There's a book that changed a lot of the way I think about attention and media [0]. The book isn't very good, but it flags something relevant here. There is a huge asymmetry between the reach of a big, flashy announcement (here: bun was re-written in memory-safe rust in a couple weeks), and the relatively small reach of a correction (often just a footnote on an old article, here a GH issue).
This asymmetry is well understood by marketing and PR professionals, and actively exploited.
> a big, flashy announcement (here: bun was re-written in memory-safe rust in a couple weeks)
Did they even claim it was "memory-safe"? Every discussion of this topic has had dozens of comments noting that their vibed codebase is bursting at the seams with unaudited unsafe blocks, lightly reviewed by people who seem to not only seem to not understand Rust, but who seem incensed at the idea of needing to understand any programming language in the first place.
No, and there's been a lot of confusion about that on this website.
They did cite Rust's safety as a motivating factor for the port. That doesn't imply trying to achieve that simultaneously with the language change — which is good, because that would be insane. (Or, if you prefer, even more insane.)
You cannot faithfully port a codebase to a new language while also radically re-architecting it. You have to choose.
They want the safety benefits of Rust going forward; i.e., after it's finished, when they then write new code in Rust.
Yeah, exactly. The typical approach is to do a mechanical translation such as with rust2c, that is full of unsafe, and then gradually refactor safety in.
And the first post is about the team working on the project, with about two and a half sentences on c2rust, and making it very clear they just started.
The newer posts go into detail about the rearchitecting that follows.
A bug-for-bug port to Rust is the first step to fixing that. Assuming the port is actually 1:1 without any behavioral changes, these bugs already exist in the Zig code. The difference is now it's known where effort can be dedicated in order to one day have a memory-safe release of Bun. People have absolutely lost their mind over this and completely forgotten the benefits Rust gives you. I feel like I've gone back 10 years reading threads about the Rust port of Bun these are the exact same arguments we see from people advocating continued use of C++.
> People have absolutely lost their mind over this and completely forgotten the benefits Rust gives you.
To be fair this is the type of reaction to be expected with all of the evangelism going around touting Rust as some pure beast incapable of error.
> these are the exact same arguments we see from people advocating continued use of C++.
Not really - people pushing Rust claim that switching to their new found lord and saviour will cure all evils and save everyone. The truth is - as you're pointing out - the bugs stay, maybe even more are created too.
Ports/re-architectures/rewrites always do this - promise to end all ills, and failing to deliver.
Hmmm, given the general mood in this case, I feel like there's a lot of people keen to find any criticism of the code they can and amplify it as possible. Most of it strikes me as relatively shallow at the moment, though (that is, apart from the fact that merging such a large LLM assisted port is certainly a, uh _bold_ move (to put it lightly), there's not much that people are pointing out about the actual result that feels like it's worse than any other port in progress, but there is definitely a lot of hay being made about any issue that is found).
You could view it as a specific application of the quote.
In your quote, there is no time-dependency between the lie and the truth. Whereas here, it's an attractive lie (easily parsed, great narrative), followed up by truths (that need more than surface-level analysis).
I thought you were going to call out the problem in the other direction: All of these big threads on social media are missing the point that the original Zig version had a lot of memory safety problems.
The rewrite was a code translation meant to be a starting point.
> a big, flashy announcement (here: bun was re-written in memory-safe rust in a couple weeks), and the relatively small reach of a correction (often just a footnote on an old article, here a GH issue).
The Bun team never made a big announcement that the code is now memory safe. They've been clear that this is the starting point.
Anyone expecting it to be perfect immediately and to have solved all of the memory problems in the original Zig code is arguing with an announcement they imagined, not what the Bun team has said.
Did anyone try to map this code back to the original codebase to see if this memory problem exists in the original codebase?
Not just marketing and PR, the mainstream media knows that pushing out BS and then retracting it later can have lasting effects because people will remember the original article / headline, and never see the correction.
only the mainstream media knows about this? Quite odd to qualify media this way here, when most of all media uses this mechanism. We also forgot politicians who are experts in this field.
Ctrl + F "only" is only in your messaging not mine. I never said they were the only ones doing this? It's not just politicians, celebrities know about this and will use it to their advantage. Whoever makes the headlines first might have a stronger sway over their adversaries. I'm not even poking at any side in particular, this is reality across the board unfortunately. People will just blindly take and believe the primary headlines.
What I don't understand is if they were going to translate Zig to unsafe Rust, why not just build a translation tool for it? You could do a one-to-one mapping of language constructs, hardcoding patterns in your codebase, and as one friend put it "Tbh they could've just hooked up zig translate-c to c2rust". They would get deterministic translation, would probably have not been a heavy investment to build, and the output would have the same assurances as the input.
In this case, I would trust the output even less than the input. The input was memory-unsafe but hand-written. The output is memory-unsafe but also vibe-coded and has had no eyeballs on it. What is the point of abusing agentic AI for this use-case?
> "Tbh they could've just hooked up zig translate-c to c2rust".
Have you ever seen what comes out of c2rust? It's awful. It relies on a library of functions which emulate unsafe C pointer semantics with unsafe Rust.
A few years ago, when I was struggling with bugs in OpenJPEG (a JPEG 2000 decoder), someone tried running it through c2rust. The converted unsafe rust segfaulted at the same place the C code did. It's compatible, but not safe.
Main insight: don't do string manipulation in C or unsafe Rust. It's totally the wrong tool for the job.
This is awful. They have some internal string format borrowed from a Zig library where the address of the item is in the low end of a pointer and the length is at the high end. Why are they doing that in 2026? It lets you save a few bytes at best. It doesn't enforce the Rust rule that strings must be strict UTF-8. It's totally alien to the safe way Rust handles strings.
