It turns out that the underlying reason for this observation is also a nice illustration for the concept of *semantic types* that comes up in my [work on formalizing Rust]({{ site.baseurl }}{% post_url 2015-10-12-formalizing-rust %}) (or rather, its type system).
Finally, this discussion will once again lead us to realize that we rely on our type systems to provide much more than just type safety.
+**Update (Jan 11th):** Clarified the role of privacy; argued why `evil` is the problem.
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## An Example
So, this example clearly shows that to evaluate the safety of types like `Vec`, we have to look at *every single function* provided by that data structure, even if it does not contain any `unsafe` code.
-## The reason why
+## The Reason Why
Why is it the case that a safe function can break `Vec`?
How can we even say that it is the safe function which is at fault, rather than some piece of `unsafe` code elsewhere in `Vec`?
The function `evil` above violates this invariant, while all the functions actually provided by `Vec` (including the ones that are implemented unsafely) preserve the invariant.
That's why `evil` is the bad guy. (The name kind of gave it away, didn't it?)
+Some will disagree here and say: "Wait, but there is some `unsafe` code in `Vec`, and without that `unsafe` code `evil` would be all right, so isn't the problem actually that `unsafe` code?"
+This observation is correct, however I don't think this position is useful in practice.
+`Vec` with `evil` clearly is a faulty data structure, and to fix the bug, we would remove `evil`.
+We would never even think about changing the `unsafe` code such that `evil` would be okay, that would defeat the entire purpose of `Vec`.
+In that sense, it is `evil` which is the problem, and not the `unsafe` code.
+
This may seem obvious in hindsight (and it is also [discussed in the Rustonomicon](https://doc.rust-lang.org/nightly/nomicon/working-with-unsafe.html)), but I think it is actually fairly subtle.
There used to be claims on the interwebs that "if a Rust program crashes, the bug must be in some `unsafe` block". (And there probably still are.)
Even academic researchers working on Rust got this wrong, arguing that in order to detect bugs in data structures like `Vec` it suffices to check functions involving unsafe code.
That's why I think it's worth dedicating an entire blog post to this point.
But we are not done yet, we can actually use this observation to learn more about types and type systems.
-## The semantic perspective
+## The Semantic Perspective
There is another way to phrase the intuition of types having additional invariants:
The difference between `Vec` and `MyType` is no less significant than the difference between `f32` and `u32`.
It's just that the compiler has been specifically taught about `f32`, while it doesn't know enough about the semantics of `Vec`.
-## The actual scope of unsafe
+## The Actual Scope of Unsafe
At this point, you may be slightly worried about the safety of the Rust ecosystem.
I just spent two sections arguing that the Rust compiler actually doesn't know what it is doing when it comes to checking functions that work on `Vec`.
Does it sprawl through all our code, silently infecting everything we write -- or is there some limit to its effect?
As you probably imagined, of course there *is* a limit. Rust would not be a useful language otherwise.
-The scope of `unsafe` ends at the next *abstraction boundary*.
-This means that everything outside of the `std::vec` module does not have to worry about `Vec`.
-Due to the privacy rules enforced by the compiler, code outside of that module cannot access the private fields of `Vec`, and hence it cannot tell the difference between the syntactic appearance of `Vec` and its actual, semantic meaning.
+*If* all your additional invariants are about *private* fields of your data structure, then the scope of `unsafe` ends at the next *abstraction boundary*.
+This means that everything outside of the `std::vec` module does not have to worry about `Vec`.
+Due to the privacy rules enforced by the compiler, code outside of that module cannot access the private fields of `Vec`.
+That code does not have a chance to violate the additional invariants of `Vec` -- it cannot tell the difference between the syntactic appearance of `Vec` and its actual, semantic meaning.
Of course, this also means that *everything* inside `std::vec` is potentially dangerous and needs to be proven to respect the semantics of `Vec`.
## Abstraction Safety
If the type system of Rust lacked a mechanism to establish abstraction (i.e., if there were no private fields), type safety would not be affected.
However, it would be very dangerous to write a type like `Vec` that has a semantic meaning beyond its syntactic appearance.
-Since users of `Vec` can accidentally perform invalid operations, there would actually be *no bound to the scope of `unsafe`*.
+All code could perform invalid operations like `Vec::evil`, operations that rely on the assumption that `Vec` is just like `MyType`.
+There would actually be *no bound to the scope of `unsafe`*.
To formally establish safety, one would have to literally go over the entire program and prove that it doesn't misuse `Vec`.
The safety promise of Rust would be pretty much useless.
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