[kristoff_it]

To me the most interesting fact about this fork is that it has proven that Bun could have had fast builds all along.

To be fair, there are caveats still in place today: Zig incremental compilation does not yet support aarch64 and only the linux linker supports binary patching, but it's just a matter of time before all major platforms are conquered.

[dtj1123]

After the all the noise surrounding the forking of the Zig compiler in order to speed up builds, I'm really surprised that this isn't the top comment. The fact that a one-man team achieved 1s build times proves pretty conclusively that slow build times were entirely result of negligent development practices, and that the fork was a complete misallocation of time.

[hoppp]

The port was a political decision. It's now the runtime for claude code first and it must align itself in that ecosystem by becoming fully vibe code.

[IshKebab]

I'm pretty sure they forked Zig long before it supported incremental compilation. In fact, it's still experimental and not enabled by default.

[skeledrew]

A significant part of the port was also for the built-in safety. Unsafe code just... fails to compile.

Turns out there's a journey here.

[GuB-42]

GP is talking about the fork of the Zig compiler, not the rewrite of Bun into Rust.

Anthropic submitted to patch the Zig compiler to improve Bun compilation times, it was rejected. One reason it was rejected is that it contained AI-generated code, which is against the community guidelines, but more importantly, it would have been rejected regardless of AI use. They claimed a 4x improvement, something like going from 2 minutes to 30 seconds, but, according to the Zig team, it made compilation non-deterministic, and missed on the real improvement they worked on, which was sub-1s incremental builds.

"Buz" shows that the fork submitted by Anthropic wasn't needed as Bun can now compile under 1s (incrementally), which is much better than the 30s they achieved with their patch.

It is likely that this rejection, especially the "no AI" aspect of it encouraged Anthropic to move away from Zig.

[dminik]

Is there anything suggesting that Bun even attempted to submit their Zig patches? As far as I know, they were stuck 2 versions behind. With the amount of changes and Zig already working on incremental I'm not sure what the point would have been.

[kristoff_it]

Yes it was never attempted, the whole communication on social media about the AI policy was completely detached from the reality of the engineering work being done on either side.

[dtj1123]

Yep, on rereading my comment I totally failed to make this clear. Cheers.

[Philip-J-Fry]

The port was line for line and full of unsafe code. It didn't prevent any bugs. They're not using Rust for its strengths.

[skeledrew]

You should give the article[0] a read.

"A large percentage of bugs from that list are use-after-free, double-free, and "forgot to free" in an error path. In safe Rust, these are compiler errors and RAII-like automatic cleanup with Drop. Compiler errors are a better feedback loop than a style guide."

"At the time of writing, about 4% of Bun's Rust code sits inside an unsafe block (~13,000 unsafe keywords across ~27,000 lines / ~780,000 lines), and 78% of those blocks are a single line — a pointer that came from C++, or one call into a C library. I expect this number to go down over time as we refactor from a faithful Zig port (which had no greppable unsafe keyword) to idiomatic Rust, but we are going to continue using C & C++ libraries like JavaScriptCore so it will always have more unsafe than pure Rust projects."

[0] https://bun.com/blog/bun-in-rust

[bunderbunder]

That one’s interesting to me because it speaks to both sides of the debate. Yes, safe Rust is good for catching those kinds of errors at compile time. But also, it should theoretically be very easy for a compiler to avoid those problems in Zig, too, if you are using the language the way it wants to be used.

Which, from what I’ve experienced so far, does seem to take a whole lot more effort if you’re using a coding agent. I spend an incredible amount of time making sure mine doesn’t bloat our codebase with Java-flavored Python, and all the noise and defects and performance problems that it brings.

[dnautics]

> But also, it should theoretically be very easy for a compiler to avoid those problems in Zig, too, if you are using the language the way it wants to be used.

agreed. in my side project im tinkering with a memory safety checker in zig that intercepts a compiler artifact, and it works better with idiomatic zig, problematic code is when you try to write c-isms in zig -- so i basicallt tell the checker to reject c-isms that create ambiguity for safety checking.

[skeledrew]

> using the language the way it wants to be used.

This sounds like just the coding conventions dependency they're trying to avoid.

