Funny to see this now. I’ve got a private branch I am in the process of shaping up this weekend to show the compiler team. For deep nested projects like rust analyzer, if you emit the meta data about function types earlier for downstream slots to use, before successful type checking, you can start other crates earlier and use all the slots you have instead of sitting around waiting for the full type checking of the bodies (which other crates largely don’t care about). Something like 40% wall time speed up, maybe 10% to 15% if you have the parallel frontend on.
Nice, excited to see that. This is basically deeper pipelining, IIUC. Nick (the author of the blog) implemented the current version of this "emit metadata sooner," optimization and already experimented with pushing it even sooner a while back.
We have discussed this briefly on last All Hands, the problem will probably be dealing with compiler errors, when they happen in builds that already emitted metadata. I believe this was the reason why it wasn't merged in the first place.
Yes I think I read a post of his from like 2019 were something was tried like this before. My guess is that this will be an interesting and hopefully informative PR for you all to see, but will likely not be merged as is. I had to add the ability for rustc to pause after early meta data emission and that was hacked together I feel, not well designed per se.
I view this as speculative execution/compilation and just throw out any errors from a crate that depended on another crate that ultimately errors out. It has the same outcome (same errors are printed in both cases) you just have a chance of having totally wasted some cpu time (that was otherwise just sitting around though).
Sounds lovely, give it a try on the codex-rs which seems to be getting close to using 2K crates in their repository any time now. Takes like 30 minutes to compile on a my beefy machine and it's a goddamn TUI, not sure what's going on, and not too interested in diving into that beast either.
I will email you when I have something up for sure! First cargo check has it at about 28% reduced on codex rs, from 383 seconds down to 277 seconds on a 16 core machine. No -Zthreads yet though, and that typically has a similar effect and reduces the speed up the early meta data gives.
> Incremental compilation is not cleaned up. Older deps pile up and don’t get cleaned.
This is one of those problems that’s both huge/important, and probably impossible to solve. It bites me all the time, I have a measly 1TB nvme disk and I basically have to wipe my target directory and docker build cache (I have a shared cargo cache volume across builds) every other day. I’ve had this workstation for 2.5 years now and I’m honestly worried about nand endurance coming to bite me soon.
How would one even solve this? When do you know a cache entry won’t be used any more? Trace back provenance of artifacts to their sources, and if the mtimes have been bumped just delete the artifacts? It’s not like resetting source files to older mtimes is a good idea anyway…
I'm glad to see the donations from big companies to open source maintainers are making measurable difference to the Rust experience. Telling these companies that their employees spend 5% less time waiting for compilation might motivate future investment in people like Nick and the others mentioned.
I've moved from Rust to Go for most things because in the era of agents, being able to iterate quickly on a project is a huge advantage and Rust is way, way slower than Go for compilation. There are times when Rust is more appropriate, but for the vast majority of things Go is perfectly fine.
I don't really understand why Rust seems to be the go-to language for llms. Yes sure it has a good type system, but in practice llms seem to trip over it as much as I do, and given how fast llms are at writing code, that just makes compile times a much larger bottleneck.
Rust is built on the idea that code that compile must be correct, to the best of its abilities. LLMs are fast and sloppy, Rust keeps them in check by not letting them get away with preventable bugs.
Of course, Rust can't do anything for spec bugs, like making the stop light green when it should be red, but it can help with crashes and vulnerabilities.
3. https://blog.rust-lang.org/2026/08/20/supply-chain-attack-on... - the culture of trusting random strangers logging in through GitHub without even 2FA is concerning too, given that AI agents can trust some web search and install a malicious cargo package very easily.
If we compare open issues in comparable rust and Go software (eg: sharkdp/bat vs sharkdp/fd) one would find there's no persisting pattern where rust is better.
btw. That could actually make it slower to compile. The compiler uses the orphan rule to short circuit some checks IIRC.
Nevertheless, people in the project are trying to figuring out a way to relax the rule while still maintaining coherence, so this might happen some day.
there are good reasons, obviously, but it's my #1 PITA, #2 is comptime type introspection (it looks like @yara-blue is making some progress, so fingers crossed)
Rust doesn't let you write an impl a trait that wasn't declared in your crate for a type that wasn't declared in your crate. At least one of the two needs to be locally declared. This makes splitting projects into multiple crates harder than it could be, but on the other hand it makes the crates ecosystem less brittle than it otherwise would be.
Some people propose relaxing this rule for "workspaces" (local projects with multiple crates that are not published individually). I haven't researched whether that would be technically feasible.
If you built two impls, your compiler wouldn't know which to pick. Or to phrase it differently, if you wanted to be able to choose the right one, it wouldn't be 'orphan impls', it would be 'orphan impls plus some selection mechanism'. E.g. Scala implicits. And once you have Scala implicits, non-orphan impls probably start looking like a pretty sweet alternative!
Very cool projects! I agree, desktop apps are one of the areas that Rust beats Go. It's too bad though, since I feel anything with a heavy GUI like this can benefit from a rapid iteration cycle which is painful with Rust.
OpenAI is a Rust Foundation platinum level donor. They gave straight cash which can go toward paying maintainers, which is better IMO. This actually made a lot of people angry because they saw it as a risk of OpenAI gaining influence or something. Can't make everyone happy.
OpenAI also hands out free subscriptions to open source maintainers: https://developers.openai.com/community/codex-for-oss They're time limited for now, but they've been pretty generous about who gets them. Anyone who can show Rust contributions would have been able to get one before.
I tend to disregard anyone who complains about foundational open source being funded like this. As long as the donor doesn't have dictatorial control over the product, why obsess over this?
Because not having dictatorial control doesn't protect anyone from anything; politicians take 'donations' (bribes) and then argue or enact laws for what the donor wants, no dictating involved.
Or in software terms, BigCorp sponsors you to develop an open source tool fulltime. If you make the tool more useful for them while making it more complex or worse or ignoring things for others, they might fund you next year as well. If you don't develop things that help them, they might stop and send that money elsewhere and you will need to get a normal job instead and drop your tool work to part-time. Even though they aren't dictating anything, what are you incentivised to do?
There's definitely a cost in terms of features BigCo doesn't need that get deprioritized. But that work still gets done if other people need it, it's just slower, and I don't think you could ever pull off making things worse on purpose in a big community run project like Rust.
The only times this actually happened are C++ and CORBA as far as I remember. Both of them were bureaucratic design by committee efforts from the start, the people doing it weren't users but implementors looking to stop competition, and it all happened before the open source era.
Can't confirm this "pretty generous", many people with lots of contributions to popular projects got any from OpenAI, not even a response to their request.
