Function service_fn 

pub fn service_fn<C, F, Fut, O>(name: &'static str, run: F) -> FnService<C, F>
where
    C: Clone + Send + Sync + 'static,
    F: Fn(Context<C>) -> Fut + Send + Sync + 'static,
    Fut: Future<Output = O> + Send + 'static,
    O: IntoServiceOutcome + Send + 'static,

Wraps an async function as a SupervisedService.

This keeps one-off services ergonomic without widening the core trait surface or forcing callers into a monolithic service object pattern.

The resulting service can be registered with SupervisorBuilder::add or SupervisorBuilder::add_with_options.

The closure’s Context parameter determines the extracted service context. With a stateful builder, that usually means:

• Context<AppState> when the service wants the full root state
• Context<Subset> when Subset: FromSupervisorState<AppState>