The linux distribution I co-maintain uses mold as our bootstrap linker to bootstrap rust itself, and it saved us -hours- on long version-by-version build chains. Mold being in c meant we could build it very early and use it as the default linker distro wide and enjoy build speedups everywhere.
Now sadly we will have to fork and maintain the c version as mold2 forever.
Rust is not actually the right tool for all problems.
Like some other commenters, I do not understand why using a cached binary isn’t sufficient. Why not use the mold3 binary and build that first, and then cache it and never rebuild it again? I’m guessing it has to do with hermiticity and build provenance guarantees in your distro.
Do you have any reading material that you can share that might help me understand better? Docs for the distro, or an issue tracker I can search through?
In short, the entire distro is always built in one-shot at any given commit, and we can only rely on cached binaries from a past release if they or nothing in their supply chains changed. Given rustc depends on almost everything, mold3 would be built far too late to be useful for the most expensive build in the whole tree, which is rust.
Recursive dependencies would break our threat model, so we cannot use any rust tools to bootstrap rust. We bootstrap rust from llvm which we bootstrap from gcc which we bootstrap from tinycc which we bootstrap from M2Planet and so on back to 186 bytes of machine code.
Anyone getting a rust package from stagex must be able to build the entire tree up until that package and get the same hash, with no binary dependencies, thus removing any trust in maintainers.
If you can boostrap one version of stagex (so that its mold is trustworthy), I don't see why you can't use that to build another version of stagex.
As you say, you can rely on the cached binary because nothing in the old version's supply chain would change; it is pinned, and as long as you can build that, everything is fine.
We use the llvm linker, lld, to link mold at the earliest point we can, then use mold exclusively after that. We are an LLVM native distro so we build llvm right away as the system compiler used to build the whole tree instead of gcc.
LLD would work fine for the whole tree, and did previously, but is much much slower than mold which is why we switched to mold.
Having to build all dependencies of rust including python and openssl and everything else before being able to use mold erases most of the full-tree build speedups as the path to rust is already about 80% of the full tree build time.
We are a distro that mandates independently verified 100% deterministic builds from source for any given release commit of the tree, so we have to build the whole tree several times for every release.
Can't you just keep using Mold2 for the initial toolchain bootstrap and then build mold3 with Rust as part of the "final" set of toolchains that are to be used to compile everything beyond that? It isn't perfect but it's congruent with how many other components in most open reproducible bootstrapping flows work; ie using GCC 4.4 or whatever just to bootstrap GCC 10. It's annoying but it only needs to be done once and then you just use it to bootstrap newer components forever. Doesn't help with compile times, though.
I would agree if not for the fact rust depends on almost the entire tree, so doing what you propose would increase our full tree verification time from 6ish hours to 12ish hours erasing any wins mold was giving us in the first place.
It is of extreme relevance for us as we have to re-bootstrap every time we change any dependency of rust, and rust depends on almost everything that is expensive to build in a toolchain tree.
I have been thinking about Rust bootstrapping recently. Couldn't a Rust compiler without borrow checker be put together relatively easily?
Assuming the source code contains no issues (which can be checked later once the Rust compiler is built), one could leave that piece behind, and take the shortest path from .rs to executed code (C transpilation, or even an interpreter).
Our tree is entirely reproducible and hermetic. In fact we are the only distro that does this 100%.
The problem is changing any dependency of rust, even python or perl or musl or openssl or the llvm stack or any dependencies of the llvm stack have the potential of resulting in a different hash for rust.
Any time a dependency is changed all decedents must be rebuilt. Which we must do very frequently. That is where mold saved us a ton of time.
That's why you build a stage2 compiler at least with stage0 being the latest prebuilt.
Your stage1 will be based on whatever version is your stage0, but your stage2 should end up identical to any other build with any other stage0. So you don't have to rebuild the full chain all the time.
And then preferably you build a stage3 with FDO, you can easily get 20% more speed with a good corpus of tests.
Do you have at least a distributed cache with I guess sccache to save time for each rebuild?
If the need is well justified, maybe there is great chance to ask adding #[no_mangle] and extern C support? Since release is very fresh. If that is causing the problem. Or is some dependency the issue?
We regularly and redundantly full source bootstrap our entire tree from 186 bytes of machine code due to a supply chain security policy that forbids trusting any single human or computer.
Bootstrapping rust with mold2 is going to be the likely play. My complaint is that now we must maintain mold2 forever now as mold3 rust edition is now incompatible with most of our tree which is made up of dependencies of rust.
Hopefully not much. I am just lamenting it is an extra dependency and chore we must carry and think about forever, and will have no official upstream support.
You could use https://github.com/thepowersgang/mrustc to get relatively modern Rust compiler (1.90 currently apparently, but every now and then that is updated). Then build newer rustc from there to reach the current 1.99.
But I don't get why some people are obsessed with bootstrapping. Yes it is good to be able to do it, but it isn't something you need to do regularly.
Especially since rust had a much better cross compilation story than C or C++ (not as good as go or zig though), so you don't need to bootstrap on a new architecture, just cross compile to it. Furthermore, new architectures for hosting a compiler (as opposed to just a target, like microcontrollers) is a rare event. Just something that happens every few years.
> But I don't get why some people are obsessed with bootstrapping.
It only matters if you have supply chain attacks in your threat model. Given they are up 400x since 2019, they should probably be in almost every threat model. Most distros operate on the honor system and that is not going to survive the post AI world.
That's why you have signed packages that everyone can rebuild and verify from the very early stages of bootstrapping to speed up the process and get back to a synchronization point that is reasonable.