> The converted unsafe rust segfaulted at the same place the C code did. It's compatible, but not safe
That is indeed the point of c2rust. It gives you a baseline that is semantically identical to the original codebase, and with that passing the full test suite, bug-for-bug, you can then start gradually adopting rusty idioms to improve the memory safety of the codebase.
What comes out of c2rust is not intended for human consumption. It's more verbose than the original and harder to work on, but no safer. You lose the C idioms that people understand, while not gaining Rust idioms. It's like working on compiler-generated assembly code by hand.
2022 discussion on HN.[1]
There's a DARPA funded effort called TRACTOR, Translate All C To Rust, which has funded some efforts to develop a usable translator.[2] It's about 10 months after award, with no reported progress. I've been checking the personal sites of the academics involved, and they barely mention the project, although $5 million has been allocated to it.[3] The approach comes from U.C. Berkeley - let the LLM generate slop, check it using formal methods.[4] Not expecting near-term results.
Sure, but the point remains. They could've used Claude to build a Zig to Rust converter, ended up with something that was both deterministic _and_ beneficial to the wider community.
I mean, LLMs have been really good at translating code for a while now, which is why I'm more surprised that others are surprised this happened. They claim its a marketing trick despite the fact that they have to manage and maintain a fork of Zig if they don't switch languages.
“Tbh they could've just hooked up zig translate-c to c2rust”
This doesn’t work like you think it does. These things are full of errors and make the code very verbose and hard to reason about. It works with small apps, not entire rewrites.
That would have been the proper way to port a codebase to another language, by parsing the syntax tree and applying deterministic and verified transformations.
>What I don't understand is if they were going to translate Zig to unsafe Rust, why not just build a translation tool for it?
This would require experience, judgement, and most of all humility. Some vibecoders, in all their hubris, try to build a Tower of Babel with non-deterministic models and are shocked when the project inevitably fails.
said it in another comment [0] - that the whole rewrite thing is just a marketing exercise by LLM merchants to try sell you plebs that their wares "work"
This doesn't seem surprising, given the straight translation that they prompted.
Couldn't a case be made that it's better to get Bun to the to the language with the stronger type system first and, once there, use that stronger type system as leverage for these kinds of improvements as a follow-on effort? It seems preferable to requiring perfection on the very first step.
> Couldn't a case be made that it's better to get Bun to the to the language with the stronger type system first and, once there, use that stronger type system as leverage for these kinds of improvements as a follow-on effort? It seems preferable to requiring perfection on the very first step.
This is what they are doing.
They are working through the issues as they come in.
It's not surprising that a mostly straightforward translation to (partly unsafe) Rust exhibits UB.
What is a bit disappointing is that the Rust code apparently has APIs that aren't marked unsafe but may cause UB anyway. When doing this kind of translation, I'd always err on the side of caution and start by marking all/most things unsafe. Or prompt the slopbots to do the same I guess.
Then you can go in and verify the safety of individual bits step by step.
I was a little shocked that they could get it fully working in a week to be honest. My side project is a very similar ambition (https://tsz.dev) but I am in no way claiming success. i keep adding more and more tests to ensure things works. Even after all of TypeScript's own tests pass I am finding bugs which I was totally expecting.
The bar for matching tsc's behavior is really _really_ high. see:
I suspect they've been planning this and experimenting for many months. Along with the large existing test suite, they have lots of tooling for parallelizing agents and an unlimited token budget. So don't feel too bad..
I'm stunned that it went from 'this is an experiment' to merging a ~million lines of (likely) unreviewed code in a week. I have nothing against using agents but to rush something like this and leave the community blindsided seems extremely ameteurish. Like something you'd expect a bright eyed graduate engineer to do.
tsz for me is an experiment to see how can this kind of work be done better. With a slight difference that tsz is not a direct port and it's a different architecture. I'm also not claiming to have answers but I've learned a ton. A few things that works
- Test before code, Bun had lots of test so that's good but maybe they could start by asking Mythos to write like 20k additional tests that pass on Zig Bun first.
- Deterministic anti-slop features. LLMs love to solve the problem in the wrong abstraction layer or place. There are many ways to catch this with deterministic tests. I do this in tsz a lot
- Roadmap that constantly evolving by humans.
- Taking a pause and looking how the progress is going and undoing slop
The issue isn't the existence of undefined behavior that miri would catch. The issue is exposing an API that allows undefined behavior from safe code - which miri only catches if you go write the test that proves it.
This isn't an all together unreasonable thing to happen during an initial port of code from an unsafe language. You can, and the bun team seems to be, go around later and make sure that the functions where you wrap unsafe code does so correctly. Temporarily in a porting stage incorrectly marking some unsafe functions as safe isn't a real issue. It's a bit strange to merge it into the main repo in this state, but not a wholly unreasonable thing to do if the team has decided that they're definitely doing this. The only real issue would be if they made an actual release with the code in this state.
It's also a bit unfortunate that they didn't immediately set up their tests to run in miri if only because LLMs respond so well to good tests - I know they didn't do this not because of this github issue (which doesn't demonstrate that) but because there's another test [1] that absolutely does invoke undefined behavior that miri would catch. Though the code it's testing doesn't actually appear to be used anywhere so it's not much of a real issue. That said it's obviously early in the porting process... maybe they'll get around to it (or just get rid of all this unsafe code that they don't actually need).
[1] https://github.com/oven-sh/bun/blob/4d443e54022ceeadc79adf54... - the pointers derived from the first mutable references are invalidated by creating a new mutable reference to the same object. In C terms think of "mutable reference" as "restrict reference which a trivial mutation is made through". It's easy to do this properly, derive all the pointers from the same mutable reference, it just wasn't done properly.