LLMs are useful in engineering even if the PRs don't include LLM-written code.
>I’m still writing all my own code and text, because (a) that’s paramount, and (b) the project policy requires it, but I had useful LLM analysis assistance on several of the PRs mentioned in this post.
This is one of the most important objectives in software today.
Rust is the best language to serialize agentic LLM output to. It's native, well constructed, low-defect due to design. It's also easy for humans to read and debug if necessary.
The biggest problem with Rust is the compile times. The cycle has to get faster. And we need to start thinking about making the artifact cache non-blocking so multiple agents can work simultaneously - that'll be a big task, but essential if we want to speed up work on one machine rather than spinning up clusters of agent sandboxes (the alternative, perhaps superior solution).
Doing stuff isn't free. For instance, Go compiles relatively quickly for a modern language, but the biggest reason for that is that it does less stuff than most compilers... less optimization, less checking, and some stuff built into the language to avoid some of the problems with having to read lots of headers just to compile a file and other ways of doing less stuff, but mostly the key is it does less stuff, in both the good and bad senses of that.
If you want something like Rust that offers guarantees and checks and cross-checks by the boatload, it adds up. Macros, monomorphization, implicit code generation with traits and all those other things add up too. And you can't always get O(n) or O(n log n) code to implement those checks. Maybe it can be sped up and maybe there's tricks here or there, but at the Pareto frontier, a language that has more checks will be slower to compile than one that has fewer.
And that's not a bad thing or a deficit in Rust, it's just the nature of the beast.
Instead of Go, you could have reached out to complex languages with fast compilation times like D, OCaml, Haskell, Ada, Delphi, C++.
All of them have alternative implementations with fast compilation times.
D, use dmd for fast development workflows, gdc or ldc for the ultimate performance at the expense of compilation times.
OCaml, use the REPL or bytecode interpreter for fast development times, the full blow compiler for ultimate performance.
Haskell, use the REPL, GHCi for the fast development cycles, GHC for the release build.
Ada and Delphi, have had fast implementations since forever, although Ada/SPARK is indeed somehow expensive.
C++, yes it isn't a mistake. Use Live++, VS hot reload, coupled with binary libraries, or a REPL like CINT (nee ROOT), binary libraries for dependencies, incremental compilation and incremental linking for the development workflow.
The problem with Rust isn't the language itself, rather the ecosystem currently lacking such kind of options being available.
including c++ with all of those qualifiers would be like me saying rust is fast with sccache, subsecond, cranelift, and making every single module a separate crate.
The difference being the adoption culture across the ecosystem.
C++ game engines also don't need Bevy like tutorials, because most studios aren't compiling them from scratch, and tools like Live++ or VC++ hot reload are relatively easy to use.
unreal engine is compiled from scratch (depending on your definition of "from scratch") and has notoriously long compile times. heck you have to recompile the editor to make certain gameplay code changes. and it's by far the most popular AAA game engine on the planet. if it was so easy to fix with your suggestions they would have done it already.
but hey, just add subsecond and switch to cranelift. problem solved :)
Unreal Engine has an installer, and they have done my suggestions, as they are the main customers of Live++ [0], advocates of tooling like Blueprints and now Verse for doing full games.
Many studios have delivered games with little to no changes to the underlying C++ code.
so... c++ compile times are fast, because you can simply not compile c++ and instead use blueprints? seems odd since we're talking about c++/rust compile times. turns out you can make rust compile times infinitely faster by not compiling it too...
but when you do have to compile c++, it can be real slow. maybe not as slow as rust, but it's not in the same league as eg. go, c#, zig, etc.
ue5's hot reload is by no means perfect either - many gameplay code changes require recompiling the editor. exposing c++ properties on a blueprint? recompile. modify a constructor? recompile. changing parameters, return types, or adding/removing UFUNCTION or UPROPERTY macros? recompile. you can simply search the web to read the experiences of thousands of ue devs complaining about slow compile times and workflows. it's the same with unity btw, search "reloading domain unity".
also verse is not in unreal engine. no studio is using verse for ue games. not sure why you threw that in.
i get you love c++ and dislike rust, but you aren't really making a good case for c++ when you group it in with other languages that have fast compile times ootb, make arguments that involve not using c++ (like blueprints, lol), and rely on brittle third-party hot-reloading solutions.
I threw that in, because if you knew Unreal Engine, you would be aware of the roadmap for Unreal 6.
No one is saying that C++ compile times are anywhere close to Delphi or D, although they are much better when using modules actually.
Rather that C++ ecosystem has a developer culture and tools, that makes compiling from scratch and pure distribution of source code for what is a compiled language, something rare, thus despite its slowness, the average impact is less felt than on Rust projects.
Rust wants badly to have its cake and eat it too. I get it, but it's not the sensibilities I have. The rustc compiler, if it really can achieve everything all at once, will be a beast like none other, making C++ compilers look simple by comparison.
I'd love something halfway. Go is maybe a bit radical in some regards, but also, with the news of the new SIMD package for Go, it has occured to me just how little I missed having things like, say, autovectorization.
(I know also that some people have tried halfway, but the big thing is figuring out how to keep a relatively simple type system that can still support a borrow checker. Even if there is some middleground, is it truly worth it? As nice as it sounds, I've been more skeptical. Go seems to exist in a very narrow space where its simplifications barely can be made to work.)
And since you bring up Go and contrast the compile time with Rust, it's been experienced at Google (and also Volvo and other places) that Rust and Go teams are as productive whereas C++ is less than half as productive: https://www.youtube.com/watch?t=27012&v=6mZRWFQRvmw&feature=...
So the focus on Rust compile time is misplaced. It's not a big deal in terms of overall productivity.
I don't think that follows - just that compile times aren't the only thing that matters. Maybe rust could be twice as productive as go if compile times went to zero for instance (or maybe not - just disputing that the evidence proves the claim here).
But it can't go to zero, that was the point -- at some level you have to pay for what you get. I'm not saying the compile times couldn't be better, but Rust can't be Go in terms of compile times if it wants to offer the guarantees and options it does. Meanwhile if Go wants to offer more options and stronger guarantees, it won't be able to preserve the short compile times everyone points to.
The Cranelift backend is extremely fast. Most of the slowness is all the crazy optimizations that LLVM does + other things (generating debug info etc).
For symbol heavy projects, linking is a surprising bottleneck.
Some ideas to speed up compilation by not evaluating items that are not used might pan out significantly for big crates in your dep tree (that behavior might never be stable because that would allow items with compile errors in a crate that would still let your application compile, which is against the Rust approach). The same work to do that would also allow overlapping of crate evaluation between different rustc instances called by cargo, as it would require partial evaluation of crates (to do name res only and gather the symbols needed from its deps).
