This post floats a variation of boats’ UnpinCell proposal that I’m calling MinPin.¹ MinPin’s goal is to integrate Pin into the language in a “minimally disruptive” way² – and in particular a way that is fully backwards compatible. Unlike Overwrite, MinPin does not attempt to make Pin and &mut “play nicely” together. It does however leave the door open to add Overwrite in the future, and I think helps to clarify the positives and negatives that Overwrite would bring.

In July, boats presented a compelling vision in their post pinned places. With the Overwrite trait that I introduced in my previous post, however, I think we can get somewhere even more compelling, albeit at the cost of a tricky transition. As I will argue in this post, the Overwrite trait effectively becomes a better version of the existing Unpin trait, one that effects not only pinned references but also regular &mut references. Through this it’s able to make Pin fit much more seamlessly with the rest of Rust.

What would you say if I told you that it was possible to (a) eliminate a lot of “inter-method borrow conflicts” without introducing something like view types and (b) make pinning easier even than boats’s pinned places proposal, all without needing pinned fields or even a pinned keyword? You’d probably say “Sounds great… what’s the catch?” The catch it requires us to change Rust’s fundamental assumption that, given x: &mut T, you can always overwrite *x by doing *x = /* new value */, for any type T: Sized. This kind of change is tricky, but not impossible, to do over an edition.