Distros that are not fully hermetic and don't have reproducible packages (and there are many layers of reproducibility) will certainly have issues, but that's not a huge problem as long as you have a documented path to getting back to the current state. It doesn't need to be the fastest path, just a verifiable chain of trust.
I'm a bit confused why a decision to decrease the maintenance burden of Mold by switching languages makes Rust the wrong tool here? Fearless concurrency sounds like a huge benefit for what they're doing, given the resources they have.
It's simply an ordering problem. When building the entire world from scratch, usually the C and C++ toolchains are built near the very first, and Rust toolchains built somewhat later. Anything written in Rust must come after the Rust toolchain is built. You need a linker as part of your C++ toolchain, so it must be written in a language ready to go at that point. If it is written in C, you are done. If it is written in Rust you have to wait. So a Rust-based linker can no longer be used from that very, very early C bootstrap.
It's not a big deal for normal users, where you have Rust ready to go. Kind of a bummer in this case, but this is a specialized one.
You could. But now you are trusting an entirely new toolchain that can build the Rust-based linker, even if it is statically linked. This toolchain is much larger than a comparatively small and much more easily understood C bootstrap toolchain.
This effectively triples or quadruples (maybe even more) the amount of code you need to trust for the cold bootstrap.
Too many years at Google where the two terms are most often used interchangeably; which is imprecise, but most of the time it doesn't matter. Forgive me for being imprecise here.
The fact remains that rust adds yet another toolchain to the boot process that needs trust and verification.
Sorry to break it to everyone but Rust depends on python which depends on perl and openssl and the majority of any lean full source bootstrapped toolchain tree.
Unless the gcc rust engine is mature any time soon (lol), we have no path to use rust until very late game in a distro build.
The earliest we can bootstrap a go compiler is about 10 minutes. It builds directly from tinycc. Add 6 hours for our fastest compile of the shortest path to rust, with 192 cores.
I am a rust fan too, but it is the worst language to bootstrap, which is why for systems programming I still must often revert to C to have a small and reviewable and fast to build dependency surface.
LLVM is the default yes, there is also an experimental cranelift backend, which is all Rust. Not sure if it is good enough to build the compiler itself using it.
(There is also a GCC backend called codegen_gcc that is pretty far along. And a separate reimplementation of both the frontend and backend using gcc and C++, called gccrs, which is not nearly as far along.)
I'm pretty sure it does not support enough features to build the compiler (e.g. inline asm is not supported at all), and it will also be very, very slow to unusable.
One of the explicit goals for Mold 3.0 is to promote its use as the default linker in Linux distributions, and bootstrapping complexity is certainly worthy of consideration in that realm.
> We will then conduct extensive compatibility testing and work closely with Linux distribution developers to make it practical for them to adopt mold as /usr/bin/ld. Making this happen is one of our highest priorities for mold 3.x.
We are one of only two distros that uses mold as the default system linker. Unfortunately that will be stuck at mold2 until the full tree is built, but we can swap to mold3 for end user consumers of the tree. Sucks that we have to maintain and use both now.
There are classes of virus that are hard to detect. One is a compiler virus that passes itself from compiler to compiler. You only get rid of the vector by bootstrapping from 0.
We only re-bootstrap the layers which had dependencies change under them. Early parts of the tree rarely change so we often do not have to rebuild these across releases, but very late tree things like rust depend on almost everything and something in the rust dependency graph changes almost every release.
Using binaries from past releases is a strict downgrade in terms of verification speed, as it means a new independent reproducible build verifier must now build both trees, doubling the release verification time, and erasing any wins mold3 could otherwise offer.
Google can rely on lots of centralized internal provenance tooling to prove cached binaries are not tampered with to other Googlers but when the goal is proving end to end full source bootstrapped build integrity to any interested user from the public in the least time possible, the requirements are significantly higher.
Sure but that's a compiler bootstrapping problem. It doesn't answer the question: why do you need to bootstrap the toolchain to build the distro? You can reuse a trusted toolchain that's been safely bootstrapped .
>Rust is not actually the right tool for all problems.
"my problems (that are not mold's) are not solved by mold. How dare mold make those decisions?"
Maybe you should rewrite more of your linux distribution in rust so it's available earlier in the build process and get back to it being the default linker.
No, they were never one of mold's goals. You don't get to assign solutions to people _and then blame them_. Hyrum's Law being a load bearing part of your infrastructure isn't Mold's problem.
I can't really agree with that. If you ship your linker with a disclaimer saying 'don't use this for early stage stuff, one day we might decide to rewrite the whole codebase in a language that won't be available early enough' then you'd have a point. But if you didn't that is precisely the kind of use case where mold shines, and so inevitably it will be adopted. Your downstream should be precious. At a minimum you should communicate such intentions, do so timely and hear what others have to say, even if afterwards you decide to push on.
Pretty much every piece of open source software ships with a legal document saying that the software is provided as-is without any warranty or guarantee of functionality (i.e. https://github.com/rui314/mold/blob/main/LICENSE ). Unless I had a support agreement with the maintainers that superseded that document, I would not expect any special treatment from upstream. Personally, whenever I add a new dependency, I do it with the knowledge that I may have to either replace it or take on maintenance myself in the future.
I actually made a Linux distribution that relies on this being your latest comment, since you didn't say this wouldn't always be your latest comment. If you post any newer ones, it will break.
Are you seriously arguing that every open source project needs to anticipate every single way any of their users might use their software in the future, and also any future change they might want to make to the software, and write up a doc of disclaimers and caveats?