Another thing is that stable rust doesn't treat macros as idempotent (because that wasn't a requirement from the start, there are crates that do dynamic IO to generate types), but if they are then incr comp can be faster by not evaluating them unnecessarily.
I know people are working on a bunch of different strategies to improve both first and incremental compile times, and I'm looking forward to the fruit of their labor.
Compile times are rarely the bottleneck for me, but that doesn't mean I won't welcome any improvements on that front.
> For symbol heavy projects, linking is a surprising bottleneck.
Is that still true with modern linkers like `wild`? Seems like it can link Chromium in 1-2s.
> Another thing is that stable rust doesn't treat macros as idempotent
This seems absolutely insane to me given how pervasive macros are in Rust (I wonder how much of incremental compilation time is just repeated serde derives?). Obviously we can't just blindly treat all macros as idempotent, but an opt-in attribute on a macros that pinky-promises that it is seems like it ought to be pretty easy to implement?
Maybe somebody's looked into it and it doesn't help much? But I've heard (unverified) rumours of the opposite.
> Is that still true with modern linkers like `wild`? Seems like it can link Chromium in 1-2s.
Wild improves things significantly, but it really depends on the kind of project. For some a third of the time can be linking. And not everyone configures their environment to use a different linker. Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
I fully expect that if wild delivers an effective incremental linking architecture, then rustc will be able to produce the patches directly (instead of wild having to produce patches from two versions of the object files), which would mean both that rustc is producing less LLVM bytecode and that wild gets to do what it (will) do best and make linking as fast as mechanically possible.
> This seems absolutely insane to me given how pervasive macros are in Rust
Part of the problem I see is that crates will have to opt-in (and we might be able to change the default over an edition boundary) to get the perf benefit, and it might require a granularity lower than "a crate" (which would complicate implementation). I haven't seen additional discussions around these design considerations (needed to stabilize), which will need to happen before people can see progress.
Sadly, Rust is, like many open source projects, a show-up-o-cracy: you need a motivated (group of?) individual(s) to deliver a feature to fruition, and if the person driving a feature is fine with using nightly for their purposes, and only needs a subset of a feature, they will drive the feature to that state (lets say 80% completion), and then the feature will linger (until someone else with enough motivation to see the other 80% through shows up). This is exacerbated because the project is unwilling to have 80% solutions on stable unless the state is very clearly not going to preclude other future work, or the path to completion is 100% visible (but if that were the case, then it would have been completed already). So we end up with situations like the Allocator APIs.
(I did a quick test, and I'm seeing between 15-20% for https://github.com/servo/stylo which is a macro-heavy crate. I guess it only matter for crates you're actually editing though, macro will already be cached as part of "entire crate is unchanged")
> Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
Yeah, although I think there's more of a trade-off there. Alternative allocators can add significant amounts of compile time. Whereas, modulo maturity, I think a faster linker is more of a pure win. I would imagine we will make wild the default linker at some point if development continues on the trajectory it seems to be on.
I’m not sure if Cranelift support will ever become stable. It’s really complex, it has tradeoffs like you mentioned, and it requires its own backend to be maintained. TPDE looks like something that could more realistically be stabilized as it just slots into the existing LLVM support.
Zero-cost abstractions aren't zero cost in compilation time. High-level abstractions translate to a lot of boilerplate that the compiler has to optimize out.
In unoptimized builds often the linker is the bottleneck. Rust/Cargo can parallelize most of the build, generating tons of code and debug info, but then the poor linker has to consume all of it at once. The object/exe formats were designed in ancient times, so they're hard to build incrementally or in parallel (some linkers are trying).
Is there any experimentation with new ABIs? I mean certain linker flags like --f-lto literally hijack the linker protocol to dump an AST into the backend.
At least for the fully static binary part of rust, there should be some optimizations there w.r.t. compilation. Sure you're not going to interface with shared libraries well but maybe a small experimental feature for fully owned projects? Idk.
Rust already uses MIR-based libraries for LTO, and uses thinLTO by default.
ABIs won't help automatically. The overhead comes mainly from generics and macros. Cross-crate monomorphic generics and AST macros are impossible to express in an ABI. Turning them into polymorphic usually wouldn't remove the many abstraction layers that need to be peeled off, and only make them cost at runtime too.
So far MIR optimizations were helpful. Rust can simplify high-level MIR quicker than LLVM can simplify the same code after it's been lowered to lots of low-level instructions.
I once heard (and don't know if it's still true) that the biggest compile-time sinks are macros and codegen.
Both are kind of outside the Rust compiler's influence. Macros can be almost arbitrarily complex: you pay for what you order. Codegen is LLVM, and that's a fixed choice. You can use Cranelift to get around it, but then you pay elsewhere.
Also, generics and monomorphization regularly come up in these discussion, while common wisdom seems to be that cost for the additional static analysis over other languages like C++ is no a major contributor.
Regardless, it's nice to see performance improvements in the compiler, even if you have to cooperate to benefit from them (e.g. by keeping your macros light and use less generics).
First of all, the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"[0].
Now, is rustc slower than e.g. clang? by how much? why?
Those are different (and complicated) questions. It really depends on what you're compiling, but I'd say rustc can be 1-5x slower (maybe more at times?).
The reasons are many and varied, but in general rust compilation is slower because the compiler is doing way more things compared to C (monomorphization, complex trait resolution + type inference, borrow checker..)
[0]: Also I'd argue that "slow" without a concrete point of reference is a meaningless term in this context.
The go compiler is considered fast, yet Google spends effort speeding up the go compiler. Not to the same extent, but Google has a lot of go code, so it does make a difference.
Ultimately it's the fact that compile time was not a first-class consideration during the design of Rust's important features. There's only so much you can do to mitigate the consequences.
Compilation times were a consideration, but they were a subservient consideration to the prime considerations of 1) being as memory-safe as GC'd languages, 2) exploring the limits of statically-guaranteed thread safety, and 3) being as fast and memory-efficient as C and C++. If Rust had been willing to compromise on those goals and instead had just, for example, used a virtual machine with pervasive garbage collection and been designed for dynamically-dispatched generics then you'd get a lot of compilation time reductions for free, but the world didn't need another Java, it needed a more secure systems language to stand up against C++ where every previous challenger had failed.
