Struggling to make lifetimes happy for async with temporary borrow of other data during build

[vlovich]

I'm trying to do something that looks something like this:

#[self_referencing]
struct MyStructInner {
      owned: SomeStruct,
      #[borrows(owned)]
      #[not_covariant]
      referencing: SomeStructRef<'this>,
}

struct MyStruct(MyStructInner);
  
impl MyStruct {
      async fn new(owned: SomeStruct, helper: &SomeOtherStruct) -> Result<Self> {
            Ok(MyStruct(
               MyStructInnerAsyncTryBuilder {
                  owned,
                  referencing_builder: move |owned| Box::pin(async move { SomeStructRef::try_new(owned, helper).await? } )
               }.try_build().await?
            ))
      }
} 

This throws a compiler error saying that the lifetime of helper may outlive the future because of the pinned box. I typed this in GitHub's text editor so apologies if there's bugs in the above code (tried to distill down my actual code).

Is this a scenario that ouroboros doesn't handle well yet? I'm a little surprised the borrow lifetime is thought to extend past the await of the build.

[someguynamedjosh]

Unfortunately ouroboros internally requires that the future returned by the builder lives for at least the 'this lifetime. I went in and tried playing around with it to get it to accept any other lifetime bound, but it would always throw borrow check errors on simple test cases. If you would like to try looking for a possible solution yourself, the code ouroboros generates for constructing asynchronously looks like this:

async fn new_async(
    data: i32,
    dref_builder: impl for<'this> ::core::ops::FnOnce(
        &'this i32,
    ) -> ::core::pin::Pin<
        ::ouroboros::macro_help::alloc::boxed::Box<
            dyn ::core::future::Future<Output = &'this i32> + 'this,
        >,
    >,
) -> () {
    todo!()
}

A simple test case which breaks if I do things like replace + 'this with some lifetime parameter on the async fn itself is as follows:

async fn example() {
    new_async(12, |data| Box::pin(async move { data })).await;
}