pub fn peek(&self) -> Option<&Fut::Output> {
        if let Some(inner) = self.inner.as_ref() {
            match inner.notifier.state.load(SeqCst) {
                COMPLETE => unsafe { return Some(inner.output()) },
                POISONED => panic!("inner future panicked during poll"),
                _ => {}
            }
        }
        None
    }
Returns [`Some`] containing a reference to this [`Shared`]'s output if
it has already been computed by a clone or [`None`] if it hasn't been
computed yet or this [`Shared`] already returned its output from
[`poll`](Future::poll).
futures-util/src/future/future/shared.rs