Type Alias IoTask 

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

A unit of I/O work handed from a background I/O thread to the main-thread event loop. It is a boxed closure so host.rs stays agnostic of net/http: the I/O thread captures only plain Send data (bytes, ids, TcpStreams) and the closure runs the JS-touching dispatch on the main thread (where the thread-local host lives). I/O threads NEVER touch the host directly.

Aliased Type

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