PS. Spamming github just makes people less likely to work in the open. Please don't. We can all judge this work just fine on third party sites.
PPS. And we might want to withhold judgement until it's in a published state. Judging intermediate working states doesn't seem terribly fair or interesting to me.
That kind of error was expected. I don't see it as an issue against the rewrite. They kept the stable versions on Zig in case ppl needs stability. Eventually, the errors will get fixed.
That kind of error was entirely avoidable. There are well-known tools in the Rust ecosystem that detect this kind of error and while the tools do not detect all instances of UB caused by mistakes in unsafe blocks, it's still considered good practice to run them.
Indeed. My point is that just using the standard tools in the Rust ecosystem - like miri - would have trivially uncovered this error before it made it to the mainline.
In any practical application there'll be a known set of errors and I'm generally fine merging code that has known deficiencies. But personally, I'd not condone merging anything that causes UB. It undermines such a fundamental guarantee of the language that it should be detected and eliminated. And bun certainly rises to the level of software where I'd expect that the project runs all available tooling to detect such cases. Especially if you LLM - code it. "Do not produce UB" should be part of the test harness.
I'm curious, but unable to ascertain, does the same problem exist in the original Zig code? Is this an issue introduced by the translation to Rust? Because if it is a problem that can be replicated in both code bases, it seems a point in Rust's favor, that the issue is easily identifiable with tools that exist in its ecosystem.
I'm also curious about that. One thing to keep in mind: the invariants you have to uphold in unsafe blocks are quite stringent. I expect that in some instances the Rust code has new UB due to this.
Not a single person on the Bun team nor Anthropic has yet done anything egregious to market this as anything but a swap to a more memory-safe language with better compiler guarantees.
Thus far most of the buzz and marketing has been entirely negative from people who are against AI.
My take is that most of the buzz is also tied to recent negative opinions of Anthropic themselves due to some of their recent decisions.
I dunno, I am commenting on it mainly because I find the intensity of the anger and accusations of bad faith to be pretty out of proportion with what's actually happening, and I kind of value pushing back on such things to try to moderate the tone of the discussion (not as a devil's advocate thing per se, but more I am more likely to comment if I feel like the average vibe is unreasonable).
The HN guidelines specifically ask you not to do what you're doing, and say what to do if you have a genuine concern:
> Please don't post insinuations about astroturfing, shilling, brigading, foreign agents, and the like. It degrades discussion and is usually mistaken. If you're worried about abuse, email hn@ycombinator.com and we'll look at the data.
If you can increase your reach, why not do it? Also, HN has better reputation among tech circles than Reddit and is less niche than other resources. Modern marketing hits everything.
HN is enormously influential for programmers and employees within the tech industry. Who happen to be exactly who Anthropic, and other AI companies, desperately need adoption from...
I think HN has an outsized influence in the industry, for its size. There are a lot of big tech employees and startup founders reading it. Account purchasing absolutely happens, I've discovered and gotten banned at least a dozen years-old accounts that were blatantly sold and puppeteered by bots in the past. The comments aren't obviously bot-written this time around, so I can't conclusively prove it happened in this case, but it is a thing that happens in general and something to be aware of. There's also vote selling to promote things onto the front page. Given how cheap shilling on HN is, and the fact that many will perceive it to be organic while always viewing straightforwards ads skeptically, I wouldn't be surprised if the cost:effectiveness ratio probably beats any other form of advertising.
Certainly disagree with "AIs are not good at writing Rust". We can discuss the pros and cons of AI coding in general but in my experience they do just as well with Rust as any other language. If anything I'm impressed with how seamlessly the models can work with Rust's ownership model.
Sure. I'm completely unaffiliated and think Zig's AI stance is ridiculous & politically-motivated and a port is absolutely justified if they will not budge. Apparently I am deeply in the minority.
The no-AI policy of the Zig compiler project is for the compiler, other projects can do whatever they want.
Bun's fork of Zig was just an unsound hack that at best would have produced a strictly inferior speedup compared to our current work with incremental compilation, which is already plenty usable:
> An example of this is the changes to type resolution which happened in the 0.16.0 release cycle—these didn’t affect users too much, but had big implications for the compiler implementation. Before those changes, the compiler’s behavior was often highly dependent on the order in which types and declarations were semantically analyzed by the compiler. Some orders might result in successful compilation, while others give compile errors. Single-threaded semantic analysis prevented these bugs from causing user-facing non-determinism. The rewritten type resolution semantics were designed to avoid these issues, but Bun’s Zig fork does not incorporate the changes (and has not otherwise solved the design problems), which means their parallelized semantic analysis implementation will exhibit non-deterministic behavior. That’s pretty much a non-starter for most serious developers: you don’t want your compilation to randomly fail with a nonsense error 30% of the time.
There is a reason why, zig is upholding the quality and they hate it.
Zig rejected Bun's proposed contribution because it was a bad contribution, which they explained at length. Zig should not be made to "budge" on bad contributions. It seems you think Zig is unreasonable for rejecting bad code that happens to also be AI-generated, but believe it's reasonable for a project to be forced to accept bad code because it is AI?
This case is wild and seems to perfectly encapsulate all the problems people complain about with vibecoded projects.
The "rewrite it in rust" commit is +1M lines of code. Humans haven't looked at that in depth. In about a week, they saw the tests passed and pushed it to main. Now people have started to look through it and are pointing out glaring issues. And the solution is just going to be "feed it to another AI and ask it to fix it".
