Comment by steveklabnik
1 day ago
It’s a ten year old bug because it has never been found in the wild, ever, in those ten years. Low impact, high implementation effort bugs take less priority than bugs that affect real users.
The project is adopting Ferrocene for the spec.
Ferrocene is intended to document the behavior of the current version of the rustc compiler, so it's just an effort to formalize "the compiler is the language".
Yes, the soundness hole itself is low impact and doesn't need to be prioritized but it undermines the binary "Zig is definitively not memory-safe, while safe Rust, is, by definition, memory-safe" argument that was made in response to me. Now you're dealing with qualitative / quantitative questions of practical impact, in which my original statement holds: "Zig is less memory safe than Rust, but more than C/C++. Neither Zig nor Rust is fundamentally memory safe."
You can of course declare that Safe Rust is by definition memory safe, but that doesn't make it any more true than declaring that Rust solves the halting problem or that it proves P=NP. RustBelt is proven sound. Rust by contrast, as being documented by Ferrocene, is currently fundamentally unsound (though you won't hit the soundness issues in practice).
I believe these two statements should show the fundamental difference:
- If a safe Rust program exhibits a memory safety problem, it is a bug in the Rust compiler that is to be fixed - If a Zig program exhibits a memory safety problem, it is a bug in the Zig program that needs to be fixed, not in the compiler
Wouldn't you agree?
> Ferrocene is intended to document the behavior of the current version of the rustc compiler, so it's just an effort to formalize "the compiler is the language".
I must admit I haven't read the specification, but I doubt they attempt to be "bug for bug" compatible in the sense that the spec enumerates memory safety bugs present in the Rust compiler. But am I then mistaken?
No, I don't agree. A compiler bug is something that gets fixed in a patch release after it's reported, or perhaps some platform-specific regression that gets fixed in the next release after it's reported. What we're discussing by contrast is a soundness hole in the language itself - one which will most likely require breaking changes to the language to close (i.e. some older programs that were perfectly safe will fail to compile as a side effect of tightening up the Rust language to prevent this soundness hole).
As to the Ferrocene specification, it explicitly states "Any difference between the FLS and the behavior of the Rust compiler is considered an error on our part and the FLS will be updated accordingly."
Proposals to fix the soundness hole in Rust either change the variance rules themselves, or require where clauses in certain places. Either of these changes would require corresponding changes to chapter 4 of the Ferrocene specification.
4 replies →
> If a safe Rust program exhibits a memory safety problem, it is a bug in the Rust compiler that is to be fixed - If a Zig program exhibits a memory safety problem, it is a bug in the Zig program that needs to be fixed, not in the compiler
That is the absolute best description of memory safety I’ve heard expressed.