replace unfold with from_fn

[cramertj]

a la https://doc.rust-lang.org/std/iter/fn.from_fn.html

[cramertj]

aaaand unfortunately this doesn't work out today because the returned async body can't reference upvars from the closure passed in, so we still need unfold. :(

[danielhenrymantilla]

FWIW, we can finally implement this on 1.85.0 onwards 🙂, courtesy of AsyncFnMut() -> Option<Item>, whose lifetime semantics are exactly what we need here

[cramertj]

@danielhenrymantilla Unfortunately, I believe it's still not possible to spell this API without return type notation or type alias impl trait.

Without these features, there's no way to spell this struct:

pub struct FromFn<'a, Item, F: AsyncFnMut() -> Option<Item>> {
    func: F,
    ongoing_future: Option<F::CallRefFuture<'a>>, // `CallRefFuture` aka `F::async_call_mut(...)` isn't stable
}

[danielhenrymantilla]

Do we necessarily need it to be named? The closure param inside it will remain hard-to-name, so I'd imagine we can bite the bullet and just involve an -> impl Stream API:

use ::futures::stream::{self, Stream};

fn from_fn<Item>(
    f: impl AsyncFnMut() -> Option<Item>,
) -> impl Stream<Item = Item> {
    stream::unfold(f, |mut f| async move {
        Some((f().await?, f))
    })
}