The entire codebase is slop now. Nobody knows what it does. It manages to pass some tests, but its largely a black box just on the basis of humans haven't read it yet. The code isn't guaranteed to be anything close to 1:1 with the old codebase. Its probably vaguely shaped like the old codebase, but new bugs could be there, old bugs could be there, nobody knows anything yet.
Its going to be interesting to see how recoverable this is. They are almost certainly going to just hand every file to an AI, say "look for soundness issues and fix them" and then what? If AI is making huge,sweeping changes to the code so frequently that humans can't keep up, is that really maintainable? The only solution appears to be "even more AI" while anybody that looks closely gets scared away by the too-large-to-comprehend-and-entirely-slop codebase.
This kind of thing has been happening with many smaller projects already, but now its a larger project and happening in a much more public way, with the intent to replace human-written, mostly-understood code with slop. I suspect the same thing, with the same problems, is happening inside all the largest companies, just not quite as obviously.
Sorry wasn't there a post literally like a week ago about this being a long term experimental branch and how we needed to not kick the hatchling while it's an egg?
Man that issue got way too many comments from non-contributors. I agree that this shouldn't have been merged in in it's current state, but that doesn't mean posting about it on GitHub is a worthwhile way to fix the problem.
Maybe they want a quick switchover and the UB is replicating existing problems so it is net neutral for the codebase (but positive future coz developers can do future work on rust without synchronizing two codebase? ).
exactly. If they wanted to iterate on their port they would add lifetime annotations here, which are the tool Rust be uses to ensure safety. They're just kicking the unsafety block down the road. This accomplishes nothing and is not how you get Rust to deliver its safety promise.
I think the only way to interpret a one million line LLM-generated diff with no proper reviews as an employee of Anthropic is that my company no longer has an interest in understanding, or even looking at, its own code.
I'd be concerned that by jumping onboard with this sort of development process I'd lose touch with how to engineer software in a detail-oriented or remotely rigorous way.
It also makes me question what sort of value the entire Bun project ever had if a drop-in replacement can just be thrown in here like it's nothing. Why do we need all these JS runtimes again?
The AI bubble is so large that we've also forgotten how useless and dumb a lot of software engineering labor was even before LLMs came along. We were already in a bubble.
All that is to say, I think it's useful to reframe some conversations about AI as, "if AI can accomplish this task, was it ever actually valuable?" I think for some specific things, the answer will be yes, but the tech industry has been huffing its own farts for so long I really don't think anyone has sight anymore of what's economically valuable in a ground truth sense. Much like LLMs themselves, this confusion pollutes the entire well of discourse about their economic utility.
This was 100% a predictable outcome after Bun was acquired. Of course they were going to do something like this.
What would have been significantly better is just rewriting Claude in a language that's actually well suited to what it's doing in the first place (which could well be Rust, Codex is written in it as prior art). It's funny how the vibe coding promoters are keen on things like this, rewriting other codebases as fast as possible with little quality checking, but they are still defensive of their own code.
Step 1: Vibe-code a buggy, poorly-performing, 500k+ LoC desktop-installed monstrosity in TypeScript to implement a trivial TUI. Proudly note that you’re meeting a 16ms frame budget … for a trivial chat UI.
Step 2: Purchase an entire company for a product that, if you squint, might help paper over the entirely predictable problems that arise from using the wrong tools to implement the wrong architecture, because surely the solution isn’t reevaluating your original engineering choices.
Step 3: Perform a buggy, vibe-code rewrite of the tool you just bought. A tool you only need because — for whatever internal political reasons — sunk cost means you can only keep digging.
"unsafe" is a promise to the compiler that you're going to ensure invariants that the compiler can't check. Rust only promises to eliminate UB if the invariants are held. You can still get UB by violating that promise, as this bug demonstrates.
> I thought the unsafety couldn't "spread" like that in Rust.
The goal of a library is to provide the encapsulation such that the unsafety doesn't spread.
If undefined behavior occurs, the fault lies with whoever wrote `unsafe { ... }` in the body of a function. If I write "unsafe" in order to call an unsafe library function, and I don't meet the library function's pre-requisites, then it's my fault. If the library internally writes "unsafe" in order while providing a safe wrapper, and I never actually wrote `unsafe { ... }`. If neither I nor the library wrote `unsafe { ... }`, then it is the fault of the compiler.
Using "in safe Rust" means that `unsafe` doesn't occur either in the user code nor in the library. In this context, since we've heard how many uses of `unsafe { ... }` exist in the Bun rewrite, I'd read "in safe Rust" to mean "without calling any functions marked as unsafe".
That is not Rusts guarantee. The guarantee is that safe rust cannot in itself introduce UB - UB can only ever be introduced in unsafe blocks, but it can then materialize in safe code.
I can't tell if you're trolling but `unsafe { crash() }` is safe from the compiler's perspective. Otherwise you wouldn't be able to achieve anything in 'safe' rust, even print to stdout.
I think its a good question, just because the whole UB thing is such an ideological shibboleth.
Maybe its better to think about this in the reverse, where C and C++ has 'defined behavior', but unsafe rust intentionally does not, its just whatever the complier and platform lets you get away with. Ultimately its still just a computer which stores values in memory and jumps to subroutines.
it's more straightforward to write safe rust when rust owns everything, In real world you often are interfacing with underlying libs or systems etc, which you need to treat as invariants but also handle yousrelf manually to make guarantees to compiler. unsafe exists in tons of codebases it's just you have to make sure you encapsulate it properly, which is what this bug is.