No, Rust is not at the paretto frontier for your mentioned 1, 2, 3 and also 4 compilation speed. It could have all of those things and also just have faster compilation. Rust devs have talked about how they have some regrets about not optimizing more for compilation, but instead another attribute got optimized in terms of paretto efficiency - the language's development time. Arguably I think that is the single worst trait you can optimize for in language development, because by saving a little time developing the language you cost humanity, let's say hundreds of millions of hours of development time downstream from you, with millions of users being less productive.
Its similar to C++ compilers, there is no one reason. It is a language that tries to optimize a lot, its a big language, it does safety checks, it uses llvm which is a bit slow, its a language that makes use of generics which generate extra code etc etc.
C++ compilers have it better, despite the fame, because on the C++ world, people like their binary libraries (not compiling the world from scratch on each git clone), there better support for incremental compilation, and incremental linking, there are interpreters and hot code reload tooling available.
Some C++ compilers and linkers have better support for incremental compilation and linking, and do it more fine grained than Rust, e.g. VC++.
Some tooling ideas go all the way back to when C++ vendors started adopting ideas from Smalltalk and Lisp, e.g. Energize C++ or Visual Age for C++ v4.
Or build systems like ClearMake from ClearCase, where the object files and binary libraries are shared across everyone on the cluster with the same views (ClearMake speak for what files/branches are selected).
This is not actually the main reason, most of the time.
Generics/monomorphization and how iterators work results in a lot of compiler bytecode that has to be churned through. More bytecode = longer compilation. It increases the size of the (debug) binaries, the debuginfo in general, causes performance issues with debug binaries in some situations unless you bump the optimization level, causes more IO, etc.
It's unlikely that many people are using Rust without using Option<T> or Result<T, U> a fair bit. Idiomatic Rust fundamentally uses a lot of generics. And a basic for loop expands into quite a lot of intermediate representation due to Iterator.
Other languages support generics, monomorphization, and iterators (e.g. Zig or D), but they're not as slow as Rust to compile, what's the reason for that?
It's likely easier to answer for specific languages
All three of those words can mean "just like Rust" but equally "Not at all like Rust" for different languages.
Two examples to contrast: In C++ the iterators are basically a pointer analog (in some cases they're just literally pointers) and that's a very difference "feature" but it's still definitely iterators. In Ginger Bill's Odin, the iterators are a function, possibly generic, which returns a pair, the next item and a boolean telling you whether the iterator was exhausted.
Trait solving isn't a bottleneck for Rust compilation unless you're doing some extremely cursed things like trying to implement Doom in the type system. At the end of the day it's still mostly that Rust just generates a ton of IR for LLVM to chew on (because of monomorphization), then LLVM takes a while to process all of it (because LLVM is designed primarily to produce high-performance code, often resulting in reasonable tradeoffs against compilation speed), and then the linker takes a while to connect it all together. With an alternative backend to LLVM (e.g. Cranelift) you could choose to design it with a greater emphasis on compilation speed (likely trading off generated code quality in the process). And with a more radical vertically-integrated model where Rust controls the linker you could do in-place linking and nearly completely skip the final step (at least for debug builds), but that's a big change compared to the classic C-style build model.
I finally gave in to the rust hype and built a new project in it (or my agent fleet did). Almost instant regret. My poor machine with just 2TB HDD and 24 cores was almost immediately crippled by agents each working in their own sandbox, each one building and compiling
Most projects my fleet works on are in other languages (TS, Go, Elixir) and can comfortably handle 10+ agents working in parallel, but not rust. I had to cap the fleet to 5 workers and build a dedicated resource monitor to step in and tidy up every time the disk almost filled up
That and general progress on building is far slower, with far more time spent building and testing than any other language I use.
Ended up rebuilding in Go, the perf gains weren't worth it
You should have set up https://github.com/mozilla/sccache at minimum. Setting up compiler caching is routine for people coming from older compiled language backgrounds but it's often missed by people coming from languages like JS/TS who have never been exposed to these ideas.
There's a new project called kache https://github.com/kunobi-ninja/kache that takes this even further and handles more edge cases with worktrees, but it's newer.
If you were pulling the entire project fresh and rebuilding everything in every worktree all the time without any caching then it would be frustrating. You could have asked your agent to set up basic caching and solved most of your problem in minutes.
Cargo could definitely improve with some inter-project sharing by default, but simultaneously working with a bunch of workspaces of the same project is a very new phenomena...
You really shouldn't build software with rust if your workflow involves 10+ agents. Why aren't you using Python or Typescript for things like that?
Rust is what you use when you want to maximize on the runtime performance. It will build software that requires significantly less memory and maximize the CPU that will save time and energy. It's a system language for writing software meant to access the hardware.
Yes this seems like the main way to go. With dependency injection using "dyn" traits on top of that, it seems that this mostly solves the compilation issues with Rust. It might be better for tests and agents to split code in small chunks too.
I am experimenting with this in my project right now. I haven't reached a verdict yet, but in theory it sounds good. I think you're still paying the linking cost though?
Splitting out a project into multiple crates reduces what needs to be recompiled during development. I tend to break out areas of my application that aren't going to change regularly so I'm only having to rebuild the main application.
In a production scenario where you're probably building your application in some CI environment, unless you cache the release build artifacts to re-use when applicable, the entire project will be rebuilt every run.
It’s honestly my biggest issue right now during development. I’m refactoring into a “vertical slice” architecture and then will try crating by feature… fingers crossed.
I think there are a few more. Especially cranelift, but I would not use it for production stuff and it is missing a lot of llvm intrinsics that will just trap if you are trying to use them. But for most it seems to be just fine for regular dev builds with a HUGE speedup.
I recall a talk about makepad.dev, I think it was by Rik Arends, that explained how they achieved outstanding compilation times in rust for makepad.
I can't find that talk again, but it was quite interesting: the rust compiler is fast, but often times it has to perform a lot of unnecessary checks because crates contain more stuff than needed. By stripping unnecessary work, the makepad team made building pretty fast.
Incremental seems to keep winning the easy wins, so the interesting part is whether the remaining compile-time still lives in the same places as last year.
It’s not compiling C code, so that’s not really possible to answer. C is uniquely fast to compile, especially compared with more featureful languages like C++ and Rust.
In practice, C++ and Rust feel pretty similar, mostly due to the kind of code people tend to write in both languages (favoring static dispatch, generics/templates, etc.).
Rephrased question: How does the Rust compiler's speed compare to the C compiler GCC's speed with respect the language each is compiling, i.e., rust and C, respectively
I don't understand why developers are interested in Rust.
- Raw performance doesn't actually matter for the vast majority of use cases. Performance does not equate scalability. Moreover, differences usually disappear in practice once you've actually developed the software because time and storage complexity of operations usually dwarf constant resource costs. Furthermore, do you realize how much more computational resources AI uses compared to a classic piece of software? Trying to optimize tools in the age of AI is like putting lipstick on a pig.
