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For example, with this >> version you could stash a Cell> (even >> with Gary's suggested Core<'_> change) and then observe that reference >> on Drop of Data, which seems unsound to me. > > The Core<'_> change seems unrelated, Core is not Sync, so you can't store= it in > the first place. > > Otherwise, I don't see how one can exploit this. The formal invariance of > Cell> is not practically exploitable be= cause > the framework controls what lifetimes are available and everything reacha= ble > through 'bound outlives the data; something that is shorter lived than 'b= ound > can't be stored either. The current code is problematic as the `unbind` callback have `Pin<&'bound Self::Data<'bound>>`. I think we've discussed this last week a= nd the conclusion is that this isn't sound, and the callback should have `Pin<&Self::Data<'bound>>` instead (the outer lifetime should be callback-scoped)`. Did you forget to make the change? > > Bringing back ForLt for this reason seems undesirable, as it has a real d= esign > cost. If we have the `#[variance_check]` macro (like I mentioned in another threa= d), we could use that to check instead, but it'll still be unsound if lifetime = of driver data escapes to devres due to destruction order. Best, Gary