Unsafe code can break certain invariants of Rust, as `unsafe` is just a compiler "hold my beer" flag, which is why you're meant to do safety checks in your safe interface around unsafe code. If the unsafe code is wrapped in a way that does no guarding (or does something stupid in general), it is technically marked safe (because you said "rustc, hold my beer" as `unsafe` is also a contract) despite actually being unsafe
Rust has lots of undefined behavior, in general a broadly similar set to that which exists in C. What Rust does that is different is that to trigger undefined behavior, you need to execute unsafe code. (This isn't the same as saying that you have to be in unsafe code--you can violate a precondition in unsafe code and have the UB itself trigger in safe code).
I'm sure there have been attempts at defining a language that has no UB, but afaik all meaningful languages have UB in some dark corner or enumerated explicitly. For example, Java thread execution order is UB.
In this context "UB" means something different than how you're using it. The UB being mentioned here is the "nasal demons" form, i.e., programs which contain undefined behavior have no defined meaning according to the language semantics.
What you're talking about is probably better described in this context as "unspecified behavior", which is behavior that the language standard does not mandate but does not render programs meaningless. For example, IIRC in C++ the order in which g(), h(), and i() are evaluated in f(g(), h(), and i()) is unspecified - an implementation can pick any order, and the order doesn't have to be consistent, but no matter the order the program is valid (approximately speaking).
So this "unspecified behavior" might turn into the more nasal demon type when g(), h() and i() share mutable state and assume some particular sequential order of execution. No?
It is only allowed in unsafe blocks. As long as the unsafe blocks are few and well understood then Rust programmers can contain this to a small well defined portion of a program.
Unsafe Rust allows you to tell the compiler “hold my beer”. It’s a concession to the reality that the normal restrictions of Rust disallow some semantically valid programs that you might otherwise want to write. The safeguards work great in most cases, but in some they’re overly restrictive.
In practice, the overwhelming majority of code is able to be written in safe Rust and the compiler can have your back. The majority of the rest is for performance reasons, interacting with external functions like C libraries over FFI, or expressing semantics that safe Rust struggles with (e.g., circular references).
OK but the title says "in safe Rust". Am I misunderstanding something? All the replies here are saying how it's allowed in unsafe Rust, which is not what the title says.
If code in an unsafe block triggers undefined behavior, then the assumptions the compiler makes regarding safety will no longer be true, and purely safe code (code with no unsafe blocks) is no longer guaranteed to be safe. This is what's happening in the example the person on Github wrote in the issue.
Unsafe blocks are you saying to the compiler 'trust me bro, I know this is safe'. But often that relies on some property of the code being true in order for it to actually be safe. Generally speaking, the expectation in rust is that you either encapsulate the code that enforces whatever property you are relying on behind a safe interface, so that it's not possible for other code to use it unsafely, or that you mark the interface itself unsafe so that it's obvious that the code using that interface needs to maintain that property itself. Rust code that doesn't do this will generally be considered buggy by most rust programmers (e.g. if you find a use of safe interfaces in the stdlib that causes a memory safety violation, then you should file a ticket with the rust team), but this is essentially only a social convention of where the blame lies for a bug, not something that compiler itself can enforce (and, for example, you can violate memory safety in rust with only safe std interface by abusing OS interfaces like /proc/self/mem but this is something that most people don't think can be reasonably fixed). The main reason that rust as a language is better in this regard is that it gives much better tools for being able to express that safe interface without giving up performance and that it has the means to mark and encapsulate this safe/unsafe distinction.
Here's some links on this topic which have some examples:
Dumbest point ever.
There is no value for this issue.
I don't agree with the way they did the rewrite, but they did the rewrite, and this post contributes nothing, beside making the author seem childish.
If it had any real contribution I would have waved it off, but it really doesn't.
This tribalism and "I'm better than you"-ism and the same reason everybody hated the stack overflow community, and the rust community as well.
I agree. I'm as skeptical as many commenters but I also think the degree of polarization in HN around this technology and the degree to which people are calling those with different views shills or naysayers is pretty sad.
So many people are fundamentally misunderstanding everything about this rewrite.
In fact using the word "rewrite" itself is pretty inaccurate.
As has been mentioned the goal was a port so they "could" eventually rewrite most of it to be idiomatic rust. The main benefit of this now is the compiler and being able to use these tools to fix issues that were already being hidden when it was in zig.
If you go into this codebase expecting to see idiomatic rust and get angry when it's not there, you are going in with the entirely incorrect attitude.
It's understandable how people see it as AI slop or whatever given the division among developers at the moment. But please see it for what it is instead of just jumping to conclusions.
> As has been mentioned the goal was a port so they "could" eventually rewrite most of it to be idiomatic rust.
They may have said that, but quite clearly the value they actually get out of it is getting the headline "AI reimplements complex, broadly used software in 2 weeks, but makes it way better because it's rust now" in front of a million people's eyes, only 1% of whom will ever find out it was mostly fluff
> quite clearly the value they actually get out of it is getting the headline
This is entirely disingenuous. Jarred has already made it clear what value they get out of moving off of Zig. Yes they used AI heavily to attempt this goal but I don't see what the big issue is. They haven't even released it yet and Anthropic themselves have said 0 about this.
The "headlines" thus far are really just people completely uninvolved with Bun and with all to gain by perpetrating "AI BAD".
My honest take: the big issue isn't "what if it goes wrong" its the fear that a migration of this size works out of the box and being done almost entirely by AI.
This asymmetry is well understood by marketing and PR professionals, and actively exploited.
[0] https://en.wikipedia.org/wiki/Trust_Me,_I%27m_Lying
Did they even claim it was "memory-safe"? Every discussion of this topic has had dozens of comments noting that their vibed codebase is bursting at the seams with unaudited unsafe blocks, lightly reviewed by people who seem to not only seem to not understand Rust, but who seem incensed at the idea of needing to understand any programming language in the first place.