This is ridiculous. The amount of entitlement I'm reading here is gross. This is how you burn out maintainers and drive them away from open source.
> Your downstream should be precious.
No. Every open source maintainer is free to decide for themselves how much or how little they are willing to bend over backward for the sake of serving all possible user needs. If you don't like that, then feel free to build whatever you need yourself, from scratch.
If not, downstream may maintain mold2 themselves forever because their _extremely specific_ use case is neither a promise nor a valuable thing for mold to maintain. Debian understood this a while ago and isn't whining when they have to maintain their own fork. Maintainers maintain.
If enough downstreamers are unhappy about choices, they are also welcome to fork, until the base project is abandoned. Or they realize their usecases are extremely narrow.
(In addition, it's an extremely hypocritical and purist demand, because I am pretty certain that their distribution has, at some point, an arbitrary executable to make a compiler from. So they're probably okay with blobs, just not that one in particular.)
Yeah, for stuff we wanna rely on, this should ideally be the default. I'd go one step further and say no breaking changes past 1.0.0 at all. Instead people should favor creating entirely new projects (forks or not) and jump over to those, leaving the old one behind, if they want to do massive changes to something.
Of course, no one would be forced to do this, but it feels like if more did this, long-term supporting stuff that depends on those things would be a lot easier, if things could just be instead of changing under our feet all the time. Thank god for Nix and NixOS, even with their warts.
Luckily Mr Clanker can do most of the work for you at least (I myself have already had Claude/Codex rewrite a couple of Rust projects in C, worked great)
Was the conversion assisted? I get the sense that Mold's author is an extremely competent guy and it would be quite feasible for someone like him to do it on his own.
Oh what? That was not on my bingo card for 2026. Mold already was incredible when it still was in C++ and I assume it is even better now that it is in rust
If it was transpiled there would be what GNU calls a "preferred form" of the linker source in which it wasn't written in Rust.
This is true for - as an example - the WUFFS GIF decoder. You can get C which decodes GIFs and was transpiled from WUFFS, but that's awful code and nobody wants to modify that code, whereas the WUFFS source code for the decoder is fine.
When we look at rui314's changes to mold today after 3.0 release, they just modify the mold source code in Rust, as you'd expect if this was in fact now written in Rust.
Bounds checking is like 1% of the safety that Rust guarantees you. That's a false-equivalence. The bigger ones are around memory safety like use-after-free, double-free, and memory issues in the face of concurrency. Zig does not help you with that.
Bound checks, which are part of what you get with Rust, also exist in ReleaseSafe Zig, that is true. But if that is what you wanted to say, your statement is very confusing and also does not answer the GP. If you meant to say ReleaseSafe and Rust are equivalent, then this is just false. ReleaseSafe still has many UBs (most importantly use after free and double free), not to mention that Rust solves many problems at compile time and Zig only at runtime.
Sorry, I agree I was unclear and your restatement accurately reflects my intent. My biggest concern with Zig safety is that ReleaseFast turns asserts into assumptions, which is equivalent to injecting UB at every failed assert site. I doubt many users are aware of that "feature".
Zig makes you choose that at build time. You can use safe builds for development to catch bugs, and fast builds for release. You can even use safe builds in of the release, and optimize hot paths with fast builds.
In practice, projects written in Zig very much can choose both.
That’s not entirely true. There’s a third choice. You can go the TigerBeetle route (TigerStyle). Zig is extremely well suited for software where you just avoid dynamic memory allocation all together. It’s extremely fast, very effective and arguably very safe. But not suitable for all applications obviously.
There’s also another caveat that there’s an effort to build in build time static memory safety checks in a way that’s more general than Rusts borrow checker, through a kind of plug in system rather than forced into the language. It’s not part of mainline Zig yet but this seems to be the direction Andrew wants to go.
I'm curious about motivation - bounds checks for corrupted inputs seems like it would be one, but it also seems that fixing corrupted input handling in a C/C++ code base would not be too hard, and probably less of an effort? So why did you choose the rewrite?
And second, did you use any AI tools for the rewrite?
> fixing corrupted input handling in a C/C++ code base would not be too hard
The best programmers on the planet have tried and failed with this task for 50 years now, so I don't think this is true.
The main disadvantage of Rust right now is not supporting some more obscure platforms, but because mold wouldn't support them anyway I don't see that as a problem.
> The main disadvantage of Rust right now is not supporting some more obscure platforms
Last time I checked, I got impressed by the wide platform support, once you go down the tier list (https://doc.rust-lang.org/nightly/rustc/platform-support.htm...). What "obscure platform" specifically are you thinking about, that is currently missing from those lists?
LLVM doesn't have an equivalent annotated list, but it is missing alpha, bfin, c6x, fr30, frv, gcn, h8300, ia64 (aka Itanium), lm32, m32c, m32r, mcore, mep, microblaze, mmix, mn10300, moxie, nds32, nios2, pa (aka PA-RISC), pdp11, pru, rl78, rs6000, rx, sh (aka SuperH), storm16, v850, vax, and visium. For its part, LLVM does have some targets that GCC doesn't have (mostly related to GPU compilation).
The "big" targets that GCC has that LLVM lacks are Alpha, Itanium, PA-RISC, and SuperH, with Itanium being sufficiently weird that it's pretty firmly in the "fuck this" category from a maintainer's perspective, and people are actively ripping out support for it.
Just to be clear, I think this is a very small disadvantage. GCC and therefore C/C++ supports some old stuff like SuperH, Intel Itanium, PA-RISC and a bunch of microcontroller archs that LLVM does not.
