StreamExt::scan is barely useful due to lifetime issue

[jerry73204]

Continuing #2046, the lifetime issue is still a headache, that the async block basically cannot access the state when it's executed. Currently, I stick to futures::stream::unfold workaround to bind streams with states.. IMO, the future block of scan could own the state and returns to stream just like what unfold did . Perhaps we can make this change, or are there any other considerations for this design?

[ivormetcalf]

I just ran into this problem and wholeheartedly agree. Something like:

fn scan<S, B, Fut, F>(self, initial_state: S, f: F) -> Scan<Self, S, Fut, F>
where
    F: FnMut(S, Self::Item) -> Fut,
    Fut: Future<Output = Option<(B, S)>>, 

might perhaps be a better design.

[jerry73204]

@ivormetcalf you can use code block quote for better readability.

[hjfreyer]

Does anyone know of a library that implements @ivormetcalf 's proposed signature?