[nromiun]

> It's possible for a Rust program to be technically compilable but still semantically wrong.

This was my biggest problem when I used to write Rust. The article has a small example but when you start working on large codebases these problems pop up more frequently.

Everyone says the Rust compiler will save you from bugs like this but as the article shows you can compile bugs into your codebase and when you finally get an unrelated error you have to debug all the bugs in your code. Even the ones that were working previously.

> Rust does not know more about the semantics of your program than you do

Also this. Some people absolutely refuse to believe it though.

[MrJohz]

I think the key idea is that Rust gives you a lot of tools to encode semantics into your program. So you've got a much greater ability for the compiler to understand your semantics than in a language like JavaScript (say) where the compiler has very little way of knowing any information about lifetimes.

However, you've still got to do that job of encoding the semantics. Moreover, the default semantics may not necessarily be the semantics you are interested in. So you need to understand the default semantics enough to know when you need something different. This is the big disadvantage of lifetime elision: in most cases it works well, but it creates defaults that may not be what you're after.

The other side is that sometimes the semantics you want to encode can't be expressed in the type system, either because the type system explicitly disallows them, or because it doesn't comprehend them. At this point you start running into issues like disjoint borrows, where you know two attributes in a struct can be borrowed independently, but it's very difficult to express this to the compiler.

That said, I think Rust gives you more power to express semantics in the type system than a lot of other languages (particularly a lot of more mainstream languages) which I think is what gives rise to this idea that "if it compiles, it works". The more you express, the more likely that statement is to be true, although the more you need to check that what you've expressed does match the semantics you're aiming for.

[LtdJorge]

Yes, that's a very common misconception.

Of course, if your program compiles, that doesn't mean the logic is correct. However, if your program compiles _and_ the logic is correct, there's a high likelihood that your program won't crash (provided you handle errors and such, you cannot trust data coming from outside, allocations to always work, etc). In Rust's case, this means that the compiler is much more restrictive, exhaustive and pedantic than others like C's and C++'s.

In those languages, correct logic and getting the program to compile doesn't guarantee you are free from data races or segmentation faults.

Also, Rust's type system being so strong, it allows you to encode so many invariants that it makes implementing the correct logic easier (although not simpler).

[irishcoffee]

> However, if your program compiles _and_ the logic is correct, there's a high likelihood that your program won't crash (provided you handle errors and such, you cannot trust data coming from outside, allocations to always work, etc).

That is one hell of a copium disclaimer. "If you hold it right..."

[kibwen]

Rust certainly doesn't make it impossible to write bad code. What it does do is nudge you towards writing good code to a noticeably appreciable degree, which is laudable compared to the state of the industry at large.

[irishcoffee]

Rust is just a tool. It’s as fallible as any other tool. I wish we took it off the pedestal and treated it as such.

[steveklabnik]

Are all tools equal in all dimensions or can they be compared for fitness of purpose?

[estebank]

A hand saw, a table saw and a SawStop are all tools, but they have different characteristics even though they all are meant to cut the same wood.

[irishcoffee]

Ada, C, and lisp are all tools, but they have different characteristics even though they are all meant to cut through the same problems.

[wakawaka28]

>In those languages, correct logic and getting the program to compile doesn't guarantee you are free from data races or segmentation faults.

I don't believe that it's guaranteed in Rust either, despite much marketing to the contrary. It just doesn't sound appealing to say "somewhat reduces many common problems" lol

>Also, Rust's type system being so strong, it allows you to encode so many invariants that it makes implementing the correct logic easier (although not simpler).

C++ has a strong type system too, probably fancier than Rust's or at least similar. Most people do not want to write complex type system constraints. I'm guessing that at most 25% of C++ codebases at most use complex templates with recursive templates, traits, concepts, `requires`, etc.

[simonask]

Comparing type systems is difficult, but the general experience is that it is significantly easier to encode logic invariants in Rust than in C++.

Some of the things you can do, often with a wild amount of boilerplate (tagged unions, niches, etc.), and some of the things are fundamentally impossible (movable non-null owning references).

C++ templates are more powerful than Rust generics, but the available tools in Rust are more sophisticated.