However, there is now a Rust codegen plugin for GCC, so even this disadvantage is now basically moot.
I think the claim is that GCC supports a few obscure platforms that LLVM doesn't. Don't ask me which, that is just the claim that I heard multiple times. So it's not Rust that limits platforms, but LLVM. And they say the GCC backend efforts of Rust are meant to deal with that.
There are many. Two big ones are Alpha and PA-RISC. NetBSD and Linux continue to support both. Linux distro choices are pretty much limited to Gentoo though.
1. You've got the causality backwards. LLVM backends don't come for free, you have to write code to enable support for them.
2. LLVM does not support as many backends as GCC does, so even if you did get 100% of the LLVM supported backends up and running, you'd still be missing some.
Worth adding to this: there may also be code needed in the rust part (not just the llvm part) to enable support for a target triplet. There certainly will be for the OS and libc part of the triplet: what specific syscalls exist and should be used to implement the standard library on a particular flavour of BSD etc.
But even for the architecture part there can be differences. For example, my understanding is that a lot of the calling convention details need to be handled by the frontend in LLVM, leading rustc to duplicate logic from clang here. Things like varargs FFI with C code can be particularly gnarly.
My very naive understanding is that a part of what makes mold fast is concurrency, which I'd expect to be a lot more error-prone in C/C++. Not having to worry about data races might give more confidence with trying out more complex techniques for how to split up work in a way that ends up making things faster
It's possible to write safe code in C or C++ but it's extremely difficult to read existing code and prove it's safe, without using as much effort as it takes to write it in the first place. This includes the code you wrote last month whose surrounding code has changed. And you have to be right every time while the attacker only needs you to be wrong once. The problem is not writing the code, it is continually verifying it.
As someone who worked professionally with both C++ and Rust, I mostly agree with this, but I would say that writing the concurrent code in C++ is also hard.
The only way I found that works reliably is stick to a small well defined set of mostly safe primitives. E.g. at work we use message passing / event bus architecture everywhere, which works great for a robotics / industrial context. But even then, if you somehow mess up and have a variable accessed from event handlers in two different threads, it is tough to spot other than if you get lucky and observe it with a build using TSAN.
With rust that class of mistakes is just entirely eliminated, which makes it easier to to concentrate on the hard things that actually matter (like the domain specific logic).
I suspect fearless concurrency is another motivating factor. Better perf can be achieved by squeezing more parallelism, but without borrow checking it's difficult to do fine-grained parallelism correctly.
Apparently the old mold version had a couple of plain C source files in the source tree (both vendored 3rd party libs but also in the 'regular' source code), so technically C/C++ is correct in this case ...and looks like the Rust version also has some (very minimal) C code left (looks like mostly varargs stuff, I guess Rust doesn't have a concept of varargs?), so it could be called a C/Rust project ;)
The linux distribution I co-maintain uses mold as our bootstrap linker to bootstrap rust itself, and it saved us -hours- on long version-by-version build chains. Mold being in c meant we could build it very early and use it as the default linker distro wide and enjoy build speedups everywhere.
Now sadly we will have to fork and maintain the c version as mold2 forever.
Rust is not actually the right tool for all problems.
Like some other commenters, I do not understand why using a cached binary isn’t sufficient. Why not use the mold3 binary and build that first, and then cache it and never rebuild it again? I’m guessing it has to do with hermiticity and build provenance guarantees in your distro.
Do you have any reading material that you can share that might help me understand better? Docs for the distro, or an issue tracker I can search through?
https://stagex.tools
In short, the entire distro is always built in one-shot at any given commit, and we can only rely on cached binaries from a past release if they or nothing in their supply chains changed. Given rustc depends on almost everything, mold3 would be built far too late to be useful for the most expensive build in the whole tree, which is rust.
Recursive dependencies would break our threat model, so we cannot use any rust tools to bootstrap rust. We bootstrap rust from llvm which we bootstrap from gcc which we bootstrap from tinycc which we bootstrap from M2Planet and so on back to 186 bytes of machine code.
Anyone getting a rust package from stagex must be able to build the entire tree up until that package and get the same hash, with no binary dependencies, thus removing any trust in maintainers.
What _is_ your threat model?
If you can boostrap one version of stagex (so that its mold is trustworthy), I don't see why you can't use that to build another version of stagex.
As you say, you can rely on the cached binary because nothing in the old version's supply chain would change; it is pinned, and as long as you can build that, everything is fine.
Why not use LLD for linking Rust? You already must have the LLVM for Rust to be buildable, so that do not add any dependency.
We use the llvm linker, lld, to link mold at the earliest point we can, then use mold exclusively after that. We are an LLVM native distro so we build llvm right away as the system compiler used to build the whole tree instead of gcc.
LLD would work fine for the whole tree, and did previously, but is much much slower than mold which is why we switched to mold.
Having to build all dependencies of rust including python and openssl and everything else before being able to use mold erases most of the full-tree build speedups as the path to rust is already about 80% of the full tree build time.
We are a distro that mandates independently verified 100% deterministic builds from source for any given release commit of the tree, so we have to build the whole tree several times for every release.
Can't you just keep using Mold2 for the initial toolchain bootstrap and then build mold3 with Rust as part of the "final" set of toolchains that are to be used to compile everything beyond that? It isn't perfect but it's congruent with how many other components in most open reproducible bootstrapping flows work; ie using GCC 4.4 or whatever just to bootstrap GCC 10. It's annoying but it only needs to be done once and then you just use it to bootstrap newer components forever. Doesn't help with compile times, though.