They did cite Rust's safety as a motivating factor for the port. That doesn't imply trying to achieve that simultaneously with the language change — which is good, because that would be insane. (Or, if you prefer, even more insane.)
You cannot faithfully port a codebase to a new language while also radically re-architecting it. You have to choose.
They want the safety benefits of Rust going forward; i.e., after it's finished, when they then write new code in Rust.
The newer posts go into detail about the rearchitecting that follows.
To be fair this is the type of reaction to be expected with all of the evangelism going around touting Rust as some pure beast incapable of error.
> these are the exact same arguments we see from people advocating continued use of C++.
Not really - people pushing Rust claim that switching to their new found lord and saviour will cure all evils and save everyone. The truth is - as you're pointing out - the bugs stay, maybe even more are created too.
Ports/re-architectures/rewrites always do this - promise to end all ills, and failing to deliver.
In your quote, there is no time-dependency between the lie and the truth. Whereas here, it's an attractive lie (easily parsed, great narrative), followed up by truths (that need more than surface-level analysis).
The rewrite was a code translation meant to be a starting point.
> a big, flashy announcement (here: bun was re-written in memory-safe rust in a couple weeks), and the relatively small reach of a correction (often just a footnote on an old article, here a GH issue).
The Bun team never made a big announcement that the code is now memory safe. They've been clear that this is the starting point.
Anyone expecting it to be perfect immediately and to have solved all of the memory problems in the original Zig code is arguing with an announcement they imagined, not what the Bun team has said.
Did anyone try to map this code back to the original codebase to see if this memory problem exists in the original codebase?
In this case, I would trust the output even less than the input. The input was memory-unsafe but hand-written. The output is memory-unsafe but also vibe-coded and has had no eyeballs on it. What is the point of abusing agentic AI for this use-case?
Have you ever seen what comes out of c2rust? It's awful. It relies on a library of functions which emulate unsafe C pointer semantics with unsafe Rust.
A few years ago, when I was struggling with bugs in OpenJPEG (a JPEG 2000 decoder), someone tried running it through c2rust. The converted unsafe rust segfaulted at the same place the C code did. It's compatible, but not safe.
Main insight: don't do string manipulation in C or unsafe Rust. It's totally the wrong tool for the job.
This is awful. They have some internal string format borrowed from a Zig library where the address of the item is in the low end of a pointer and the length is at the high end. Why are they doing that in 2026? It lets you save a few bytes at best. It doesn't enforce the Rust rule that strings must be strict UTF-8. It's totally alien to the safe way Rust handles strings.
[1] https://github.com/oven-sh/bun/blob/main/src/bun_core/string...
That is indeed the point of c2rust. It gives you a baseline that is semantically identical to the original codebase, and with that passing the full test suite, bug-for-bug, you can then start gradually adopting rusty idioms to improve the memory safety of the codebase.
2022 discussion on HN.[1]
There's a DARPA funded effort called TRACTOR, Translate All C To Rust, which has funded some efforts to develop a usable translator.[2] It's about 10 months after award, with no reported progress. I've been checking the personal sites of the academics involved, and they barely mention the project, although $5 million has been allocated to it.[3] The approach comes from U.C. Berkeley - let the LLM generate slop, check it using formal methods.[4] Not expecting near-term results.
[1] https://news.ycombinator.com/item?id=30169263
[2] https://csl.illinois.edu/news-and-media/translating-legacy-c...
[3] https://chandrasekaran-group.github.io/
[4] https://metalift.pages.dev/
They did ;) a highly dynamic one...
This doesn’t work like you think it does. These things are full of errors and make the code very verbose and hard to reason about. It works with small apps, not entire rewrites.
AI disproportionately empowers the stupid and evil
This would require experience, judgement, and most of all humility. Some vibecoders, in all their hubris, try to build a Tower of Babel with non-deterministic models and are shocked when the project inevitably fails.
[0]: https://news.ycombinator.com/item?id=48078224
Couldn't a case be made that it's better to get Bun to the to the language with the stronger type system first and, once there, use that stronger type system as leverage for these kinds of improvements as a follow-on effort? It seems preferable to requiring perfection on the very first step.
This is what they are doing.
They are working through the issues as they come in.
What is a bit disappointing is that the Rust code apparently has APIs that aren't marked unsafe but may cause UB anyway. When doing this kind of translation, I'd always err on the side of caution and start by marking all/most things unsafe. Or prompt the slopbots to do the same I guess.
Then you can go in and verify the safety of individual bits step by step.
The bar for matching tsc's behavior is really _really_ high. see:
https://github.com/type-challenges/type-challenges
I'm not against using LLMs to write a lot of code. But verification should be 100x more robust now that we can output code at this rate.
- Test before code, Bun had lots of test so that's good but maybe they could start by asking Mythos to write like 20k additional tests that pass on Zig Bun first.
- Deterministic anti-slop features. LLMs love to solve the problem in the wrong abstraction layer or place. There are many ways to catch this with deterministic tests. I do this in tsz a lot
- Roadmap that constantly evolving by humans.
- Taking a pause and looking how the progress is going and undoing slop
- Fuzztest(https://github.com/google/fuzztest) style "trying to break things" with the powers of LLM
The issue isn't the existence of undefined behavior that miri would catch. The issue is exposing an API that allows undefined behavior from safe code - which miri only catches if you go write the test that proves it.
