forum: https://internals.rust-lang.org/t/sharing-for-a-lifetime/6675
---
-This post is about an aspect of the [RustBelt paper]({{ site.baseurl }}{% post_url 2017-07-08-rustbelt %}).
+This post is about an aspect of the [RustBelt paper]({% post_url 2017-07-08-rustbelt %}).
Concretely, I'd like to share some of our thoughts on the nature of types and shared references.
Let's see how well this goes. :)
-Shared references are an extremely powerful mechanism in the Rust type system, and we've had quite some trouble finding a good way of handling them in our [formal model]({{ site.baseurl }}{% post_url 2015-10-12-formalizing-rust %}).
+Shared references are an extremely powerful mechanism in the Rust type system, and we've had quite some trouble finding a good way of handling them in our [formal model]({% post_url 2015-10-12-formalizing-rust %}).
In this post, I will present the model that we came up with.
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Let's get started by discussing what a "model" of a type looks like, and what's so hard about handling shared references.
### What's in a Type?
-I've already [written]({{ site.baseurl }}{% post_url 2015-10-12-formalizing-rust %}) [about]({{ site.baseurl }}{% post_url 2016-01-09-the-scope-of-unsafe %}) the idea of "semantic types", which essentially boils down to the concept that types can impose *additional invariants* on their fields, like `self.cap >= self.len` (for `Vec`) or `!is_zero(self.0)` (for `NonZero`).
+I've already [written]({% post_url 2015-10-12-formalizing-rust %}) [about]({% post_url 2016-01-09-the-scope-of-unsafe %}) the idea of "semantic types", which essentially boils down to the concept that types can impose *additional invariants* on their fields, like `self.cap >= self.len` (for `Vec`) or `!is_zero(self.0)` (for `NonZero`).
So, whenever I say "(semantic) type", I really mean "a type's (syntactic) definition (aka the data layout) **and** the additional invariants that the module imposes on this type".
Two types may be syntactically equal but have different invariants, and then clearly casting from one to the other would be horribly unsafe -- so, these would be completely different types despite their syntactic similarity.
What really matters for a type is the answer to the question: *Given some sequence of bytes, is this sequence a valid inhabitant of the type?*[^1]
Either way, this model has certainly helped me not only in the formal work on RustBelt, but also in thinking about the Rust type system itself.
For example, I briefly wondered why `Cell` is not covariant.
-Indeed, if `U <: T`, then it is perfectly safe to turn a `Cell<T>` into a `Cell<U>` -- that is to say, whenever some sequence of bytes is valid at `Cell<T>`, it is also valid at `Cell<U>`.
+Indeed, if `T <: U`, then it is perfectly safe to turn a `Cell<T>` into a `Cell<U>` -- that is to say, whenever some sequence of bytes is valid at `Cell<T>`, it is also valid at `Cell<U>`.
(In fact, the owned invariant for `Cell<T>` is *exactly the same* as the one for `T`!)
That's precisely what we need for covariance, right?
Well, not exactly -- because our types have two invariants, we also have to require that whenever a pointer is a shared `Cell<T>`, it is a shared `Cell<U>`, and that is decisively *not* true!
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