I would agree if not for the fact rust depends on almost the entire tree, so doing what you propose would increase our full tree verification time from 6ish hours to 12ish hours erasing any wins mold was giving us in the first place.
Agreed -- not my wheelhouse, but couldn't you just build rust earlier in your process using LLD, then build mold, then build everything else?
Would that meant that building rust is slower, but everything else is the same, and you don't have to maintain a fork of another complex project?
Rust requires python and perl and musl and openssl and 80% of the entire wall time of building the whole tree.
Rust is the single most expensive thing to bootstrap in any given linux distro.
It is the thing you need mold the most for to speed things up.
Hopefully, someone can do some work on decreasing the requirements needed to build rust itself and ease bootstrapping.
Because mold is faster. Linking is a significant bottleneck when building large projects.
The complaint is about bootstrapping, where speed is of little relevance.
It is of extreme relevance for us as we have to re-bootstrap every time we change any dependency of rust, and rust depends on almost everything that is expensive to build in a toolchain tree.
But we're just talking for bootstrapping Rust here.
I have been thinking about Rust bootstrapping recently. Couldn't a Rust compiler without borrow checker be put together relatively easily?
Assuming the source code contains no issues (which can be checked later once the Rust compiler is built), one could leave that piece behind, and take the shortest path from .rs to executed code (C transpilation, or even an interpreter).
This exists: https://github.com/thepowersgang/mrustc.
It seems like you would benefit from having reproducible and hermetic builds with a good caching layer so you don't do the same work again and again.
Then you can just use the latest built version to build mold and the next version of rust.
Our tree is entirely reproducible and hermetic. In fact we are the only distro that does this 100%.
The problem is changing any dependency of rust, even python or perl or musl or openssl or the llvm stack or any dependencies of the llvm stack have the potential of resulting in a different hash for rust.
Any time a dependency is changed all decedents must be rebuilt. Which we must do very frequently. That is where mold saved us a ton of time.
That's why you build a stage2 compiler at least with stage0 being the latest prebuilt.
Your stage1 will be based on whatever version is your stage0, but your stage2 should end up identical to any other build with any other stage0. So you don't have to rebuild the full chain all the time.
And then preferably you build a stage3 with FDO, you can easily get 20% more speed with a good corpus of tests.
Do you have at least a distributed cache with I guess sccache to save time for each rebuild?
If the need is well justified, maybe there is great chance to ask adding #[no_mangle] and extern C support? Since release is very fresh. If that is causing the problem. Or is some dependency the issue?
The need here is bootstrapping. If mold is written in Rust you cannot even compile it.
Even Rust is written with Rust. But yeah, if you want to bootstrap absolutely from the beginning, I see the issue.
We regularly and redundantly full source bootstrap our entire tree from 186 bytes of machine code due to a supply chain security policy that forbids trusting any single human or computer.
Could you not either bootstrap rust with lld or bootstrap rust with mold2?
Bootstrapping rust with mold2 is going to be the likely play. My complaint is that now we must maintain mold2 forever now as mold3 rust edition is now incompatible with most of our tree which is made up of dependencies of rust.
How much work is involved with mantaining a linker in deep maintenance mode?
Hopefully not much. I am just lamenting it is an extra dependency and chore we must carry and think about forever, and will have no official upstream support.
how's bootstrapping rust story nowadays?
You could use https://github.com/thepowersgang/mrustc to get relatively modern Rust compiler (1.90 currently apparently, but every now and then that is updated). Then build newer rustc from there to reach the current 1.99.
But I don't get why some people are obsessed with bootstrapping. Yes it is good to be able to do it, but it isn't something you need to do regularly.
Especially since rust had a much better cross compilation story than C or C++ (not as good as go or zig though), so you don't need to bootstrap on a new architecture, just cross compile to it. Furthermore, new architectures for hosting a compiler (as opposed to just a target, like microcontrollers) is a rare event. Just something that happens every few years.
> But I don't get why some people are obsessed with bootstrapping.
It only matters if you have supply chain attacks in your threat model. Given they are up 400x since 2019, they should probably be in almost every threat model. Most distros operate on the honor system and that is not going to survive the post AI world.
That's why you have signed packages that everyone can rebuild and verify from the very early stages of bootstrapping to speed up the process and get back to a synchronization point that is reasonable.
Distros that are not fully hermetic and don't have reproducible packages (and there are many layers of reproducibility) will certainly have issues, but that's not a huge problem as long as you have a documented path to getting back to the current state. It doesn't need to be the fastest path, just a verifiable chain of trust.
I'm a bit confused why a decision to decrease the maintenance burden of Mold by switching languages makes Rust the wrong tool here? Fearless concurrency sounds like a huge benefit for what they're doing, given the resources they have.
It's simply an ordering problem. When building the entire world from scratch, usually the C and C++ toolchains are built near the very first, and Rust toolchains built somewhat later. Anything written in Rust must come after the Rust toolchain is built. You need a linker as part of your C++ toolchain, so it must be written in a language ready to go at that point. If it is written in C, you are done. If it is written in Rust you have to wait. So a Rust-based linker can no longer be used from that very, very early C bootstrap.
It's not a big deal for normal users, where you have Rust ready to go. Kind of a bummer in this case, but this is a specialized one.
Can't you just use a statically linked mold binary for the initial bootstrap, just like you use some kind of pre-existing, basic C compiler?
You could. But now you are trusting an entirely new toolchain that can build the Rust-based linker, even if it is statically linked. This toolchain is much larger than a comparatively small and much more easily understood C bootstrap toolchain.
