Function generator 

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

To build generator, pass an async function with two parameters: First parameter is Yielder – context handle to yield generated item The second parameter – it’s a Resume value passed to the Generator::resume() method at the first time