From: Ralf Jung Date: Mon, 11 Jan 2016 13:41:58 +0000 (+0100) Subject: stress privacy a little more X-Git-Url: https://git.ralfj.de/web.git/commitdiff_plain/577f74665aa17da5e15692f593502ae9d5c957cf?ds=sidebyside;hp=a81aa96fb6fb24f7614f6bbdfaff511d590b5576 stress privacy a little more --- 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 995cb61..403d6b3 100644 --- a/personal/_posts/2016-01-09-the-scope-of-unsafe.md +++ b/personal/_posts/2016-01-09-the-scope-of-unsafe.md @@ -130,9 +130,10 @@ Or, to put it slightly differently: If the scope of `unsafe` grows beyond the sy 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 @@ -143,7 +144,8 @@ This nicely brings us to another important point, which I can only glimpse at he 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.