This effectively triples or quadruples (maybe even more) the amount of code you need to trust for the cold bootstrap.
Why do you keep saying C? Mold was C++ with some dependencies like TBB or zstd.
Too many years at Google where the two terms are most often used interchangeably; which is imprecise, but most of the time it doesn't matter. Forgive me for being imprecise here.
The fact remains that rust adds yet another toolchain to the boot process that needs trust and verification.
Maybe that's about to flip. Maybe Rust will be built first, followed by C.
Sorry to break it to everyone but Rust depends on python which depends on perl and openssl and the majority of any lean full source bootstrapped toolchain tree.
Unless the gcc rust engine is mature any time soon (lol), we have no path to use rust until very late game in a distro build.
The earliest we can bootstrap a go compiler is about 10 minutes. It builds directly from tinycc. Add 6 hours for our fastest compile of the shortest path to rust, with 192 cores.
I am a rust fan too, but it is the worst language to bootstrap, which is why for systems programming I still must often revert to C to have a small and reviewable and fast to build dependency surface.
Rust requires LLVM (or GCC) which requires C++, not just C.
LLVM is the default yes, there is also an experimental cranelift backend, which is all Rust. Not sure if it is good enough to build the compiler itself using it.
(There is also a GCC backend called codegen_gcc that is pretty far along. And a separate reimplementation of both the frontend and backend using gcc and C++, called gccrs, which is not nearly as far along.)
I'm pretty sure it does not support enough features to build the compiler (e.g. inline asm is not supported at all), and it will also be very, very slow to unusable.
Maybe it should be rewritten in Rust then
One of the explicit goals for Mold 3.0 is to promote its use as the default linker in Linux distributions, and bootstrapping complexity is certainly worthy of consideration in that realm.
> We will then conduct extensive compatibility testing and work closely with Linux distribution developers to make it practical for them to adopt mold as /usr/bin/ld. Making this happen is one of our highest priorities for mold 3.x.
We are one of only two distros that uses mold as the default system linker. Unfortunately that will be stuck at mold2 until the full tree is built, but we can swap to mold3 for end user consumers of the tree. Sucks that we have to maintain and use both now.
Have you considered using Eurydice?
I am not finding any linkers by that name.
It's a rust to C transpiler, I was thinking it could let you use a C version of the mold linker without maintaining one yourself.
Interesting idea! Will look into this.
Why do you need to bootstrap a toolchain to bootstrap a distro?
I know this is common but it seems like either an aesthetic decision, or glibc cruft.
There are classes of virus that are hard to detect. One is a compiler virus that passes itself from compiler to compiler. You only get rid of the vector by bootstrapping from 0.
No, you can do bootstrapping and save binaries for reuse with hash verification. Android did that for its Rust toolchain: https://cs.android.com/android/platform/superproject/main/+/...
Bootstrapping at every build does not save you from the threat you think it does.
We only re-bootstrap the layers which had dependencies change under them. Early parts of the tree rarely change so we often do not have to rebuild these across releases, but very late tree things like rust depend on almost everything and something in the rust dependency graph changes almost every release.
Using binaries from past releases is a strict downgrade in terms of verification speed, as it means a new independent reproducible build verifier must now build both trees, doubling the release verification time, and erasing any wins mold3 could otherwise offer.
Google can rely on lots of centralized internal provenance tooling to prove cached binaries are not tampered with to other Googlers but when the goal is proving end to end full source bootstrapped build integrity to any interested user from the public in the least time possible, the requirements are significantly higher.
Sure but that's a compiler bootstrapping problem. It doesn't answer the question: why do you need to bootstrap the toolchain to build the distro? You can reuse a trusted toolchain that's been safely bootstrapped .
Because no other trusted toolchains exist under a threat model that trusts no single person or computer. We -are- the trusted toolchain.
>Rust is not actually the right tool for all problems.
"my problems (that are not mold's) are not solved by mold. How dare mold make those decisions?"
Maybe you should rewrite more of your linux distribution in rust so it's available earlier in the build process and get back to it being the default linker.
They were solved by mold. And then mold decided to un-solve them.
The mold developers are not responsible for what people want to do with their software.
No, they were never one of mold's goals. You don't get to assign solutions to people _and then blame them_. Hyrum's Law being a load bearing part of your infrastructure isn't Mold's problem.
I can't really agree with that. If you ship your linker with a disclaimer saying 'don't use this for early stage stuff, one day we might decide to rewrite the whole codebase in a language that won't be available early enough' then you'd have a point. But if you didn't that is precisely the kind of use case where mold shines, and so inevitably it will be adopted. Your downstream should be precious. At a minimum you should communicate such intentions, do so timely and hear what others have to say, even if afterwards you decide to push on.
Pretty much every piece of open source software ships with a legal document saying that the software is provided as-is without any warranty or guarantee of functionality (i.e. https://github.com/rui314/mold/blob/main/LICENSE ). Unless I had a support agreement with the maintainers that superseded that document, I would not expect any special treatment from upstream. Personally, whenever I add a new dependency, I do it with the knowledge that I may have to either replace it or take on maintenance myself in the future.
I actually made a Linux distribution that relies on this being your latest comment, since you didn't say this wouldn't always be your latest comment. If you post any newer ones, it will break.
Edit: darn, it broke
Are you seriously arguing that every open source project needs to anticipate every single way any of their users might use their software in the future, and also any future change they might want to make to the software, and write up a doc of disclaimers and caveats?
This is ridiculous. The amount of entitlement I'm reading here is gross. This is how you burn out maintainers and drive them away from open source.
