From: Ralf Jung Date: Mon, 11 Jan 2016 14:18:14 +0000 (+0100) Subject: Argue more why `evil` is the problem X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/aace0ce10b5f3b7cff0440393134cb95359836e9?ds=sidebyside;hp=ba2d965151a9e8312f4538380392e5b82e52fffd Argue more why `evil` is the problem --- diff --git a/ralf/_posts/2016-01-09-the-scope-of-unsafe.md b/ralf/_posts/2016-01-09-the-scope-of-unsafe.md index 403d6b3..757bd73 100644 --- a/ralf/_posts/2016-01-09-the-scope-of-unsafe.md +++ b/ralf/_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. + ## An Example @@ -60,6 +62,12 @@ 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.