[Dylan16807]

I'm optimistic for that refactoring. But if one in 30 lines sprinkled all over everywhere is unsafe then you haven't actually done much to improve safety. When you have a large percent of fully safe functions and modules, that's when you're making a difference. Counting the lines is not a very helpful metric.

[skeledrew]

> one in 30 lines

> Counting the lines is not a very helpful metric.

Seems self-contradictory.

And they did say they're working to reduce the unsafe code over time. That huge percentage of memory bugs that they were able to better triage and fix due to the port was also them improving safety.

[sfink]

The way I'm thinking of it is that they ported Bun from Zig to Unsafe Rust, and are still working on now porting it from Unsafe Rust to Rust (not 100% safe, but a reasonable amount).

[atombender]

The original Zig code is metaphorically one big unsafe block. Even if the Rust port is made up of 3% unsafe blocks, that still means 97% of the original Zig code has been made safe (in the Rust sense).

[voidmain]

The safety of safe rust is created by extremely strong and subtle invariants that unsafe rust must maintain perfectly (or all bets are off). The idea that a program with tens of thousands of lines of vibe coded unsafe Rust is memory safe is mind boggling.

[formerly_proven]

IME agents are extremely happy to write more or less convincing // SAFETY: comments (most Rust code in the training set has these just before unsafe { - also it often works in unit tests). However, miri remains broadly unimpressed by comments.

[ammar2]

Of note, even that 97% of original code that is now "safe in the rust sense" could violate invariants through safe blocks that cause memory-safety issues. I can't say exactly how the LLM-ported Rust code made use of unsafe but see https://www.ralfj.de/blog/2016/01/09/the-scope-of-unsafe.htm...

[jmull]

Not every line of zig is "unsafe".

[atombender]

No, but when the compiler isn't enforcing anything, how do you know which ones are safe and which ones are not? If we ignore declarations etc. and only consider statement lines that read or write memory, then any such line of Zig (or C, or C++, or any other unsafe language) can be classified as a potentially unsafe — not just today, but in the future, since safety bugs can be generated upstream by giving out pointers that aren't valid or concurrency-safe.

[audunw]

Since they did a one-to-one translation to start with, hypothetically, if these were the true numbers, you could probably get a static analysis tool to guarantee you that 97% of the zig code was safe and tell you which of the 3% were unsafe. Zig has a lot more syntax and conventions to encourage safe code compare to C after all. It just doesn’t have way to mark which parts are safe or unsafe.

An LLM could probably also trivially give you accurate reviews saying which parts were unsafe and in need of tests or reviews. I mean, considering their LLM budgets they could probably have had nightly reviews running every night for years before spending more than their Rust rewrite.

Not that I think Rust rewrite was a bad idea. Rust is a good fit for this kind of project. I say that as a Zig enthusiast. It’s just that their stated motivation and reported results are kinda BS. If they just wrote “we just like Rust and thought it’d be cool to see of LLM could do the whole rewrite”, and left it at that, I think it’d be a more honest description of the motivation. The rest is just rationalisation.

[Zakis1]

No you cannot get a static analysis tool to guarantee 97% of zig code was safe. Well, you can - the tool is called the borrow checker.

Do you know what's infinitely cheaper and faster than an LLM running nightly reviews of all of the code and infact mathematically provable that the code (and any new code) is memory safe? It's called the borrow checker.

[samwillis]

They have made it very clear that the mechanical translation is the starting point, and are refactoring to remove as many unsafe declarations as possible.

[insanitybit]

They turned every hidden memory safety bug into a grep'able memory safety bug. That's step 1 and a massive win.

[neuronexmachina]

That may have been the case for the initial iteration a few months ago, but is it still true?

[ozgrakkurt]

This is an example of technical problems being easy but organization being hard imo. It is stupid if you think about it from a theoretical perspective but they didn’t want to use this feature for some reason.

Probably because they did their fork of zig for fast compilation before and they didn’t want to just discard it and use this

[janpio]

Do the caveats mean that it could _not_ actually have had fast builds on all the platforms it supports? (Honestly asking if I connected the dots correctly here, this is not a snarky "actually" rethorical question...)