write way too long post on the unsafe code model

[web.git] / personal / _posts / 2016-01-09-the-scope-of-unsafe.md

diff --git a/personal/_posts/2016-01-09-the-scope-of-unsafe.md b/personal/_posts/2016-01-09-the-scope-of-unsafe.md
index 403d6b3eab6c6e0f7e477e3a1998666d721334ad..59a318b3ac1226b3fdcc1bbd907b893b4396f92d 100644 (file)
--- a/personal/_posts/2016-01-09-the-scope-of-unsafe.md
+++ b/personal/_posts/2016-01-09-the-scope-of-unsafe.md
@@ -14,6 +14,8 @@ What I am saying is that the scope of `unsafe` is larger than the `unsafe` block
 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.
+
 <!-- MORE -->
 
 ## An Example
@@ -49,7 +51,7 @@ Oops!
 
 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`?
@@ -60,13 +62,19 @@ More precisely speaking, `ptr` points to an array of type `T` and size `cap`, of
 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:
 
@@ -121,7 +129,7 @@ This is just as bad as a function assigning `*f = 42u32` to a `f: &mut f32`.
 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`.