It's like letting a wet dog (who'd just been swimming in a nearby swamp) run loose inside your hermetically sealed cleanroom.

"You can take five actions in unsafe Rust that you can’t in safe Rust, which we call unsafe superpowers. Those superpowers include the ability to:

    Dereference a raw pointer
    Call an unsafe function or method
    Access or modify a mutable static variable
    Implement an unsafe trait
    Access fields of a union

In addition, unsafe does not mean the code inside the block is necessarily dangerous or that it will definitely have memory safety problems: the intent is that as the programmer, you’ll ensure the code inside an unsafe block will access memory in a valid way.

People are fallible, and mistakes will happen, but by requiring these five unsafe operations to be inside blocks annotated with unsafe you’ll know that any errors related to memory safety must be within an unsafe block. Keep unsafe blocks small; you’ll be thankful later when you investigate memory bugs."

But using ordinary module encapsulation and private fields, you can scope the code that needs to uphold those preconditions to a particular module.

So the "trusted computing base" for the unsafe code can still be scoped and limited, allowing you to reduce the amount of code you need to audit and be particularly careful about for upholding safety guarantees.

Basically, when writing unsafe code, the actual unsafe operations are scoped to only the unsafe blocks, and they have preconditions that you need to scope to a particular module boundary to ensure that there's a limited amount of code that needs to be audited to ensure it upholds all of the safety invariants.

Ralf Jung has written a number of good papers and blog posts on this topic.

   int average(int x, int y) {
       return (x+y)/2;
   }

I might be old, but more than 10 years ago, hardly anyone talked about UB in C and C++ programming. In the last 10 years, it is all the rage, but seems to add very little to the conversation. For example, if you program C or C++ with the Win32 API, there are loads of weird UB-ish things that seem to work fine.

This is not how compilers work. Optimization happens based on language semantics, not on what platforms do.

UB in C is often found where different real hardware architectures had incompatible behavior. Rather than biasing the language for or against different architectures they left it to the compiler to figure out how to optimize for the cases where instruction behavior diverge. This is still true on current architectures e.g. shift overflow behavior which is why shift overflow is UB.

  int average(int x, int y) {
    long sum = (long)x + y;
    if(sum > INT_MAX || sum < INT_MIN)
        return -1; // or any value that indicates an error/overflow
  
    return (int)(sum / 2);
  }

There is no guarantee that sizeof(long) > sizeof(int), in fact the GNU libc documentation states that int and long have the same size on the majority of supported platforms.

https://www.gnu.org/software/libc/manual/html_node/Range-of-...

> return -1; // or any value that indicates an error/overflow

-1 is a perfectly valid average for various inputs. You could return the larger type to encode an error value that is not a valid output or just output the error and average in two distinct variables.

AI and C seem like a match made in hell.

That used to be the case for 32-bit platforms, but most 64-bit platforms in which GNU libc runs use the LP64 model, which has 32-bit int and 64-bit long. That documentation seems to be a bit outdated.

(One notable 64-bit platform which uses 32-bit for both int and long is Microsoft Windows, but that's not one of the target platforms for GNU libc.)

average(INT_MAX,INTMAX) should return INT_MAX, but it will get that wrong and return -1.

average(0,-2) should not return a special error-code value, but this code will do just that, making -1 an ambiguous output value.

Even its comment is wrong. We can see from the signature of the function that there can be no value that indicates an error, as every possible value of int may be a legitimate output value.

It's possible to implement this function in a portable and standard way though, along the lines of [0].

[0] https://stackoverflow.com/a/61711253/ (Disclosure: this is my code.)

That won't occur on an 'LP64' platform, [0] but we should aim for proper portability and conformance to the C language standard.

[0] https://en.wikipedia.org/wiki/64-bit_computing#64-bit_data_m...

C libraries tend to support a lot of exotic platforms. zlib for example supports Unicos, where int, long int and long long int are all 64 bits large.

