That's a good question!
It seems perfectly fine to change `insert` to take `&Pin<Entry<T>>`.
I can't come up with any counter-example.
-However, the formal model also cannot justify this variant of `insert`: As we defined previously, `Pin<'a, T>.shr` just uses `T.shr`.
+However, the formal model also cannot justify this variant of `insert`: Our model as defind previously defines `Pin<'a, T>.shr` in terms of `T.shr`.
That made it easy to justify [`Pin::deref`](https://doc.rust-lang.org/nightly/std/mem/struct.Pin.html#method.deref).
-However, it also means `&Pin<T>` and `&&T` *are the same type*.
-The two invariants are equivalent.
+However, as a consequence, `Pin<'a, T>.shr` and `(&'a T).shr` are literally the same invariant, and hence `&Pin<T>` and `&&T` *are the same type*.
We could literally write functions transmuting between the two, and we could justify them in my model.
Clearly, `insert` taking `entry: &&T` would be unsound as nothing stops the entry from being moved later:
{% highlight rust %}
{% endhighlight %}
This shows that the version of `insert` that takes a shared reference cannot be sound in the model.
-I do have an idea for how to solve this problem: Introduce a *fourth* "mode" or typestate, the "shared pinned" state, with an accompanying invariant.
+Notice that this is a consequence of a choice I made when building the model, namely the choice to define `Pin<'a, T>.shr` in terms of `T.shr`.
+This does *not* show that `&Pin<T>` and `&&T` have to be the same type given the public API and the contract they provide.
+Different choices in the model could lead to a different situation.
+The problem is, how else *could* we define `Pin<'a, T>.shr`, if we do not want to use `T.shr`?
+What *is* the invariant of a shared reference to a pinned reference?
+
+I do have an idea for how to answer this question: Introduce a *fourth* "mode" or typestate, the "shared pinned" state, with an accompanying invariant.
However, I previously argued strongly against introducing such a fourth state, on the grounds that three typestates is already complicated enough.
In fact, an earlier version of the `Pin` API used to have two kinds of pinned references (shared and mutable) reflecting the two distinct "shared pinned" and "(owned) pinned" typestates.
The shared variant got subsequently removed, and I feel somewhat guilty about that now because I strongly pushed for it.
That just seems like the worst of both worlds.
However, now we already have a [version of the futures crate](https://aturon.github.io/2018/04/06/futures2/) using the revised `Pin`, so I don't know if changing it again is realistic. :/
+(**Update:** Seems like [there may be breaking changes in future future versions anyway](https://www.reddit.com/r/rust/comments/8ac85w/futures_02_is_here/dwxkhvl/), so maybe the ship has not yet sailed after all. **/Update**)
I feel bad about that. Can we still fix this before everything gets stabilized?
Others [have](https://github.com/rust-lang/rfcs/pull/2349#issuecomment-373206171) [argued](https://github.com/rust-lang/rfcs/pull/2349#issuecomment-378555691) for a shared pinned reference after it got removed from the API, and I argued against them as I did not understand how it could be useful.
Thanks for not being convinced by me!
I spelled out the details [in the RFC issue](https://github.com/rust-lang/rfcs/pull/2349#issuecomment-372109981).
So, if we want to declare that shared pinning is a typestate in its own right---which overall seems desirable---do we want it to be restricted like this due to an implementation detail of arbitrary self types?
-**Update:** @Diggsey [points out](https://github.com/rust-lang/rfcs/pull/2349#issuecomment-379230538) that we can still have a `PinRefCell` with a method like `fn get_pin(self: Pin<PinRefCell<T>>) -> Pin<T>`, as long as the `PinRefCell` never gives out mutable references. So it turns out that combining interior mutability and pinning should work fine, but having both pinned and unpinned interior mutability in the same type is where things start to go wrong due to `Pin::deref`. **/Update**
+**Update:** @Diggsey [points out](https://github.com/rust-lang/rfcs/pull/2349#issuecomment-379230538) that we can still have a `PinRefCell` with a method like `fn get_pin(self: Pin<PinRefCell<T>>) -> Pin<T>`, as long as the `PinRefCell` never gives out mutable references. So it turns out that combining interior mutability and pinning should work fine. Later, @glaebhoerl suggested we can even [combine `RefCell` and `PinRefCell` into one type if we dynamically track the pinning state](https://internals.rust-lang.org/t/safe-intrusive-collections-with-pinning/7281/11?u=ralfjung). **/Update**
## 4 Conclusion
I am impressed by the creativity that went into coming up with these APIs, and looking forward to analyzing more of them in the future.
The situation around shared pinning is still open, and it seems we need to have a discussion about what model we ultimately want to adopt---which code we ultimately want to be legal---and whether we want to change the `Pin` types before stabilization.
-Unfortunately I am four days late in my race against the [first significant user of this API](https://aturon.github.io/2018/04/06/futures2/).
Anyway, as usual, please [let me know what you think](https://internals.rust-lang.org/t/safe-intrusive-collections-with-pinning/7281)!
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