Support splitting into multiple objects?

[fzyzcjy]

Hi thanks for the helpful package!

When implementing https://github.com/fzyzcjy/flutter_rust_bridge, I am considering the following scenario. Firstly a simple one:

struct One { .. }
struct Two<'a> { .. }
fn compute(one: &'a One) -> Two<'a> { .. }

I have a Arc<One> as input, and want to provide something as if Arc<Two> as output, but it should have no lifetime specifier by putting Arc<One> inside it.

// pseudo code for both self_cell and ouroboros
struct TwoWithOwner {
  one: Arc<One>,
  two: Two<'this>,
}

let input: Arc<One> = ...;
let output: Arc<TwoWithOwner> = Arc::new(TwoWithOwnerBuilder {
  one: input,
  two_builder: |one| compute(one),
}.build());

Now comes the problem. Suppose the return type is a Vec:

fn compute(one: &'a One) -> Vec<Two<'a>> { .. }

And for the same input, I hope to provide Vec<Arc<Two>> as output (again without lifetimes).
If using the exact same code, I will only get something like:

struct VecTwoWithOwner {
  one: Arc<One>,
  two: Vec<Two<'this>>,
}

In other words, an Arc<VecTwoWithOwner> object, instead of Vec<Arc<TwoWithOwner>>.

It would be great if there are some API that can split an Arc<VecTwoWithOwner> object into multiple Vec<Arc<TwoWithOwner>> objects. From my naive thoughts, maybe there can be something like a transform function. Or, there may be better solutions for the scenario above.

Cross-posted in: Voultapher/self_cell#58