- Exposing an unsafe API and relying on the caller to manually uphold invariants

- Doing things like defensive copying at a performance cost

In many cases Rust gives you the best of both worlds: sharing memory liberally while still having the compiler enforce correctness.

And even if you try to provide some kind of safer abstraction, you're limited by the much more primitive type system, that can't distinguish between owned types, unique borrows, and shared borrows, nor can it distinguish thread safety properties.

So you're left to convention and documentation for that kind of information, but nothing checking that you're getting it right, making it easy to make mistakes. And even if you get it right at first, a refactor could change your invariants, and without a type system enforcing them, you never know until someone comes along with a fuzzer and figures out that they can pwn you

In practice (in both languages) you check what the actual unsafe code does (or "all" code in C's case), note code that depends on external actors for safety (it's not all C code, nor is it all unsafe Rust blocks), and check their callers (and callers callers, etc).

From my past experiences with Rust, the team never had to think about data race once, or mutable volatile globals. And we all there suffered from those decades ago with C and sometimes C++ as well.

You like those and don't want to migrate? More power to ya! But badmouthing Rust with what seem fairly uninformed comments is just low. Inform yourself first.

That's only true at the same level of scrutiny as "all C operations can cause undefined behaviour, regardless of what they are", which I find similarly shallow.

And there is not many things we have statistics on in CS, but memory vulnerabilities being absolutely everywhere in unsafe languages, and Rust cleaning up the absolute majority of them even when only the new parts are written in Rust are some of the few we do know, based on actual, real life projects at Google/Microsoft among others.

A memory safe low-level language is as novel as it gets. Rust is absolutely not just hype, it actually delivers and you might want to get on with the times.

    > absolutely laughable c-strings that perform terribly

Rust's type system (including ownership and borrowing, Sync/Send, etc), along with it's privacy features (allowing types to have private fields that can only be accessed by code in the module that defined them) allows you to create fully safe interfaces around code that uses unsafe; there is provably no combination of uses of the interface which lead to undefined behavior.

Now, yeah, it's possible to also use unsafe in Rust just for applying a local optimisation. And that has fewer benefits than a fully encapsulated safe interface, though is still easier to audit for potential UB than C.

So you're right that it's on a continuum, but the distinction between safe and unsafe code means you can more easily find the specific places where UB could occur, and the encapsulation and type system makes it possible to create safe abstractions over unsafe code.

Sure, you can technically just write your own vulnerability for your own program and inject it at an unsafe and see the whole world crumble... but the exact same is true for any form of FFI calls in any language. Is Java memory safe? Yeah, just because I can grab a random pointer and technically break anything I want won't change that.

The fact that a memory vulnerability error may either appear at no place at all OR at the couple hundred lines of code thorough the whole project is a night and day difference.

Unsafe code can be incorrect (or unsound), and needs to be careful about it. Part of being careful is that safe code can call the unsafe code in a way that triggers that unsoundness; in that way, safe code can cause undefined behaviour in unsafe code.

It's not always the case that this is possible; there are unsafe blocks that don't need to depend on safe code for its correctness.

So, in theory, unsafe rust opens the floodgates. In practice, though, you can use small fragments of unsafe code that programmers can fairly easily check to be safe.

Then, once you’ve convinced yourself that those fragments are safe, you can be assured that your whole program is safe (using ‘safe’ in the rust sense, of course)

So, there may be some small islands of unsafe code that require extra attention from the programmer, but that should be just a tiny fraction of all lines, and you should be able to verify those islands in isolation.

This is where the rubber hits the road. Rust does not allow you to do this, in the sense that this is possibly undefined behavior. That "possibly" is why the compiler allows you to write this code, because by saying "unsafe", you are promising that this specific arbitrary address is legal for you to write to. But that doesn't mean that it's always legal to do so.

A simple example might be modifying a const value deep down in some class, where it only becomes apparent later in the program’s execution. Hence their analogy of the wet dog in a clean room - whatever beliefs you have about the structure of memory in your entire program, and guaranteed by the compiler, could have been undone by a rogue unsafe.