- It's more verbose than most other languages; waste of tokens, context window, time and AI reasoning capacity. The strain most devs feel when reading Rust, also impacts LLMs. It obfuscates essential logic and replaces it with ceremony.
- The Rust training set is far smaller than other popular programming languages. It's also quite different from other programming languages so it probably doesn't benefit as much from cross-language patterns; in fact, could be problematic.
- You have to wait for a compiler; more wasted time which the AI agent could have been using to write code.
It's like TypeScript. Devs became obsessed with TypeScript. Everyone was sure that it would be better for AI. Not the case. LLMs essentially never make type errors in JavaScript (or TypeScript). Static typing is completely redundant. These are the most trivial kinds of errors to avoid. Seriously, try Claude with vanilla JavaScript on Node.js. TypeScript is worse with AI because people who used it tended to over-engineer, also a lot of TypeScript is ceremonial and performative complexity intended for a bureaucratic audience; those are the main patterns which AIs picked up from their training set.
Seriously, try working with a TypeScript codebase and notice how Claude invents all these contrived abstractions, spread out over a huge number of files with unclear separation of concerns and full of lengthy, highly contrived comments.
It's the same issue with all these over-engineered languages which were developed for people who were not good at coding and needed a whole bunch of extra safety features in order to produce functioning software. It's like training wheels on a bike; great to learn in the early stages but if you want to learn how the pros do it, you've got to look at the people who compete in the Olympics; and note that there's a reason none of them still have training wheels on their bikes!
The EverInitializedPlaces example really stands out. Going from ~1.5M to ~90K apply_effects_in_block calls by changing the CFG traversal is a reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
It also seems like the new Polonius/trait-solver work is pushing compiler performance toward a more interesting problem: doing expensive analysis only when it is actually needed.
4.57% mean wall-time reduction across 629 benchmarks in two months is pretty remarkable. Great progress.
> reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
Isn't this true for most optimizations, not just in compilers? My usual goto process for optimizing is "Find stuff we're doing that we don't have to do, re-evaluate what data structures we use and then re-evaluate what algorithms we use" basically, with minor changes depending on the results. Served me well so far, and haven't (intentionally) written any compilers.
Funny to see this now. I’ve got a private branch I am in the process of shaping up this weekend to show the compiler team. For deep nested projects like rust analyzer, if you emit the meta data about function types earlier for downstream slots to use, before successful type checking, you can start other crates earlier and use all the slots you have instead of sitting around waiting for the full type checking of the bodies (which other crates largely don’t care about). Something like 40% wall time speed up, maybe 10% to 15% if you have the parallel frontend on.
Nice, excited to see that. This is basically deeper pipelining, IIUC. Nick (the author of the blog) implemented the current version of this "emit metadata sooner," optimization and already experimented with pushing it even sooner a while back.
We have discussed this briefly on last All Hands, the problem will probably be dealing with compiler errors, when they happen in builds that already emitted metadata. I believe this was the reason why it wasn't merged in the first place.
Yes I think I read a post of his from like 2019 were something was tried like this before. My guess is that this will be an interesting and hopefully informative PR for you all to see, but will likely not be merged as is. I had to add the ability for rustc to pause after early meta data emission and that was hacked together I feel, not well designed per se.
I view this as speculative execution/compilation and just throw out any errors from a crate that depended on another crate that ultimately errors out. It has the same outcome (same errors are printed in both cases) you just have a chance of having totally wasted some cpu time (that was otherwise just sitting around though).
Sounds lovely, give it a try on the codex-rs which seems to be getting close to using 2K crates in their repository any time now. Takes like 30 minutes to compile on a my beefy machine and it's a goddamn TUI, not sure what's going on, and not too interested in diving into that beast either.
This thing is a monster, blows out the disk space on a Claude code web session. Will run it on my test server later and let you know.
Now I want to test it too! Need any testers with Threadripper systems to really see what it can do? ;) Email in profile
I will email you when I have something up for sure! First cargo check has it at about 28% reduced on codex rs, from 383 seconds down to 277 seconds on a 16 core machine. No -Zthreads yet though, and that typically has a similar effect and reduces the speed up the early meta data gives.
It’s insane. 45 minutes to build debug from scratch. Linking is super slow even in debug and that’s without LTO.
Incremental compilation is not cleaned up. Older deps pile up and don’t get cleaned.
Run out of disk space? Oh it’s just the 200+ GB codex-rs target folder.
Forget about doing worktrees.
Rust has a lot of work to do.
> Incremental compilation is not cleaned up. Older deps pile up and don’t get cleaned.
This is one of those problems that’s both huge/important, and probably impossible to solve. It bites me all the time, I have a measly 1TB nvme disk and I basically have to wipe my target directory and docker build cache (I have a shared cargo cache volume across builds) every other day. I’ve had this workstation for 2.5 years now and I’m honestly worried about nand endurance coming to bite me soon.
How would one even solve this? When do you know a cache entry won’t be used any more? Trace back provenance of artifacts to their sources, and if the mtimes have been bumped just delete the artifacts? It’s not like resetting source files to older mtimes is a good idea anyway…
> Linking is super slow even in debug
Linking is normally slower in debug btw (because of the debug info)
Use the split-debuginfo setting in your Cargo.toml to have the DWARF bypass the linker (on Linux, YMMV on other platforms).
A full debug build of the DMD D compiler takes 3 seconds on my Mac Mini.
I'm glad to see the donations from big companies to open source maintainers are making measurable difference to the Rust experience. Telling these companies that their employees spend 5% less time waiting for compilation might motivate future investment in people like Nick and the others mentioned.
Really nice to see that the 5% speedup is despite making the borrow checker better, validating code that previously would have tripped it up.
Sometimes we really can have our cake and eat it too.
I've moved from Rust to Go for most things because in the era of agents, being able to iterate quickly on a project is a huge advantage and Rust is way, way slower than Go for compilation. There are times when Rust is more appropriate, but for the vast majority of things Go is perfectly fine.
I don't really understand why Rust seems to be the go-to language for llms. Yes sure it has a good type system, but in practice llms seem to trip over it as much as I do, and given how fast llms are at writing code, that just makes compile times a much larger bottleneck.
Rust and LLMs are a good match.
Rust is built on the idea that code that compile must be correct, to the best of its abilities. LLMs are fast and sloppy, Rust keeps them in check by not letting them get away with preventable bugs.
Of course, Rust can't do anything for spec bugs, like making the stop light green when it should be red, but it can help with crashes and vulnerabilities.
