Type Alias DeferredSink 

pub type DeferredSink = Box<dyn FnOnce(u32) -> Result<Box<dyn Sink>, String> + Send>;

A sink built at start, on the worker that owns its task, knowing its task id — for a sink that touches a shared resource (a durable file sink’s directory) and must be opened by its owner only, and one that acks its barriers from another thread under its own id. A builder that fails fails the start before any task runs, exactly as a deferred operator’s does.

Aliased Type

pub struct DeferredSink(/* private fields */);