Rust encourages using unsafe to "teach" the language new design patterns and data structures; and uses this heavily in its standard library. For example, the Vec type is a wrapper around a raw pointer, length, and capacity; and exposes a safe interface allowing you to create, manipulate, and access vectors with no risk of pointer math going wrong -- assuming the people who implemented the unsafe code inside of Vec didn't make a mistake, the external, safe interface is guaranteed to be sound no matter what external code does.

Think of unsafe not as "this code is unsafe", but as "I've proven this code to be safe, and the borrow checker can rely on it to prove the safety of the rest of my program."

    > For example, the Vec type is a wrapper around a raw pointer, length, and capacity; and exposes a safe interface allowing you to create, manipulate, and access vectors with no risk of pointer math going wrong -- assuming the people who implemented the unsafe code inside of Vec didn't make a mistake, the external, safe interface is guaranteed to be sound no matter what external code does.

Instead, all that functionality is written as Rust code in the standard library, such as Vec. This is what I mean by using unsafe code to "teach" the borrow checker: the language itself doesn't have any notion of growable arrays, so you use unsafe to define its semantics and interface, and now the borrow checker understands growable arrays. The alternative would be to make growable arrays some kind of compiler magic, but that's both harder to implement correctly and not generalizable.

> you can do exactly the same in C by using an opaque pointer to protect the data structure. Then you write a bunch of functions that operate on the opaque pointer. You can use assert() to protect against unreasonable inputs.

That's true and that's a great design pattern in C as well. But there are some crucial differences:

- Rust has no undefined behavior outside of unsafe blocks. This means you only need to audit unsafe blocks (and any invariants they assume) to be sure your program is UB-free. C does not have this property even if you code defensively at interface boundaries.

- In Rust, most of the invariants can be checked at compile time; the need for runtime asserts is less than in C.

- C provides no way to defend against dangling pointers without additional tooling & runtime overhead. For instance, if I write a dynamic vector and get a pointer to the element, there's no way to prevent me from using that pointer after I've freed the vector, or appended an element causing the container to get reallocated elsewhere.

Rust isn't some kind of silver bullet where you feed it C-like code and out comes memory safety. It's also not some kind of high-overhead garbage collected language where you have to write unsafe whenever you care about performance. Rather, Rust's philosophy is to allow you to define fundamental operations out of small encapsulated unsafe building blocks, and its magic is in being able to prove that the composition of these operations is safe, given the soundness of the individual components.

The stdlib provides enough of these building blocks for almost everything you need to do. Unsafe code in library/systems code is rare and used to teach the language of new patterns or data structures that can't be expressed solely in terms of the types exposed by the stdlib. Unsafe in application-level code is virtually never necessary.

By the way, the rust compiler does generate such code because under the hood LLVM runs an autovectorizer when you turn on optimizations. However, for the autovectorizer to do a good job you have to write code in a very special way and you have no way of controlling whether or not it kicked in and once it did that it did a good job.

There’s work on creating safe abstractions (that also transparently scale to the appropriate vector instruction), but progress on that has felt slow to me personally and it’s not available outside nightly currently.

    > However, for the autovectorizer to do a good job you have to write code in a very special way

There's even unsafe usage in the standard library and it's used a lot in embedded libraries.

Sorry but horrible comparison ;)

If you need to rely on unsafe in a memory-safe language for performance reasons, then there is a issue with the language compiler at that point, that needs to be fixed. Simple as that.

The whole memory-safety is the bread and butter of the language, the moment you start to bypass it for faster memory operations, you can start doing the same in any other language. I mean, your literally bypassing the main selling point of the language. \_00_/

It actually means "Rust needs to interface with many other systems that are not as stringent as it". Your interpretation has nothing to do with what's actually going on and I am surprised you misinterpreted the situation as hugely as you did.

...And even if everything was written in Rust, `unsafe` would still be needed because the lower you get [to the kernel] you get more and more non-determinism at places.

This "all or nothing" attitude is boring and tiring. We all wish things were super simple, black and white, and all-or-nothing. They are not.