Real world is far messier than that. Eg:
1. https://www.phoronix.com/news/Ubuntu-Rust-Coreutils-Audit - file system and OS aren't always neatly represented in tYpE sYsTeM.
2. https://blog.cloudflare.com/18-november-2025-outage/ - bad programmers can write bad code in any language.
3. https://blog.rust-lang.org/2026/08/20/supply-chain-attack-on... - the culture of trusting random strangers logging in through GitHub without even 2FA is concerning too, given that AI agents can trust some web search and install a malicious cargo package very easily.
If we compare open issues in comparable rust and Go software (eg: sharkdp/bat vs sharkdp/fd) one would find there's no persisting pattern where rust is better.
I nearly did this, but then I swapped to writing native desktop apps and egui - Rust is still worth it.
I've stopped using Tauri and gone with 100% egui. It's cross platform and excellent, and if you give it design constraints it will look beautiful.
Check out my 100% adobe clean room reimplementations:
https://github.com/storytold/filmcraft
https://github.com/storytold/photocraft
https://github.com/storytold/drawcraft (going to rename this vectorcraft)
The #1 thing for the Rust project to do is make Rust faster to compile.
Rust is the agentic AI language. It just needs to lean in and go faster.
> The #1 thing for the Rust project to do is make Rust faster to compile.
#0 get rid of the orphan rule
btw. That could actually make it slower to compile. The compiler uses the orphan rule to short circuit some checks IIRC.
Nevertheless, people in the project are trying to figuring out a way to relax the rule while still maintaining coherence, so this might happen some day.
there are good reasons, obviously, but it's my #1 PITA, #2 is comptime type introspection (it looks like @yara-blue is making some progress, so fingers crossed)
BTW: cheers, we should grab a beer some day :)
We should ;)
What's the orphan rule?
Rust doesn't let you write an impl a trait that wasn't declared in your crate for a type that wasn't declared in your crate. At least one of the two needs to be locally declared. This makes splitting projects into multiple crates harder than it could be, but on the other hand it makes the crates ecosystem less brittle than it otherwise would be.
Some people propose relaxing this rule for "workspaces" (local projects with multiple crates that are not published individually). I haven't researched whether that would be technically feasible.
If you built two impls, your compiler wouldn't know which to pick. Or to phrase it differently, if you wanted to be able to choose the right one, it wouldn't be 'orphan impls', it would be 'orphan impls plus some selection mechanism'. E.g. Scala implicits. And once you have Scala implicits, non-orphan impls probably start looking like a pretty sweet alternative!
It absolutely should be relaxed for workspaces.
Workspaces are a great way to architect larger projects and monorepos, and they'd at least be internally consistent.
Looks great. It's early but how well do they work? Comparable to FOSS apps yet?
Also it says egui is immediate mode, does that use a lot of {C,G}PU or have any other issues?
Are agents able to control these well? I saw vector craft mention MCP and a command api. Whats the command api? No CLI it sounds like.
Working on it! That's the intent for all of these.
Going to do Toon Boom and a few others too and build them as a suite.
Very cool projects! I agree, desktop apps are one of the areas that Rust beats Go. It's too bad though, since I feel anything with a heavy GUI like this can benefit from a rapid iteration cycle which is painful with Rust.
Wow those are some nice applications! Thanks for sharing this!
These are great
!!! WOW !!!
I'm surprised the OpenAI Codex team doesn't donate like 10B tokens or something to the Rust team for performance
OpenAI is a Rust Foundation platinum level donor. They gave straight cash which can go toward paying maintainers, which is better IMO. This actually made a lot of people angry because they saw it as a risk of OpenAI gaining influence or something. Can't make everyone happy.
OpenAI also hands out free subscriptions to open source maintainers: https://developers.openai.com/community/codex-for-oss They're time limited for now, but they've been pretty generous about who gets them. Anyone who can show Rust contributions would have been able to get one before.
I tend to disregard anyone who complains about foundational open source being funded like this. As long as the donor doesn't have dictatorial control over the product, why obsess over this?
Because not having dictatorial control doesn't protect anyone from anything; politicians take 'donations' (bribes) and then argue or enact laws for what the donor wants, no dictating involved.
Or in software terms, BigCorp sponsors you to develop an open source tool fulltime. If you make the tool more useful for them while making it more complex or worse or ignoring things for others, they might fund you next year as well. If you don't develop things that help them, they might stop and send that money elsewhere and you will need to get a normal job instead and drop your tool work to part-time. Even though they aren't dictating anything, what are you incentivised to do?
There's definitely a cost in terms of features BigCo doesn't need that get deprioritized. But that work still gets done if other people need it, it's just slower, and I don't think you could ever pull off making things worse on purpose in a big community run project like Rust.
The only times this actually happened are C++ and CORBA as far as I remember. Both of them were bureaucratic design by committee efforts from the start, the people doing it weren't users but implementors looking to stop competition, and it all happened before the open source era.
Can't confirm this "pretty generous", many people with lots of contributions to popular projects got any from OpenAI, not even a response to their request.
That's awesome
LLM written code is disallowed by the rust project policy
https://forge.rust-lang.org/policies/llm-usage.html
LLMs are useful in engineering even if the PRs don't include LLM-written code.
>I’m still writing all my own code and text, because (a) that’s paramount, and (b) the project policy requires it, but I had useful LLM analysis assistance on several of the PRs mentioned in this post.
from the article
It doesn't seem to be quite that simple?
https://forge.rust-lang.org/policies/llm-usage.html#experime...
Both OpenAI and Anthropic have robust programs for free tokens that you can apply to if you are a significant enough oss maintainer.
What you suggest may thus be more or less already happening.
This is one of the most important objectives in software today.
Rust is the best language to serialize agentic LLM output to. It's native, well constructed, low-defect due to design. It's also easy for humans to read and debug if necessary.
The biggest problem with Rust is the compile times. The cycle has to get faster. And we need to start thinking about making the artifact cache non-blocking so multiple agents can work simultaneously - that'll be a big task, but essential if we want to speed up work on one machine rather than spinning up clusters of agent sandboxes (the alternative, perhaps superior solution).
If you have fast enough hardware then Rust compile times are already not that big of a deal.
Why is the compiler slow in the first place? I have no rust knowledge, how slow us slow, lets say in comparison to a c compiler?
What is the performance killer?
Doing stuff isn't free. For instance, Go compiles relatively quickly for a modern language, but the biggest reason for that is that it does less stuff than most compilers... less optimization, less checking, and some stuff built into the language to avoid some of the problems with having to read lots of headers just to compile a file and other ways of doing less stuff, but mostly the key is it does less stuff, in both the good and bad senses of that.