[sunshowers]

Note that while C++ templates are more powerful than Rust generics at being able to express different patterns of code, Rust generics are better at producing useful error messages. To me, personally, good error messages are the most fundamental part of a compiler frontend.

[wakawaka28]

Concepts make it possible to generate very clear (even user-friendly) template errors.

[sunshowers]

True but you lose out on much of the functionality of templates, right? Also you only get errors when instantiating concretely, rather than getting errors within the template definition.

[wakawaka28]

I don't agree that Rust tools are more sophisticated and they definitely are not more abundant. You just have a language that is more anal up front. C++ has many different compilers, analyzers, debuggers, linting tools, leak detectors, profilers, etc. It turns out that 40 years of use leads to significant development that is hard to rebuild from scratch.

I seem to have struck a nerve with my post, which got 4 downvotes so far. Just for saying Rust is not actually better than C++ in this one regard lol.

[simonask]

I think it’s because you misread my comment by skimming over the important parts.

This isn’t about tooling, it’s about language features and type systems.

[littlestymaar]

I don't think anyone believes the “if it compile it works” phrase literally.

It's just that once it compiles Rust code will work more often than most languages, but that doesn't mean Rust code will automatically be bug free and I don't think anyone believes that.

[emilbratt]

Yeah, even the official Rust book points it out and if my memory serves me right (not punintended) also gives an example in the form of creating a memory leak (not to be confused with memory unsafe).

[spookie]

A memory leak can be unsafe though.

[201984]

Then why is Box::leak not marked unsafe?

[estebank]

"unsafe" or `unsafe`? One is the general meaning of the word, the latter is "it invokes undefined-behavior".

[spookie]

As "unsafe". An example would be of how AMD GPUs some time ago didn't free a programs' last rendered buffers and you could see the literal last frame in its entirety. Fun stuff.

Could've been clearer above.

[yuriks]

That is not a memory leak though! That's using/exposing an uninitialized buffer, which can happen even if you allocate and free your allocations correctly. Leaking the buffer would prevent the memory region from being allocated by another application, and would in fact prevent that from happening.

This is also something that Rust does protect against in safe code, by requiring initialization of all memory before use, or using MaybeUninit for buffers that aren't, where reading the buffer or asserting that it has been initialized is an unsafe operation.

[embedding-shape]

There are definitively people in the ecosystem who peddle sentiments like "Woah, Rust helps so much that I can basically think 'if this compiles, everything will work', and most of the times it does!", and that's the confusing part for many people. Examples found in 30 seconds of searching:

- https://bsky.app/profile/codewright.bsky.social/post/3m4m5mv...

- https://bsky.app/profile/naps62.bsky.social/post/3lpopqwznfs...

[muixoozie]

I read the comments you linked and don't really think they literally believe Rust is magic. I dunno though I guess I could imagine a vibe coder tacitly believing that. Not saying you're wrong. I just think most people say that tongue in cheek. This saying has been around forever in the Haskell community for decades. Feels like a long running joke at this point

[echelon]

Rust isn't magic, but it has an incredibly low software defect rate. Google published a few studies on this.

If Rust code compiles, it probably has a lower defect rate than corresponding code written by the same team in another language, all else being equal.

[muixoozie]

Agreed. I've been working professionally with Rust for a year and it is my experience that it's rock solid.

[binary132]

There is no hint of irony in the linked posts.

[kstrauser]

When I’ve said that, I’ve meant that almost the only remaining bugs were bad logic on my part. It’s free from the usual dumb mistakes I would make in other languages.

[spoiler]

I don't know the authors of those posts, so I don't want to put word in their mouth, but neither seem to be delusional about the "if it compiles, it works" phrase. The first one qualifies it with "most of the time", and the second one explicitly mentions using type state as a tool to aid correctness...

But I don't doubt there are people who take that phrase too literally, though.

[goku12]

Both examples you linked are people talking casually about the nature of Rust, rather than about the specific rule. That goes very much with your parent commenter's assertion that nobody takes it literally. The first example even starts with 'Most of the time' (this is true, though not guaranteed. I will explain further down). Human languages are imperfect and exaggerations and superlatives are common in casual communication.

But I have not seen any resource or anyone making technical points ever assert that the Rust compiler can verify program logic. That doesn't even make sense - the compiler isn't an AI that knows your intentions. Everybody is always clear that it only verifies memory safety.