Once you can internalize that you could unlock the power of encapsulation.

This isn't a wet dog in a cleanroom. This is cleanroom complex that has a very small outhouse that is labeled as dangerous.

All safe code in existence running on von Neumann architectures is built on a foundation of unsafe code. The goal of all memory-safe languages is to provide safe abstractions on top of an unsafe core.

    > What language is the JVM written in?

I like your second paragraph. It is well written.

- Java 74.1%

- C++ 14.0%

- C 7.9%

- Assembly 2.7%

And those values have been increasing for Java with each OpenJDK release.

At the same time, unsafe doesn't just turn off all compiler checks, it just gives you tools to go around them, as well as tools that happen to go around them because of the way they work. Rust unsafe is this weird mix of being safer than pure C, but harder to grasp; with lots of nuanced invariants you have to uphold. If you want to ensure your code still has all the nice properties the compiler guarantees (which go way beyond memory safety) you would have to carefully examine every unsafe block. Which few people do, but you generally still end up with a better status quo than C/C++ where any code can in principle break properties other code was trying to uphold.

Normally in safe code you can’t violate the language rules because the compiler enforces various rules. In unsafe mode, you can do several things the compiler would normally prevent you from doing (e.g. dereferencing a naked pointer). If you uphold all the preconditions of the language, safety is preserved.

What’s unfortunate is that the rules you are required to uphold can be more complex than you might anticipate if you’re trying to use unsafe to write C-like code. What’s fortunate is that you rarely need to do this in normal code and in SIMD which is what the snippet is representing there’s not much danger of violating the rules.

No, not even close. You only lose Rust's safety guarantees when your unsafe code causes Undefined Behavior. Unsafe code that can be made to cause UB from Safe Rust is typically called unsound, and unsafe code that cannot be made to cause UB from Safe Rust is called sound. As long as your unsafe code is sound, then it does not break any of Rust's guarantees.

For example, unsafe code can still use slices or references provided by Safe Rust, because those are always guaranteed to be valid, even in an unsafe block. However, if from inside that unsafe block you then go on to manufacture an invalid slice or reference using unsafe functions, that is UB and you lose Rust's safety guarantees because of the UB.

To continue the analogy of the dog, you let the dog get wet (=you use unsafe), but you put a cleaning room (=the sound and safe API) before your sealed room (=the safe code world)

Not sure why would one resulted in all. One of Rust's advantages is the clear boundary between safe/unsafe.

The usual retort to these questions is 'well, the standard library uses unsafe code, so everything would need a disclaimer that it uses unsafe code, so that's a useless remark to make', but the basic issue still remains that the only clear boundary is whether a function 'contains' unsafe code, not whether a function 'calls' unsafe code.

If Rust did not have a mechanism to use external code then it would be fine because the only sources of unsafe code would be either the application itself or the standard library so you could just grep for 'unsafe' to find the boundaries.

The point is that you don't need to. The guarantees compose.

> The usual retort to these questions is 'well, the standard library uses unsafe code

It's not about the standard library, it's much more fundamental than that: hardware is not memory safe to access.

> If Rust did not have a mechanism to use external code then it would be fine

This is what GC'd languages with runtimes do. And even they almost always include FFI, which lets you call into arbitrary code via the C ABI, allowing for unsafe things. Rust is a language intended to be used at the bottom of the stack, and so has more first-class support, calling it "unsafe" instead of FFI.

Yes, there is a boundary, and usually it's either the function itself, or all methods of an object. For instance, a function I wrote recently goes somewhat like this:

  fn read_unaligned_u64_from_byte_slice(src: &[u8]) -> u64 {
    assert_eq!(src.len(), size_of::<u64>());
    unsafe { std::ptr::read_unaligned(src.as_ptr().cast::<u64>()) }
  }

In other words, unsafe works if you use it carefully and keep it contained.

