Feature request: Add OnceCell::try_unwrap()

[manuthambi]

Add a method which consumes OnceCell and returns the element if it exists. The method signature can be very similar to Arc::try_unwrap().

pub fn try_unwrap(self) -> Result<T, OnceCell<T>>

[manuthambi]

On second thought, there isn't much point in returning an uninitialized OnceCell if the unwrap fails. So the signature could be: pub fn try_unwrap(self) -> Option<T>

[matklad]

sgmt, though, I think the name and signature should be, to map RefCell:

pub fn into_inner(self) -> Option<T>

[manuthambi]

into_inner sounds better, and it is better to be consistent with RefCell.

I haven't contributed to Rust before, but do you want me to send you a pull request?

[matklad]

Sure, that would be awesome! Here's a checklist of things to do:

• [ ] add the method to the unsync variant
• [ ] add the method to both versions of the sync variant
• [ ] write a short docstring for them, with a doc test
• [ ] write a simple smoke test