This isn't an all together unreasonable thing to happen during an initial port of code from an unsafe language. You can, and the bun team seems to be, go around later and make sure that the functions where you wrap unsafe code does so correctly. Temporarily in a porting stage incorrectly marking some unsafe functions as safe isn't a real issue. It's a bit strange to merge it into the main repo in this state, but not a wholly unreasonable thing to do if the team has decided that they're definitely doing this. The only real issue would be if they made an actual release with the code in this state.
It's also a bit unfortunate that they didn't immediately set up their tests to run in miri if only because LLMs respond so well to good tests - I know they didn't do this not because of this github issue (which doesn't demonstrate that) but because there's another test [1] that absolutely does invoke undefined behavior that miri would catch. Though the code it's testing doesn't actually appear to be used anywhere so it's not much of a real issue. That said it's obviously early in the porting process... maybe they'll get around to it (or just get rid of all this unsafe code that they don't actually need).
[1] https://github.com/oven-sh/bun/blob/4d443e54022ceeadc79adf54... - the pointers derived from the first mutable references are invalidated by creating a new mutable reference to the same object. In C terms think of "mutable reference" as "restrict reference which a trivial mutation is made through". It's easy to do this properly, derive all the pointers from the same mutable reference, it just wasn't done properly.
PS. Spamming github just makes people less likely to work in the open. Please don't. We can all judge this work just fine on third party sites.
PPS. And we might want to withhold judgement until it's in a published state. Judging intermediate working states doesn't seem terribly fair or interesting to me.
Yes, tools like Miri, which this very post is about.
I've seen large rewrites and migrations take both approaches -- in my experience, the former usually works out better.
Isn't the whole point of AI companies using Rust that it's explicit, safe, and AIs are fairly good at writing it?
Thus far most of the buzz and marketing has been entirely negative from people who are against AI.
My take is that most of the buzz is also tied to recent negative opinions of Anthropic themselves due to some of their recent decisions.
* Make a huge deal out of it how “Claude Code enabled Bun team to rewrite 1+ mil of Zig lines to Rust” and write a blogpost, VCs are salivating
* Basic checks fail
* Let Mythos rip the codebase to shreds, spend God knows how much more
* Write a separate blogpost
* Charlatans and smooth brains clap and defend against “delusional anti-AI mob”
* VCs orgasm even harder
Clap, clap, clap. That’s how you make money, folks.
And btw, we need to get rid of software engineers now.
> Please don't post insinuations about astroturfing, shilling, brigading, foreign agents, and the like. It degrades discussion and is usually mistaken. If you're worried about abuse, email hn@ycombinator.com and we'll look at the data.
It’s just a standard Pavlovian response of a bootlicker, it can also be triggered if you mention “tax the rich” and “regulate AI hyperscalers”.
"Zig, let me Ai you"
"no"
*Ai's Zig fork, suffers from memory bugs*
"Well I'm moving!"
*Ai's code into Rust, suffers from memory bugs*
Bun's fork of Zig was just an unsound hack that at best would have produced a strictly inferior speedup compared to our current work with incremental compilation, which is already plenty usable:
- June 2025 core team starts using it with the zig compiler itself https://ziglang.org/devlog/2025/#2025-06-14
- April 2026 https://ziglang.org/devlog/2026/#2026-04-08
> Zig's AI stance is ridiculous & politically-motivated
It's literally an issue with our business model to mess with our contributor pipeline, can't get more concrete than this.
https://kristoff.it/blog/contributor-poker-and-ai/
> An example of this is the changes to type resolution which happened in the 0.16.0 release cycle—these didn’t affect users too much, but had big implications for the compiler implementation. Before those changes, the compiler’s behavior was often highly dependent on the order in which types and declarations were semantically analyzed by the compiler. Some orders might result in successful compilation, while others give compile errors. Single-threaded semantic analysis prevented these bugs from causing user-facing non-determinism. The rewritten type resolution semantics were designed to avoid these issues, but Bun’s Zig fork does not incorporate the changes (and has not otherwise solved the design problems), which means their parallelized semantic analysis implementation will exhibit non-deterministic behavior. That’s pretty much a non-starter for most serious developers: you don’t want your compilation to randomly fail with a nonsense error 30% of the time.
There is a reason why, zig is upholding the quality and they hate it.
The "rewrite it in rust" commit is +1M lines of code. Humans haven't looked at that in depth. In about a week, they saw the tests passed and pushed it to main. Now people have started to look through it and are pointing out glaring issues. And the solution is just going to be "feed it to another AI and ask it to fix it".
The entire codebase is slop now. Nobody knows what it does. It manages to pass some tests, but its largely a black box just on the basis of humans haven't read it yet. The code isn't guaranteed to be anything close to 1:1 with the old codebase. Its probably vaguely shaped like the old codebase, but new bugs could be there, old bugs could be there, nobody knows anything yet.
Its going to be interesting to see how recoverable this is. They are almost certainly going to just hand every file to an AI, say "look for soundness issues and fix them" and then what? If AI is making huge, sweeping changes to the code so frequently that humans can't keep up, is that really maintainable? The only solution appears to be "even more AI" while anybody that looks closely gets scared away by the too-large-to-comprehend-and-entirely-slop codebase.
This kind of thing has been happening with many smaller projects already, but now its a larger project and happening in a much more public way, with the intent to replace human-written, mostly-understood code with slop. I suspect the same thing, with the same problems, is happening inside all the largest companies, just not quite as obviously.
1 week turnaround I guess is what they meant.
But I think their true strategy is to have AI produce "fixes" like these which will end up infecting the entire codebase: https://github.com/oven-sh/bun/pull/30728
It's been like a day since the merge, presumably such followups are coming.
I'd be concerned that by jumping onboard with this sort of development process I'd lose touch with how to engineer software in a detail-oriented or remotely rigorous way.