O_o

[1]: https://www.youtube.com/watch?v=rTo2u13lVcQ

As soon as you start playing with FFI and raw pointers in Python, NodeJS, Julia, R, C#, etc you can easily loose the nice memory-safety properties of those languages - create undefined behavior, segfaults, etc. I'd say Rust is a lot nicer for checking unsafe correctness than other memory-safe languages, and also makes it easier to dip down to systems-level programming, yet it seems to get a lot of hate for these features.

Inside that block, both yes and no. You have to enforce those nice guarantees yourself. Code that violates it will still crash.

If you smell it when you're not working on the gas lines, that's a signal.

https://en.wikipedia.org/wiki/Odorizer#Natural_gas_odorizers

> Ethanethiol (EM), commonly known as ethyl mercaptan is used in liquefied petroleum gas (LPG) and resembles odor of leeks, onions, durian, or cooked cabbage

Methanethiol, commonly known as methyl mercaptan, is added to natural gas as an odorant, usually in mixtures containing methane. Its smell is reminiscent of rotten eggs or cabbage.

...but you can still call it "mercaptan" and be ~ correct in most cases.

No need to get all moral about it.

In fact, it has already been merged two weeks ago: https://github.com/rust-lang/stdarch/pull/1714

The change is already visible on nightly: https://doc.rust-lang.org/nightly/core/arch/x86/fn._mm_xor_s...

Compared to stable: https://doc.rust-lang.org/core/arch/x86/fn._mm_xor_si128.htm...

So this should be stable in 1.87 on May 15 (Rust's 10 year anniversary since 1.0)

https://doc.rust-lang.org/std/intrinsics/simd/index.html

So I suspect it's a matter of two things:

1. You're calling out to what's basically assembly, so buyer beware. This is basically FFI into C/asm.

2. There's no guarantee on what comes out of those 128-bit vectors after to follow any sanity or expectations, so... buyer beware. Same reason std::mem::transmute is marked unsafe.

It's really the weakest form of unsafe.

Still entirely within the bounds of a sane person to reason about.

Of safe SIMD, but some stuff in core::arch is stabilized. Here's the first bit called in the example of the OP: https://doc.rust-lang.org/core/arch/x86/fn._mm_clmulepi64_si...

So many very useful features of Rust and its core library spend years in "nightly" because the maintainers of those features don't have the discipline to see them through.

When I started working in Rust, I'd want some feature or function, look it up, and find it was unstable, sometimes for years. This was frustrating at first, but then I'd go read the GitHub issue thread and find that there was some design or implementation concern that needed to be overcome, and that people were actively working on it and unwilling to stabilize the feature until they were sure it was the best possible design. And the result of that is that features that do get stabilized are well thought out, generalize, and compose well with everything else in the language.

Yes, I really want things like portable SIMD, allocators, generators, or Iterator::intersperse. But programming languages are the one place I really do want perfect to be the enemy of good. I'd rather it take 5+ years to stabilize features than for us to end up with another Swift or C++.

Sounds like the Rust's async story.

Rust uses this strategy of minimal/incremental stabilization quite often (see also: const generics, impl Trait); the difference between this and what drove me away from Swift is that MVPs aren't shipped unless it's clear that the design choices being made now will still be the right choices when the rest of the feature is ready.

Why define runtime independence as a goal, but then make it impossible to write runtime agnostic crates?

(Well, there's the "agnostic" crate at least now)

I would argue that it's the opposite of a mistake. If you standardize everything before the ecosystem gets a chance to play with it, you risk making mistakes that you have to live with in perpetuity.

Language designers admittedly should worry about constant breakage but it’s fine to have some churn, and we shouldn’t be so concerned of it that it freezes everything

The fact that the Rust maintainers allow people to put in half-baked features before they are fully designed is the biggest cultural failing of the language, IMO.

In nightly?

Hard disagree. Letting people try things out in the real world is how you avoid half-baked features. Easy availability of nightly compilers with unstable features allows way more people to get involved in the pre-stabilization polishing phase of things and raise practical concerns instead of theoretical ones.

C++ takes the approach of writing and nitpicking whitepapers for years before any implementations are ready and it's hard to see how that has led to better outcomes relatively speaking.

