Comment by hamcocar

3 days ago

I am very sorry, but you do not address that TBAA, like C has by default, generally is easier than just no aliasing, like what Rust has for mutable references. This is a major difference. C code can opt into a similar kind of aliasing, namely by using _restrict_, but that is opt-in, while it is always on for Rust.

And there is newer UB as well in Rust stdlib

https://github.com/rust-lang/rust/pull/139553

7 comments

hamcocar

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GolDDranks  2 days ago

> TBAA, like C has by default, generally is easier than just no aliasing [note: I assume no aliasing here to refer Rust's mutable references, not C's restrict]

I don't accept that statement. Rust's mutable references are statically checked, so they are impossible to get wrong (modulo compiler bugs) if you aren't using unsafe code. Using raw pointers has no no-aliasing requirements, so again, it's easier than in C.

The hard part is mixing mutable references and raw pointers. In 95% of Rust code, you are not required to do that, and you shouldn't do that. In the remaining 5%, you should understand the aliasing model. In that case, indeed, you need to know more than what TBAA requires you to know. But that's for the case where you can also _DO_ more than TBAA would allow you to do.

  • ralfj  2 days ago

    Yes, exactly. I'd be surprised if it's 5% -- that's about the ratio of all unsafe code, and most unsafe code does not mix references and raw pointers. I think it's less than 1% of the overall code that has to worry about this, but unfortunately I don't have hard data.

  • gryhili  2 days ago

    Ycombinator/Hacker News is giving me trouble regarding replying, maybe the debate is not going the way that some people want it to go. Sad.

    • tomhow  2 days ago

      We have rate-limiters for new accounts to limit the damage that can be done by spammers, trolls and flamewar participants.

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    • eru  2 days ago

      I think HN is (intentionally) delaying the ability to reply to deeply nested comments.