Add a way to resize StackFutures

It'd be helpful to have a way to shrink (or less commonly, grow) a StackFuture. For example, sometimes you have an object that implements a trait and needs to forward calls to another object that implements the same trait. Right now that's impossible with StackFuture without boxing the child future because otherwise you'd have to have a future contain a future that's the same size as itself. That said, there will usually be some extra space so we might be able to dynamically shrink a StackFuture.

The signature would probably be something like:

fn resize<const NEW_SIZE: usize>(self) -> Result<StackFuture<T, NEW_SIZE>, Self>;

Then using this would look something like:

impl Foo for MyObject {
    fn bar(&self) -> StackFuture<(), 1000> {
        match self.sub_object.bar().resize::<{500}>() {
            Ok(f) => f.await,
            Err(original) => Box::pin(original).await,
        }
    }
}

The idea here is we'd try to fit the future into a smaller container, but if it doesn't fit then the resize returns the original future and we can either decide to pay the allocation cost (as we did in this example) or give up.

Commencez par lire l’implémentation de StackFuture et les tests existants du dépôt, puis suivez la manière dont sa taille de stockage et son ownership sont représentés. Évaluez l’API resize proposée, y compris les chemins de réussite et d’erreur de la future d’origine ; le travail est considéré comme terminé lorsqu’il est possible d’essayer en toute sécurité avec un StackFuture plus petit ou plus grand sans boxing, sauf si le resizing échoue.