1. Big library/compiler does a thing, and people really like it

2. Other compilers and libraries copy that thing, sometimes putting their own spin on it

3. All the kinks get worked out and they write a white paper

4. Eventually the thing becomes standard

That way, everything in the standard library is something that is fully-thought-out and feature-complete. It also gives much more room for competing implementations to be built and considered before someone stakes out a spot in the standard library for their thing.

Are C++ features really that much better thought out? Modules were "standardized" half a decade ago, but the list of problems with actually using them in practice is still pretty damn long to the point where adoption is basically non-existent.

I'm not going to pretend to be nearly as knowledgeable about C++ as Rust, but it seems like most new C++ features I hear about are a bit janky or don't actually fit that well with the rest of the language. Something that tends to happen when designing things in an ivory tower without testing them in practice.

The fundamental problem with modules is that build systems for C++ have different abstractions and boundaries. C++ modules are like Rust async - something that just doesn't fit well with the language/system and got hammered in anyway.

The reason it seems like they come from nowhere is probably because you don't know where they come from. Most things go through boost, folly, absl, clang, or GCC (or are vendor-specific features) before going to std.

That being said, it's not just C++ that has this flow for adding features to the language. Almost every other major language that is not Rust has an authoritative specification.

Fully-thought-out and feature-complete is something that since C++17 has been hardly happening.

I don't have the personality or time to wade into committee type work, so I have no idea what it would take to get those two across the finish line, but the allocator one in particular makes me question Rust for lower level applications. I think it's just not going to happen.

If Zig had proper ADTs and something equivalent to borrow checker, I'd be inclined to poke at it more.

I don't write software targetting nightly, for good reason.

This take makes me sad. There are a lot of reasons why an open source contributor may not see something through. "Lack of discipline" is only one of them. Others that come to mind are: lack of time, lack of resources, lack of capability (i.e good at writing code, but struggles to navigate the social complexities of sheparding a significant code change), clinically impaired ability to "stay the course" and "see things through" (e.g. ADHD), or maybe it was a collaborative effort and some of the parties dropped out for any of the aforementioned reasons.

I don't have a solution, but it does kinda suck that open source contribution processes are so dependent on instigators being the responsible party to seeing a change all the way through the pipeline.

For example, it is perfectly legal to dereference a vector pointer that references illegal memory if you mask the illegal addresses. This is a useful trick and common in e.g. idiomatic AVX-512 code. The mask registers are almost always computed at runtime so it would be effectively impossible to determine if a potentially illegal dereference is actually illegal at compile-time.

I suspect we’ll be hand-rolling unsafe SIMD for a long time. The different ISAs are too different, inconsistent, and weird. A compiler that could make this clean and safe is like fusion power, it has always been 10 years away my entire career.

[1] https://doc.rust-lang.org/std/simd/index.html

Of course, in practice, even in Rust, it isn't strictly true that programs without unsafe can't crash with fatal runtime errors. There's always stack overflows, which will crash you with a SIGABRT or equivalent operating system error.

There are uses for this, especially since some code will run in environments where you can not simply handle it, but it's also just cleaner this way; you don't have to worry about the different behaviors between operating systems and possibly CPU architectures with regards to error recovery if you simply don't generate any.

Since there are these edge cases where it wouldn't be possible to handle faults easily (e.g. some kernel code) it needs to be considered unsafe in general.

In Rust, the CPU exception resulting from a stack overflow is considered safe. The compiler uses stack probing to ensure that as long as there is at least one page of unmapped memory below the stack (guard page), the program will reliably fault on it rather than continuing to access memory further below. In most environments it is possible to set up a guard page, including Linux kernel code if CONFIG_VMAP_STACK is enabled. But there are other environments where it’s not, such as WebAssembly and some microcontrollers. In those environments, the backend would have to add explicit checks to function prologs to ensure enough stack is available. I say “would have to”, not “does”: I’ve heard that on at least the microcontrollers, there are no such checks and Rust is just unsound at the moment. Not sure about WebAssembly.

