Function generator 

pub fn generator<F, Resume, Yield, Return>(
    f: F,
) -> impl Generator<Resume, Yield = Yield, Return = Return>
where
    for<'a> F: FnOnceOutput<Yielder<'a, Yield, Resume>, Out: Future<Output = Return>>,

To build generator, pass an async function with a parameter: First parameter is Yielder – context handle to yield generated item