Macro once 

macro_rules! once {
    ($e:expr) => { ... };
}

Available on crate feature tasks only.

Runs the given closure exactly once per call site.

Each invocation expands to its own static Once, so two once! calls in the same function are independent. Useful for one-time thread setup (priority, affinity) on threads that may be re-entered (e.g. tokio blocking pool).