Meanwhile, Go uses CPU exceptions to handle nil dereferences.

That said, I actually entirely forgot Go catches nil derefs in a segfault handler. I guess it's not a big deal since Go isn't really suitable for free-standing environments where avoiding CPU exceptions is sometimes more useful, so there's no particular reason why the runtime can't rely on it.

It's also possible to go a step further and practice "panic-free" Rust where you write code in such a way that it never links to the panic handler. Seems pretty hard to do, but seems like it might be worth it sometimes, especially if you're in an environment where you don't have anything sensible to do on a panic.

There was a bug open about it and the rationale was that no one with the expertise (some of these are quite arcane) was stepping up to do it. (edit: other comments in this thread suggest that this effort is now underway and first changes were committed a few weeks ago)

You can do safe SIMD using std::simd but it is nightly only at this point.

"People are fallible, and mistakes will happen, but by requiring these five unsafe operations to be inside blocks annotated with unsafe you’ll know that any errors related to memory safety must be within an unsafe block. Keep unsafe blocks small; you’ll be thankful later when you investigate memory bugs."

I hope the SIMD intrinsics make it to stable soon so folks can ditch unnecessary unsafes if that's the only issue.

I am 100% sure that the smell they add to natural gas does not smell like rotten eggs.

But it can occur naturally in natural gas.

https://en.m.wikipedia.org/wiki/Methanethiol

It's a bad comparison since CO doesn't smell, which is what makes it dangerous, while H2S is detected by our sense of smell at concentrations much lower than the toxic dose (in fact, its biggest dangers comes from the fact that at dangerous concentration it doesn't even smell anything due to our receptors being saturated).

It's not what's being put in natural gas, but it wouldn't be that dangerous if we did.

Update its how the std lib does it: https://doc.rust-lang.org/src/alloc/collections/linked_list....

Coming up with these niche examples of things you need unsafe for in order to discredit rust’s safety guarantees is just not interesting. What fraction of programmer time is spent writing custom linked lists? Surely way less than 1%. In most of the other 99%, Rust is very helpful.

You design an abstraction which is unsafe inside, and exposes a safe API to users. That is really how unsafe it meant to be used.

Of course the standard library uses unsafe. This is where you want unsafe to be, not in random user code. That's what it was made for.

a) are surprisingly nontrivial to get right,

b) have almost no practical uses, and

c) are only taught because they're conceptually nice and demonstrate pointers and O(1) vs O(n) tradeoffs.

Note that safe Rust has no problems with singly-linked lists or in general any directed tree structure.

C has to make a syscall to the kernel which ultimately results in a BIOS interrupt to implement printf, which you need for the hello world program on page 1 of K&R.

Does that mean that C has no abstraction advantage over directly coding interrupts with asm? Of course not.

That's not the case since the late 1990s. Other than during early boot, nobody calls into the BIOS to output text, and even then "BIOS interrupt" is not something normally used anymore (EFI uses direct function calls through a function table instead of going through software interrupts).

What really happens in the kernel nowadays is direct memory access and direct manipulation of I/O ports and memory mapped registers. That is, all modern operating systems directly manipulate the hardware for text and graphics output, instead of going through the BIOS.

On modern aschitectures you shouldn't use either unless you have an extremely niche use-case. They are not general use data structures anymore in a world where cache locality is a thing.

You can implement that linked list just once, audit the unsafe parts extensively, provide a fully safe API to clients, and then just use that safe API in many different places. You don't need thousands of project-specific linked list reimplementations.

It tends to be found in drivers, kernels, vector code, and low-level implementations of data structures and allocators and similar things. Not typical application code.

As a general rule it should be avoided unless there's a good reason to do it. But it's there for a reason. It's almost impossible to create a systems language that imposes any kind of rules (like ownership etc.) that covers all possible cases and all possible optimization patterns on all hardware.

