Change StreamExt::scan to pass state to closure by value

[taiki-e]

Fixes #2171

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

Related: #1609 (comment), #2254 (comment)

[taiki-e]

FYI @cramertj @olegnn

[taiki-e]

    Fut: Future<Output = Option<(S, B)>>,

Hmm, it seems this (state, item) is the reverse order of the tuples returned by unfold's closure (item, state). However, given the order of arguments of scan/forld's closure (state, item), I feel this is reasonable.

[benluelo]

Are there plans to cut a new release containing this change? Considering it was over 2 years go, I'm quite surprised it's still unreleased: https://docs.rs/futures/latest/futures/stream/trait.StreamExt.html#method.scan

[taiki-e]

This is a breaking change. Unfortunately, breaking releases of such widely used crates cannot be released often, as they have a significant impact on the ecosystem. This will be released once many (some?) other issues included in the milestone are resolved, so please consider helping to resolve the other issues included in the milestone.