> Your downstream should be precious.
No. Every open source maintainer is free to decide for themselves how much or how little they are willing to bend over backward for the sake of serving all possible user needs. If you don't like that, then feel free to build whatever you need yourself, from scratch.
>Your downstream should be precious
Is my downstream paying me for the maintenance ?
If not, downstream may maintain mold2 themselves forever because their _extremely specific_ use case is neither a promise nor a valuable thing for mold to maintain. Debian understood this a while ago and isn't whining when they have to maintain their own fork. Maintainers maintain.
If enough downstreamers are unhappy about choices, they are also welcome to fork, until the base project is abandoned. Or they realize their usecases are extremely narrow.
(In addition, it's an extremely hypocritical and purist demand, because I am pretty certain that their distribution has, at some point, an arbitrary executable to make a compiler from. So they're probably okay with blobs, just not that one in particular.)
> Your downstream should be precious.
Yeah, for stuff we wanna rely on, this should ideally be the default. I'd go one step further and say no breaking changes past 1.0.0 at all. Instead people should favor creating entirely new projects (forks or not) and jump over to those, leaving the old one behind, if they want to do massive changes to something.
Of course, no one would be forced to do this, but it feels like if more did this, long-term supporting stuff that depends on those things would be a lot easier, if things could just be instead of changing under our feet all the time. Thank god for Nix and NixOS, even with their warts.
What's that benefit of a new project and abandoning an old one when you can just as easily continue using the old version?
What? Why would mold be particularly valuable for early stage compilation, to the point that you’d explicitly cater to that user base?
Luckily Mr Clanker can do most of the work for you at least (I myself have already had Claude/Codex rewrite a couple of Rust projects in C, worked great)
And repeat that work every time you sync with upstream?
I expected this to be a multi-months rewrite, not 3 weeks. I almost forgot we live in the agents era now.
Edit: this seems to have been cooking for a while when the first commit dropped: https://github.com/rui314/mold/commit/f41bfcd5c72ca30cce6498...
Was the conversion assisted? I get the sense that Mold's author is an extremely competent guy and it would be quite feasible for someone like him to do it on his own.
> Was the conversion assisted?
Yes. Comment by mold's author:
https://www.reddit.com/r/rust/comments/1w45j6n/comment/p7ac5...
He decided to use LLM assistance exactly because he's an extremely competent guy.
Assisted, yes. Claude is a coauthor for some of the commits and I think he also said that explicitly. Vibe-coded, he claims not.
So what differentiates mold from wild now ? Is wild using different data structures?
(Wild is another fast linker, that only supports Linux)
Wild's primary purpose is incremental linking - I guess now the primary difference is a race + subtle differences in performance between projects.
wild's primary purpose is incremental linking, which it incidentally does not support at all, while still being faster than mold by a long shot.
Thanks for pointing out Wild.
Oh what? That was not on my bingo card for 2026. Mold already was incredible when it still was in C++ and I assume it is even better now that it is in rust
It was transpiled, not rewritten, rewrite implies by hand
If it was transpiled there would be what GNU calls a "preferred form" of the linker source in which it wasn't written in Rust.
This is true for - as an example - the WUFFS GIF decoder. You can get C which decodes GIFs and was transpiled from WUFFS, but that's awful code and nobody wants to modify that code, whereas the WUFFS source code for the decoder is fine.
When we look at rui314's changes to mold today after 3.0 release, they just modify the mold source code in Rust, as you'd expect if this was in fact now written in Rust.
No it doesn't.
Damn. I thought Zig would be a perfect rewrite language for Mold since it's a better C in many ways
Zig makes you choose either safe or fast, not both. With Rust you can get both (generally).
Part of the safety you get with Rust is runtime bounds checking, which is equivalent to Zig in "safe" mode.
Bounds checking is like 1% of the safety that Rust guarantees you. That's a false-equivalence. The bigger ones are around memory safety like use-after-free, double-free, and memory issues in the face of concurrency. Zig does not help you with that.
Bound checks, which are part of what you get with Rust, also exist in ReleaseSafe Zig, that is true. But if that is what you wanted to say, your statement is very confusing and also does not answer the GP. If you meant to say ReleaseSafe and Rust are equivalent, then this is just false. ReleaseSafe still has many UBs (most importantly use after free and double free), not to mention that Rust solves many problems at compile time and Zig only at runtime.
Sorry, I agree I was unclear and your restatement accurately reflects my intent. My biggest concern with Zig safety is that ReleaseFast turns asserts into assumptions, which is equivalent to injecting UB at every failed assert site. I doubt many users are aware of that "feature".
Zig makes you choose that at build time. You can use safe builds for development to catch bugs, and fast builds for release. You can even use safe builds in of the release, and optimize hot paths with fast builds.
In practice, projects written in Zig very much can choose both.
You make it sound as if all bugs could be catched in development builds.
Still doesn't have a good answer for use after free, although the allocators on the last release might help into that regard.
That’s not entirely true. There’s a third choice. You can go the TigerBeetle route (TigerStyle). Zig is extremely well suited for software where you just avoid dynamic memory allocation all together. It’s extremely fast, very effective and arguably very safe. But not suitable for all applications obviously.
There’s also another caveat that there’s an effort to build in build time static memory safety checks in a way that’s more general than Rusts borrow checker, through a kind of plug in system rather than forced into the language. It’s not part of mainline Zig yet but this seems to be the direction Andrew wants to go.
https://github.com/ityonemo/clr
When choosing a language, people don’t only look at language features but also at community adoption, the library ecosystem, industry backing etc.