Now regarding the 'most of the time' part. The part below is based purely on my experience and your mileage may vary. It's certainly possible to compile Rust programs with logical/semantic errors. I have made plenty. But the nature of C/C++ or similar manually memory-managed languages is such that you can make memory safety bugs quiet easily and miss them entirely. They also stay hidden longer.

And while logical errors are also possible, most people write and test code in chunks of sizes small enough where they feel confident enough to understand and analyze it entirely within their mind. Thus they tend to get caught and eliminated earlier than the memory safety bugs.

Now since Rust handles the memory safety bugs for you and you're reasonably good at dealing with logical bugs, the final integrated code tends to be bug-free, surprisingly more often than in other languages - but not every time.

There is another effect that makes Rust programs relatively more bug-free. This time, It's about the design of the code. Regular safe Rust, without any runtime features (like Rc, Arc, RefCell, Mutex, etc) is extremely restrictive in what designs it accepts. It accepts data structures that have a clear tree hierarchy, and thus a single-owner pattern. But once you get into stuff like cyclic references, mutual references, self references, etc, Rust will simply reject your code even if it can be proven to be correct at compile time. You have three options in that case: Use runtime safety checks (Rc, RefCell, Mutex, etc. This is slightly slower) OR use unsafe block and verify it manually, OR use a library that does the previous one for you.

Most of the code we write can be expressed in the restricted form that safe Rust allows without runtime checks. So whenever I face such issues, my immediate effort is to refactor the code in such way. I reach for the other three methods only if this is not possible - and that's actually rare. The big advantage of this method is that such designs are relatively free of the vast number of logical bugs you can make with a non-tree/cyclic ownership hierarchy. (Runtime checks convert memory safety bugs into logical bugs. If you make a mistake there, the program will panic at runtime.) Therefore, the refactored design ends up very elegant and bug-free much more often than in other languages.

[littlestymaar]

> "Woah, Rust helps so much that I can basically think 'if this compiles, everything will work', and most of the times it does!"

I think is is a fairly bad example to pick, because the fact that the person says “I can basically think” and “most of the time it does” (emphasis mine) shows that they don't actually believes it will makes bug-free programs.

They are just saying that “most of the time” the compiler is very very helpful (I agree with them on that).

[treyd]

> Some people absolutely refuse to believe it though.

Who says this? I've never seen someone argue it makes it impossible to write incorrect code. If that were the case then there's no reason for it to have an integrated unit testing system. That would be an absurd statement to make, even if you can encode the entire program spec into the type system, there's always the possibly the description of a solution is not aligned with the problem being solved.

[alkonaut]

"If it compiles it works" isn't true. But "If it compiles it won't eat your homework" sort of is.

[embedding-shape]

Neither are true, `std:fs::remove_dir_all("/home/user/homework")` will happily compile and run, no matter if that's what you wanted or not.

Rust programs can't know what you want to do, period.

[qouteall]

The better phrase is that "if it compiles, then many possible Heisenbugs vanish"

https://qouteall.fun/qouteall-blog/2025/How%20to%20Avoid%20F...

[throwawayqqq11]

They certainly know that you dont want to process garbage data from freed memory.

Soundness does not cover semantic correctness. Maybe you want to wipe $HOME.

[wakawaka28]

>They certainly know that you dont want to process garbage data from freed memory.

It depends on what you mean by "freed". Can one write a custom allocator in Rust? How does one handle reading from special addresses that represent hardware? In both of these scenarios, one might read from or write to memory that is not obviously allocated.

[ameliaquining]

Both of those things can be done in Rust, but not in safe Rust, you have to use unsafe APIs that don't check lifetimes at compile time. Safe Rust assumes a clear distinction between memory allocations that are still live and those that have been deallocated, and that you never want to access the latter, which of course is true for most applications.

[brooke2k]

You can indeed write custom allocators, and you can read to or write from special addresses. The former will usually, and the latter will always, require some use of `unsafe` in order to declare to the compiler: "I have verified that the rules of ownership and borrowing are respected in this block of code".

[alkonaut]

"If it compiles, then only logical bugs will make it eat your homework"