The way to do this while avoiding race conditions seems to be `Arc::into_inner` or `Arc::get_mut`; for instance, the docs for `Arc::try_unwrap` mention a possible race condition, and recommend using `Arc::into_inner` to avoid it: https://doc.rust-lang.org/std/sync/struct.Arc.html#method.tr...
This made me think of the `im` library[0] which provides some immutable/copy on write collections. The docs make it seem like they do some optimizations when they determine there is only one owner:
> Most crucially, if you never clone the data structure, the data inside it is also never cloned, and in this case it acts just like a mutable data structure, with minimal performance differences (but still non-zero, as we still have to check for shared nodes).
I hope this isn't prone to a similar race condition!
[0] https://docs.rs/im/15.1.0/im/index.html