[zozbot234]

That's why Cell<T> and RefCell<T> are a thing. Both allow you to mutate shared references, but disable shared access across threads. The qcell crate even includes a version of Cell/RefCell that's "branded" by a region/lifetime, just like in this proposal.

[codedokode]

As I remember, Cell only allows moving/copying data from/to cell so if you have a 128-byte object inside do you have to copy it to modify? Or this can be optimized?

[kbolino]

Yes, that's how Cell works. If you want to work with the data in place, you need a RefCell instead.

[codedokode]

But it is expensive, because it does run-time checks? Or they are optimized out?

[Measter]

RefCell does do runtime checks, but the cost is checking the counter, a conditional branch, then incrementing/decrementing the counter twice.

Because the counter is non-atomic and non-volatile the optimiser can sometimes optimise out the actual modification of the counter. It's not free, but it's not also not a huge expense.