If you want something like Rust that offers guarantees and checks and cross-checks by the boatload, it adds up. Macros, monomorphization, implicit code generation with traits and all those other things add up too. And you can't always get O(n) or O(n log n) code to implement those checks. Maybe it can be sped up and maybe there's tricks here or there, but at the Pareto frontier, a language that has more checks will be slower to compile than one that has fewer.
And that's not a bad thing or a deficit in Rust, it's just the nature of the beast.
Nah, the only reason is lack of tooling.
Instead of Go, you could have reached out to complex languages with fast compilation times like D, OCaml, Haskell, Ada, Delphi, C++.
All of them have alternative implementations with fast compilation times.
D, use dmd for fast development workflows, gdc or ldc for the ultimate performance at the expense of compilation times.
OCaml, use the REPL or bytecode interpreter for fast development times, the full blow compiler for ultimate performance.
Haskell, use the REPL, GHCi for the fast development cycles, GHC for the release build.
Ada and Delphi, have had fast implementations since forever, although Ada/SPARK is indeed somehow expensive.
C++, yes it isn't a mistake. Use Live++, VS hot reload, coupled with binary libraries, or a REPL like CINT (nee ROOT), binary libraries for dependencies, incremental compilation and incremental linking for the development workflow.
The problem with Rust isn't the language itself, rather the ecosystem currently lacking such kind of options being available.
Dlang is my weapon of choice and I prefer it to rust.
including c++ with all of those qualifiers would be like me saying rust is fast with sccache, subsecond, cranelift, and making every single module a separate crate.
The difference being the adoption culture across the ecosystem.
C++ game engines also don't need Bevy like tutorials, because most studios aren't compiling them from scratch, and tools like Live++ or VC++ hot reload are relatively easy to use.
unreal engine is compiled from scratch (depending on your definition of "from scratch") and has notoriously long compile times. heck you have to recompile the editor to make certain gameplay code changes. and it's by far the most popular AAA game engine on the planet. if it was so easy to fix with your suggestions they would have done it already.
but hey, just add subsecond and switch to cranelift. problem solved :)
Unreal Engine has an installer, and they have done my suggestions, as they are the main customers of Live++ [0], advocates of tooling like Blueprints and now Verse for doing full games.
Many studios have delivered games with little to no changes to the underlying C++ code.
[0] - https://dev.epicgames.com/documentation/unreal-engine/using-...
so... c++ compile times are fast, because you can simply not compile c++ and instead use blueprints? seems odd since we're talking about c++/rust compile times. turns out you can make rust compile times infinitely faster by not compiling it too...
but when you do have to compile c++, it can be real slow. maybe not as slow as rust, but it's not in the same league as eg. go, c#, zig, etc.
ue5's hot reload is by no means perfect either - many gameplay code changes require recompiling the editor. exposing c++ properties on a blueprint? recompile. modify a constructor? recompile. changing parameters, return types, or adding/removing UFUNCTION or UPROPERTY macros? recompile. you can simply search the web to read the experiences of thousands of ue devs complaining about slow compile times and workflows. it's the same with unity btw, search "reloading domain unity".
also verse is not in unreal engine. no studio is using verse for ue games. not sure why you threw that in.
i get you love c++ and dislike rust, but you aren't really making a good case for c++ when you group it in with other languages that have fast compile times ootb, make arguments that involve not using c++ (like blueprints, lol), and rely on brittle third-party hot-reloading solutions.
I threw that in, because if you knew Unreal Engine, you would be aware of the roadmap for Unreal 6.
No one is saying that C++ compile times are anywhere close to Delphi or D, although they are much better when using modules actually.
Rather that C++ ecosystem has a developer culture and tools, that makes compiling from scratch and pure distribution of source code for what is a compiled language, something rare, thus despite its slowness, the average impact is less felt than on Rust projects.
Rust wants badly to have its cake and eat it too. I get it, but it's not the sensibilities I have. The rustc compiler, if it really can achieve everything all at once, will be a beast like none other, making C++ compilers look simple by comparison.
I'd love something halfway. Go is maybe a bit radical in some regards, but also, with the news of the new SIMD package for Go, it has occured to me just how little I missed having things like, say, autovectorization.
(I know also that some people have tried halfway, but the big thing is figuring out how to keep a relatively simple type system that can still support a borrow checker. Even if there is some middleground, is it truly worth it? As nice as it sounds, I've been more skeptical. Go seems to exist in a very narrow space where its simplifications barely can be made to work.)
The middle ground is pretty much nim.
Yeah doing optimizations in a compiler on things like loops, addition, or string layouts is never free.
And since you bring up Go and contrast the compile time with Rust, it's been experienced at Google (and also Volvo and other places) that Rust and Go teams are as productive whereas C++ is less than half as productive: https://www.youtube.com/watch?t=27012&v=6mZRWFQRvmw&feature=...
So the focus on Rust compile time is misplaced. It's not a big deal in terms of overall productivity.
I don't think that follows - just that compile times aren't the only thing that matters. Maybe rust could be twice as productive as go if compile times went to zero for instance (or maybe not - just disputing that the evidence proves the claim here).
But it can't go to zero, that was the point -- at some level you have to pay for what you get. I'm not saying the compile times couldn't be better, but Rust can't be Go in terms of compile times if it wants to offer the guarantees and options it does. Meanwhile if Go wants to offer more options and stronger guarantees, it won't be able to preserve the short compile times everyone points to.
The Cranelift backend is extremely fast. Most of the slowness is all the crazy optimizations that LLVM does + other things (generating debug info etc).
For symbol heavy projects, linking is a surprising bottleneck.
Some ideas to speed up compilation by not evaluating items that are not used might pan out significantly for big crates in your dep tree (that behavior might never be stable because that would allow items with compile errors in a crate that would still let your application compile, which is against the Rust approach). The same work to do that would also allow overlapping of crate evaluation between different rustc instances called by cargo, as it would require partial evaluation of crates (to do name res only and gather the symbols needed from its deps).
Another thing is that stable rust doesn't treat macros as idempotent (because that wasn't a requirement from the start, there are crates that do dynamic IO to generate types), but if they are then incr comp can be faster by not evaluating them unnecessarily.
I know people are working on a bunch of different strategies to improve both first and incremental compile times, and I'm looking forward to the fruit of their labor.
Compile times are rarely the bottleneck for me, but that doesn't mean I won't welcome any improvements on that front.
> For symbol heavy projects, linking is a surprising bottleneck.
Is that still true with modern linkers like `wild`? Seems like it can link Chromium in 1-2s.