Zig isn’t even in the same league as Rust regarding these things. Zig may still be around and active 10 years from now, Rust is guaranteed to be.
Additionally, Zig just released 0.17, Rust's 1.0 was 11 years ago. The choice may have been mostly for non-technical reasons.
I'm curious about motivation - bounds checks for corrupted inputs seems like it would be one, but it also seems that fixing corrupted input handling in a C/C++ code base would not be too hard, and probably less of an effort? So why did you choose the rewrite?
And second, did you use any AI tools for the rewrite?
> fixing corrupted input handling in a C/C++ code base would not be too hard
The best programmers on the planet have tried and failed with this task for 50 years now, so I don't think this is true.
The main disadvantage of Rust right now is not supporting some more obscure platforms, but because mold wouldn't support them anyway I don't see that as a problem.
> The main disadvantage of Rust right now is not supporting some more obscure platforms
Last time I checked, I got impressed by the wide platform support, once you go down the tier list (https://doc.rust-lang.org/nightly/rustc/platform-support.htm...). What "obscure platform" specifically are you thinking about, that is currently missing from those lists?
The list of supported GCC architectures is here: https://gcc.gnu.org/backends.html
LLVM doesn't have an equivalent annotated list, but it is missing alpha, bfin, c6x, fr30, frv, gcn, h8300, ia64 (aka Itanium), lm32, m32c, m32r, mcore, mep, microblaze, mmix, mn10300, moxie, nds32, nios2, pa (aka PA-RISC), pdp11, pru, rl78, rs6000, rx, sh (aka SuperH), storm16, v850, vax, and visium. For its part, LLVM does have some targets that GCC doesn't have (mostly related to GPU compilation).
The "big" targets that GCC has that LLVM lacks are Alpha, Itanium, PA-RISC, and SuperH, with Itanium being sufficiently weird that it's pretty firmly in the "fuck this" category from a maintainer's perspective, and people are actively ripping out support for it.
Just to be clear, I think this is a very small disadvantage. GCC and therefore C/C++ supports some old stuff like SuperH, Intel Itanium, PA-RISC and a bunch of microcontroller archs that LLVM does not.
However, there is now a Rust codegen plugin for GCC, so even this disadvantage is now basically moot.
Rewrite all the things!
The GCC backend is fairly complete but still does not support everything.
yeah this makes no sense to me. Why would Rust limit the LLVM backend's ability to generate code for a particular platform?
I think the claim is that GCC supports a few obscure platforms that LLVM doesn't. Don't ask me which, that is just the claim that I heard multiple times. So it's not Rust that limits platforms, but LLVM. And they say the GCC backend efforts of Rust are meant to deal with that.
> Don't ask me which
There are many. Two big ones are Alpha and PA-RISC. NetBSD and Linux continue to support both. Linux distro choices are pretty much limited to Gentoo though.
1. You've got the causality backwards. LLVM backends don't come for free, you have to write code to enable support for them.
2. LLVM does not support as many backends as GCC does, so even if you did get 100% of the LLVM supported backends up and running, you'd still be missing some.
Worth adding to this: there may also be code needed in the rust part (not just the llvm part) to enable support for a target triplet. There certainly will be for the OS and libc part of the triplet: what specific syscalls exist and should be used to implement the standard library on a particular flavour of BSD etc.
But even for the architecture part there can be differences. For example, my understanding is that a lot of the calling convention details need to be handled by the frontend in LLVM, leading rustc to duplicate logic from clang here. Things like varargs FFI with C code can be particularly gnarly.
Yep! All good info.
My very naive understanding is that a part of what makes mold fast is concurrency, which I'd expect to be a lot more error-prone in C/C++. Not having to worry about data races might give more confidence with trying out more complex techniques for how to split up work in a way that ends up making things faster
It's possible to write safe code in C or C++ but it's extremely difficult to read existing code and prove it's safe, without using as much effort as it takes to write it in the first place. This includes the code you wrote last month whose surrounding code has changed. And you have to be right every time while the attacker only needs you to be wrong once. The problem is not writing the code, it is continually verifying it.
As someone who worked professionally with both C++ and Rust, I mostly agree with this, but I would say that writing the concurrent code in C++ is also hard.
The only way I found that works reliably is stick to a small well defined set of mostly safe primitives. E.g. at work we use message passing / event bus architecture everywhere, which works great for a robotics / industrial context. But even then, if you somehow mess up and have a variable accessed from event handlers in two different threads, it is tough to spot other than if you get lucky and observe it with a build using TSAN.
With rust that class of mistakes is just entirely eliminated, which makes it easier to to concentrate on the hard things that actually matter (like the domain specific logic).
I suspect fearless concurrency is another motivating factor. Better perf can be achieved by squeezing more parallelism, but without borrow checking it's difficult to do fine-grained parallelism correctly.
> And second, did you use any AI tools for the rewrite?
IMO, given the recent commits: almost certainly.
> in a C/C++ code base
It's like "in a Zodiac boat / aircraft carrier navy". This customary putting C and C++ into the same bucket is as amusing as it is unproductive.
Apparently the old mold version had a couple of plain C source files in the source tree (both vendored 3rd party libs but also in the 'regular' source code), so technically C/C++ is correct in this case ...and looks like the Rust version also has some (very minimal) C code left (looks like mostly varargs stuff, I guess Rust doesn't have a concept of varargs?), so it could be called a C/Rust project ;)
https://github.com/rui314/mold/tree/main/c
Unfortunately too many folks still do C style programming in C++.