It also makes me question what sort of value the entire Bun project ever had if a drop-in replacement can just be thrown in here like it's nothing. Why do we need all these JS runtimes again?
The AI bubble is so large that we've also forgotten how useless and dumb a lot of software engineering labor was even before LLMs came along. We were already in a bubble.
All that is to say, I think it's useful to reframe some conversations about AI as, "if AI can accomplish this task, was it ever actually valuable?" I think for some specific things, the answer will be yes, but the tech industry has been huffing its own farts for so long I really don't think anyone has sight anymore of what's economically valuable in a ground truth sense. Much like LLMs themselves, this confusion pollutes the entire well of discourse about their economic utility.
What would have been significantly better is just rewriting Claude in a language that's actually well suited to what it's doing in the first place (which could well be Rust, Codex is written in it as prior art). It's funny how the vibe coding promoters are keen on things like this, rewriting other codebases as fast as possible with little quality checking, but they are still defensive of their own code.
- Its a throw thing at the wall and see what sticks situation
- LLMs will improve*
- Using LLMs in an agentic way will improve (git worktrees, sliced PRs, spec driven steps)
So what happened here is a mess, but you gotta break a few eggs to make a souffle.
It's a learning step and I am glad it happened, there will be so many things to debrief from this.
I don't use Bun or Rust but fair play to them having a punt.
<Shameless plug> I have been working with Claude code to spec out and bring back to life a Spring Boot starter library for Apache Solr search
https://github.com/tomaytotomato/spring-data-solr-lazarus
There were a few points I had to steer it but the result has been a good implementation.
Step 2: Purchase an entire company for a product that, if you squint, might help paper over the entirely predictable problems that arise from using the wrong tools to implement the wrong architecture, because surely the solution isn’t reevaluating your original engineering choices.
Step 3: Perform a buggy, vibe-code rewrite of the tool you just bought. A tool you only need because — for whatever internal political reasons — sunk cost means you can only keep digging.
Step 4: ???
The goal of a library is to provide the encapsulation such that the unsafety doesn't spread.
If undefined behavior occurs, the fault lies with whoever wrote `unsafe { ... }` in the body of a function. If I write "unsafe" in order to call an unsafe library function, and I don't meet the library function's pre-requisites, then it's my fault. If the library internally writes "unsafe" in order while providing a safe wrapper, and I never actually wrote `unsafe { ... }`. If neither I nor the library wrote `unsafe { ... }`, then it is the fault of the compiler.
Using "in safe Rust" means that `unsafe` doesn't occur either in the user code nor in the library. In this context, since we've heard how many uses of `unsafe { ... }` exist in the Bun rewrite, I'd read "in safe Rust" to mean "without calling any functions marked as unsafe".
Maybe its better to think about this in the reverse, where C and C++ has 'defined behavior', but unsafe rust intentionally does not, its just whatever the complier and platform lets you get away with. Ultimately its still just a computer which stores values in memory and jumps to subroutines.
In this context "UB" means something different than how you're using it. The UB being mentioned here is the "nasal demons" form, i.e., programs which contain undefined behavior have no defined meaning according to the language semantics.
What you're talking about is probably better described in this context as "unspecified behavior", which is behavior that the language standard does not mandate but does not render programs meaningless. For example, IIRC in C++ the order in which g(), h(), and i() are evaluated in f(g(), h(), and i()) is unspecified - an implementation can pick any order, and the order doesn't have to be consistent, but no matter the order the program is valid (approximately speaking).
So this "unspecified behavior" might turn into the more nasal demon type when g(), h() and i() share mutable state and assume some particular sequential order of execution. No?
Unsafe Rust allows you to tell the compiler “hold my beer”. It’s a concession to the reality that the normal restrictions of Rust disallow some semantically valid programs that you might otherwise want to write. The safeguards work great in most cases, but in some they’re overly restrictive.
In practice, the overwhelming majority of code is able to be written in safe Rust and the compiler can have your back. The majority of the rest is for performance reasons, interacting with external functions like C libraries over FFI, or expressing semantics that safe Rust struggles with (e.g., circular references).
Here's some links on this topic which have some examples:
https://doc.rust-lang.org/nomicon/working-with-unsafe.html https://www.ralfj.de/blog/2016/01/09/the-scope-of-unsafe.htm...
And it's not like Bun when written in Zig has been a beacon of stability either. It has been segfaults all over the place.
I don't see what the big deal is here.
In fact using the word "rewrite" itself is pretty inaccurate.
As has been mentioned the goal was a port so they "could" eventually rewrite most of it to be idiomatic rust. The main benefit of this now is the compiler and being able to use these tools to fix issues that were already being hidden when it was in zig.
If you go into this codebase expecting to see idiomatic rust and get angry when it's not there, you are going in with the entirely incorrect attitude.
It's understandable how people see it as AI slop or whatever given the division among developers at the moment. But please see it for what it is instead of just jumping to conclusions.
They may have said that, but quite clearly the value they actually get out of it is getting the headline "AI reimplements complex, broadly used software in 2 weeks, but makes it way better because it's rust now" in front of a million people's eyes, only 1% of whom will ever find out it was mostly fluff
This is entirely disingenuous. Jarred has already made it clear what value they get out of moving off of Zig. Yes they used AI heavily to attempt this goal but I don't see what the big issue is. They haven't even released it yet and Anthropic themselves have said 0 about this.
The "headlines" thus far are really just people completely uninvolved with Bun and with all to gain by perpetrating "AI BAD".
My honest take: the big issue isn't "what if it goes wrong" its the fear that a migration of this size works out of the box and being done almost entirely by AI.