4 ms·

> technically Rust is still safer than C in its unsafe blocks This is quite dubious in a practical sense, since Rust unsafe blocks must manually uphold the saf

> technically Rust is still safer than C in its unsafe blocks This is quite dubious in a practical sense, since Rust unsafe blocks must manually uphold the safety invariants that idiomatic Safe Rust relies on at all times, which includes, e.g. references pointing to valid and properly aligned data, as well as requirements on mutable references comparable to what the `restrict` qualifier (which is rarely used) involves in C. In practice, this is hard to do consistently, and may trigger unexpected UB. Some of these safety invariants can be relaxed in simple ways (e.g. &Cell<T> being aliasable where &mut T isn't) but this isn't always idiomatic or free of boilerplate in Safe Rust.

Xylakant 11mo ago
It's great that the Google Android team has been tracking data to answer that question for years now and their conclusion is: ------- The primary security concern regarding Rust generally centers on the approximately 4% of code written within unsafe{} blocks. This subset of Rust has fueled significant speculation, misconceptions, and even theories that unsafe Rust might be more buggy than C. Empirical evidence shows this to be quite wrong. Our data indicates that even a more conservative assumption, that a line of unsafe Rust is as likely to have a bug as a line of C or C++, significantly overestimates the risk of unsafe Rust. We don’t know for sure why this is the case, but there are likely several contributing factors: unsafe{} doesn't actually disable all or even most of Rust’s safety checks (a common misconception). The practice of encapsulation enables local reasoning about safety invariants. The additional scrutiny that unsafe{} blocks receive. ----- From https://security.googleblog.com/2025/11/rust-in-android-move-fast-fix-things.html?m=1 https://security.googleblog.com/2025/11/rust-in-android-move...
- zozbot234 11mo ago
  > The practice of encapsulation enables local reasoning about safety invariants. > The additional scrutiny that unsafe{} blocks receive. None of this supports an argument that "unsafe Rust is safer than C". It's just saying that with enough scrutiny on those unsafe blocks, the potential bugs will be found and addressed as part of development. That's a rather different claim.
  - sophacles 11mo ago
    It actually does support it. Human attention is a finite resource. You can spend a little bit if attention in every line to scrutinize safety or you can spend a lot of time scrutinizing the places where you can't mechanically guarantee safety. It's safer because it spends the human attention resource more wisely.
  - baq 11mo ago
    literally from the quote: unsafe{} doesn't actually disable all or even most of Rust’s safety checks (a common misconception).
    - menaerus 11mo ago
      They also say The practice of encapsulation enables local reasoning about safety invariants. which is not fully correct. Undefined behavior in unsafe blocks can and will leak into the safe Rust code so there is nothing there about the "local reasoning" or "encapsulation" or "safety invariants". This whole blog always read to me as too much like a marketing material disguised with some data so that it is not so obvious. IMHO
      - deleted 11mo ago
        [deleted]
      - aw1621107 11mo ago
        > which is not fully correct. Undefined behavior in unsafe blocks can and will leak into the safe Rust code so there is nothing there about the "local reasoning" or "encapsulation" or "safety invariants". Strictly speaking, that encapsulation enables local reasoning about safety invariants does not necessarily imply that encapsulation guarantees local reasoning about safety invariants. It's always possible to write something unadvisable, and no language is capable of preventing that. That being said, I think you might be missing the point to some extent. The idea behind the sentence is not to say that the consequences of a mistake will not be felt elsewhere. The idea is that when reasoning about whether you're upholding invariants and/or investivating something that went wrong, the amount of code you need to look at is bounded such that you can ignore everything outside those bounds; i.e., you can look at some set of code in complete isolation. In the most conservative/general case that boundary would be the module boundary, but it's not uncommon to be able to shrink those boundaries to the function body, or potentially even further. This general concept here isn't really new. Rust just applied it in a relatively new context.

Xylakant 11mo ago

It's great that the Google Android team has been tracking data to answer that question for years now and their conclusion is: ------- The primary security concern regarding Rust generally centers on the approximately 4% of code written within unsafe{} blocks. This subset of Rust has fueled significant speculation, misconceptions, and even theories that unsafe Rust might be more buggy than C. Empirical evidence shows this to be quite wrong. Our data indicates that even a more conservative assumption, that a line of unsafe Rust is as likely to have a bug as a line of C or C++, significantly overestimates the risk of unsafe Rust. We don’t know for sure why this is the case, but there are likely several contributing factors: unsafe{} doesn't actually disable all or even most of Rust’s safety checks (a common misconception). The practice of encapsulation enables local reasoning about safety invariants. The additional scrutiny that unsafe{} blocks receive. ----- From https://security.googleblog.com/2025/11/rust-in-android-move-fast-fix-things.html?m=1 https://security.googleblog.com/2025/11/rust-in-android-move...

zozbot234 11mo ago
> The practice of encapsulation enables local reasoning about safety invariants. > The additional scrutiny that unsafe{} blocks receive. None of this supports an argument that "unsafe Rust is safer than C". It's just saying that with enough scrutiny on those unsafe blocks, the potential bugs will be found and addressed as part of development. That's a rather different claim.
- sophacles 11mo ago
  It actually does support it. Human attention is a finite resource. You can spend a little bit if attention in every line to scrutinize safety or you can spend a lot of time scrutinizing the places where you can't mechanically guarantee safety. It's safer because it spends the human attention resource more wisely.
- baq 11mo ago
  literally from the quote: unsafe{} doesn't actually disable all or even most of Rust’s safety checks (a common misconception).
  - menaerus 11mo ago
    They also say The practice of encapsulation enables local reasoning about safety invariants. which is not fully correct. Undefined behavior in unsafe blocks can and will leak into the safe Rust code so there is nothing there about the "local reasoning" or "encapsulation" or "safety invariants". This whole blog always read to me as too much like a marketing material disguised with some data so that it is not so obvious. IMHO
    - deleted 11mo ago
      [deleted]
    - aw1621107 11mo ago
      > which is not fully correct. Undefined behavior in unsafe blocks can and will leak into the safe Rust code so there is nothing there about the "local reasoning" or "encapsulation" or "safety invariants". Strictly speaking, that encapsulation enables local reasoning about safety invariants does not necessarily imply that encapsulation guarantees local reasoning about safety invariants. It's always possible to write something unadvisable, and no language is capable of preventing that. That being said, I think you might be missing the point to some extent. The idea behind the sentence is not to say that the consequences of a mistake will not be felt elsewhere. The idea is that when reasoning about whether you're upholding invariants and/or investivating something that went wrong, the amount of code you need to look at is bounded such that you can ignore everything outside those bounds; i.e., you can look at some set of code in complete isolation. In the most conservative/general case that boundary would be the module boundary, but it's not uncommon to be able to shrink those boundaries to the function body, or potentially even further. This general concept here isn't really new. Rust just applied it in a relatively new context.