError::ZeroDefault — default == NO_HANDLE. Use last for the no-default behavior instead.
• OperatorFactoryError::Register — Core-layer registration error (unknown / terminal-non-resubscribable source).