Opt-out of `PyCell`

[davidhewitt]

Like we have #[pyclass(unsendable)] to opt-out of Send, many users will not want their pyclass to be mutable. It would be nice if we could allow them to opt-out of having PyCell as the backend, so that they don't have to deal with the performance cost.

[davidhewitt]

Perhaps this opt-out could be #[pyclass(immutable)] ?

Though TBH I think I prefer immutable-by-default, so perhaps we should have #[pyclass(mutable)] to opt-in to PyCell!

[gilescope]

Could we couch this in terms of exclusive and non-exclusive access rather than mutability?

[davidhewitt]

First step of this in #1979 has now been merged. A future refinement will be to make mutability (and accompanying overhead) opt-in.

[davidhewitt]

I'm going to close this in favour of #4369, which I believe we plan to do soon as part of the improvements targeting freethreading.