Support async builders?

[fzyzcjy]

Hi thanks for the library! I do hope the builder function can support async functions.

Context: I am considering using this package to implement returning borrowed types in https://github.com/fzyzcjy/flutter_rust_bridge. Thus the builder can be arbitrary async code.

[Voultapher]

Hi, thanks for reaching out. Do I understand correctly, you have a dependent construction logic that involves async calls, right?

If possible, I'd like to avoid the async coloring issue of having multiple variants of all construction functions. There are also some question about variance across await points, where I'm not sure what a sound API would look like.

Would a spawn_blocking work for your use-case?

[fzyzcjy]

Hi, thanks for the reply! Ok I see...

Would a spawn_blocking work for your use-case?

Well no, because https://github.com/fzyzcjy/flutter_rust_bridge needs to support arbitrary user function. If we require users to do spawn_blocking, then indeed we lose the benefits of async (e.g. not taking a whole thread).

[Voultapher]

Could you please outline specifically what kind of API you would need.

[fzyzcjy]

Sure. For example, the README example says:

impl NewStructName {
    fn new(
        owner: Owner,
        dependent_builder: impl for<'a> FnOnce(&'a Owner) -> Dependent<'a>
    ) -> NewStructName { ... }

but now it would be great to be

impl NewStructName {
    fn new(
        owner: Owner,
        dependent_builder: impl for<'a> FnOnce(&'a Owner) -> Pin<Box<dyn Future<Output = Dependent<'a>>>> // <- support passing in async function. not necessarily have API like this.
    ) -> NewStructName { ... }

Indeed just something like https://docs.rs/ouroboros_examples/latest/ouroboros_examples/ouroboros_impl_chain/struct.Chain.html#method.new_async

[Voultapher]

It would need to async fn new( right?

[Voultapher]

Also I wonder if a minimal solution that only duplicates the new constructor is enough for a first version until people ask for more.

[Voultapher]

@steffahn what's you take, do we run into any variance/soundness issues with such an API?

[fzyzcjy]

It would need to async fn new( right?

Oh yes!

Also I wonder if a minimal solution that only duplicates the new constructor is enough for a first version until people ask for more.

Sorry not quite get it...

Btw, no worries about this issue, because if https://github.com/Voultapher/self_cell/issues/58 cannot be solved, then I unfortunately cannot use this package :( Though I think it is great and abstracts out unsafe things with well tested code!

[steffahn]

IIRC ouroboros has async construction APIs for a while already, so if we'd just mirror those, it might be sound (well.. or both are unsound). I haven't looked at the details myself yet.