Unsafe Rust is currently extremely underspecified and underdocumented, but it's designed to be far more specifiable than C. For example: aliasing rules. When and how you're allowed to alias references in unsafe code is not at all documented and under much active discussion; whereas in C pointer aliasing rules are well defined but also completely insane (casting pointers to a different type in order to reinterpret the bytes of an object is often UB even in completely innocuous cases).

Once Rust's memory model is fully specified and written down, unsafe Rust is trying to go for something much simpler, more teachable, and with less footguns than C.

Huge props to Ralf Jung and the opsem team who are working on answering these questions & creating a formal specification: https://github.com/rust-lang/unsafe-code-guidelines/issues

All languages at some point interface with syscalls or low level assembly that can be done wrong, but one of Rust's selling points is a safe wrapping of low-level interactions. Like safe heap allocation/deallocation with `Box`, or swapping with `swap`, etc. Except... here.

Why does a library like zlib need to go beyond Rust's safe offerings? Why doesn't rust provide safe versions of the constructs zlib needs?

If you want a clean crash instead on indeterministic behavior, you need to use assert like in C, but it won't save you from compiler optimization removing checks that are deemed useless (again, exactly like in C).

People seem to disagree.

Unsafe Rust Is Harder Than C

https://chadaustin.me/2024/10/intrusive-linked-list-in-rust/

https://news.ycombinator.com/item?id=41944121

Using raw pointers in unsafe Rust is easier than using raw pointers in C.

The solution is to not manipulate references in unsafe code. The problem is that in old versions of Rust this was tricky. Modern versions of Rust have addressed this by adding first-class facilities for producing pointers without needing temporary references: https://blog.rust-lang.org/2024/10/17/Rust-1.82.0.html#nativ...

This is not exactly true. Even in production code, unsafe preconditions check if you violate these rules.

Here: https://doc.rust-lang.org/core/macro.assert_unsafe_precondit... And here: https://google.github.io/comprehensive-rust/unsafe-rust/unsa...

> Safe Rust: memory safe, no undefined behavior possible. Unsafe Rust: can trigger undefined behavior if preconditions are violated.

So Unsafe Rust from a UB perspective is no different than C/C++. If preconditions are violated, UB can occur, affecting anywhere in the program. Its unclear how the compiler could check anything about preconditions in a block explicitly used to say that the developer is the one upholding the preconditions.

Only if you actively disable panics being triggered if unsafe preconditions are triggered. In most code, the program will crash instead. Enabling default panic on up violation in production code was done last year, IIRC.

> Its unclear how the compiler could check anything about preconditions

It can't. This is done at runtime, by default and without manually needed programmer interaction.

You can see an example of this in the `ptr`module, here: https://doc.rust-lang.org/beta/src/core/ptr/mod.rs.html#1071

Some are only enabled for `debug_assert` (which is enabled by default), see `ptr::read`, here: https://doc.rust-lang.org/beta/src/core/ptr/mod.rs.html#1370

However, even at runtime it can't do anything to say if (excuse the C pseudocode) *(uint32_t*)0x1C00 = 0xFE is a valid memory operations. On some systems, in some cases it might be.

What? Where did you get that impression?

> But in any case it seems like its just doing some number of asserts to validate some preconditions

Yeah, like C code normally would, just in the STD in this case.

https://doc.rust-lang.org/beta/

> Yeah, like C code normally would, just in the STD in this case.

Yes, in that manual checks are still needed. My point is unsafe code in rust is nowhere near safe and cannot be considered as safe without extensive analysis, no matter the language features used.

Some codebases, you can grep for "unsafe", find no results, and conclude the codebase is safe... if you trust its dependencies.

This is not one of those codebases. This one uses unsafe liberally, which tells you it's about as safe as C.

"unsafe behaviour is clearly marked" seems to be a thought-stopping cliche in the Rust world. What's the point of marking them, if you still have them? If every pointer dereference in C code had to be marked unsafe (or "please" like in Intercal), that wouldn't make C any better.

Rust has macros; are macros prohibited from generating unsafe blocks, so that macro invocations don't have to be suspected of harboring unsafe code?