> Another thing is that stable rust doesn't treat macros as idempotent
This seems absolutely insane to me given how pervasive macros are in Rust (I wonder how much of incremental compilation time is just repeated serde derives?). Obviously we can't just blindly treat all macros as idempotent, but an opt-in attribute on a macros that pinky-promises that it is seems like it ought to be pretty easy to implement?
Maybe somebody's looked into it and it doesn't help much? But I've heard (unverified) rumours of the opposite.
> Is that still true with modern linkers like `wild`? Seems like it can link Chromium in 1-2s.
Wild improves things significantly, but it really depends on the kind of project. For some a third of the time can be linking. And not everyone configures their environment to use a different linker. Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
I fully expect that if wild delivers an effective incremental linking architecture, then rustc will be able to produce the patches directly (instead of wild having to produce patches from two versions of the object files), which would mean both that rustc is producing less LLVM bytecode and that wild gets to do what it (will) do best and make linking as fast as mechanically possible.
> This seems absolutely insane to me given how pervasive macros are in Rust
There was some work done on this front, but I haven't kept up to date on the current status of that. There was a PR showing promise https://github.com/rust-lang/rust/pull/129102 (later landed as https://github.com/rust-lang/rust/pull/145354, 10% on a specific serde-heavy crate, reports of 32% improvements in the original PR). The tracking issue doesn't have any updates https://github.com/rust-lang/rust/issues/151364, but you can try out nightly with -Zcache-proc-macros to see what the effect could be on your projects.
Part of the problem I see is that crates will have to opt-in (and we might be able to change the default over an edition boundary) to get the perf benefit, and it might require a granularity lower than "a crate" (which would complicate implementation). I haven't seen additional discussions around these design considerations (needed to stabilize), which will need to happen before people can see progress.
Sadly, Rust is, like many open source projects, a show-up-o-cracy: you need a motivated (group of?) individual(s) to deliver a feature to fruition, and if the person driving a feature is fine with using nightly for their purposes, and only needs a subset of a feature, they will drive the feature to that state (lets say 80% completion), and then the feature will linger (until someone else with enough motivation to see the other 80% through shows up). This is exacerbated because the project is unwilling to have 80% solutions on stable unless the state is very clearly not going to preclude other future work, or the path to completion is 100% visible (but if that were the case, then it would have been completed already). So we end up with situations like the Allocator APIs.
(I did a quick test, and I'm seeing between 15-20% for https://github.com/servo/stylo which is a macro-heavy crate. I guess it only matter for crates you're actually editing though, macro will already be cached as part of "entire crate is unchanged")
> Another thing that can easily improve performance is changing the global allocator, but I've seen teams that measured 10% perf improvements in their own metrics decide against going with anything other than the "default".
Yeah, although I think there's more of a trade-off there. Alternative allocators can add significant amounts of compile time. Whereas, modulo maturity, I think a faster linker is more of a pure win. I would imagine we will make wild the default linker at some point if development continues on the trajectory it seems to be on.
Cranelift's biggest blocker for me is that it completely breaks debuggers:
https://github.com/rust-lang/rustc_codegen_cranelift/issues/...
I’m not sure if Cranelift support will ever become stable. It’s really complex, it has tradeoffs like you mentioned, and it requires its own backend to be maintained. TPDE looks like something that could more realistically be stabilized as it just slots into the existing LLVM support.
Zero-cost abstractions aren't zero cost in compilation time. High-level abstractions translate to a lot of boilerplate that the compiler has to optimize out.
In unoptimized builds often the linker is the bottleneck. Rust/Cargo can parallelize most of the build, generating tons of code and debug info, but then the poor linker has to consume all of it at once. The object/exe formats were designed in ancient times, so they're hard to build incrementally or in parallel (some linkers are trying).
Is there any experimentation with new ABIs? I mean certain linker flags like --f-lto literally hijack the linker protocol to dump an AST into the backend.
At least for the fully static binary part of rust, there should be some optimizations there w.r.t. compilation. Sure you're not going to interface with shared libraries well but maybe a small experimental feature for fully owned projects? Idk.
Rust already uses MIR-based libraries for LTO, and uses thinLTO by default.
ABIs won't help automatically. The overhead comes mainly from generics and macros. Cross-crate monomorphic generics and AST macros are impossible to express in an ABI. Turning them into polymorphic usually wouldn't remove the many abstraction layers that need to be peeled off, and only make them cost at runtime too.
So far MIR optimizations were helpful. Rust can simplify high-level MIR quicker than LLVM can simplify the same code after it's been lowered to lots of low-level instructions.
I once heard (and don't know if it's still true) that the biggest compile-time sinks are macros and codegen.
Both are kind of outside the Rust compiler's influence. Macros can be almost arbitrarily complex: you pay for what you order. Codegen is LLVM, and that's a fixed choice. You can use Cranelift to get around it, but then you pay elsewhere.
Also, generics and monomorphization regularly come up in these discussion, while common wisdom seems to be that cost for the additional static analysis over other languages like C++ is no a major contributor.
Regardless, it's nice to see performance improvements in the compiler, even if you have to cooperate to benefit from them (e.g. by keeping your macros light and use less generics).
First of all, the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"[0].
Now, is rustc slower than e.g. clang? by how much? why?
Those are different (and complicated) questions. It really depends on what you're compiling, but I'd say rustc can be 1-5x slower (maybe more at times?).
The reasons are many and varied, but in general rust compilation is slower because the compiler is doing way more things compared to C (monomorphization, complex trait resolution + type inference, borrow checker..)
[0]: Also I'd argue that "slow" without a concrete point of reference is a meaningless term in this context.
> the fact that the article talks about "speeding up" the rust compiler doesn't automatically mean that the compiler is "slow"
Well, if it weren't "slow" for some definition of "slow", nobody would bother speeding it up, would they?
The go compiler is considered fast, yet Google spends effort speeding up the go compiler. Not to the same extent, but Google has a lot of go code, so it does make a difference.
Ultimately it's the fact that compile time was not a first-class consideration during the design of Rust's important features. There's only so much you can do to mitigate the consequences.
Compilation times were a consideration, but they were a subservient consideration to the prime considerations of 1) being as memory-safe as GC'd languages, 2) exploring the limits of statically-guaranteed thread safety, and 3) being as fast and memory-efficient as C and C++. If Rust had been willing to compromise on those goals and instead had just, for example, used a virtual machine with pervasive garbage collection and been designed for dynamically-dispatched generics then you'd get a lot of compilation time reductions for free, but the world didn't need another Java, it needed a more secure systems language to stand up against C++ where every previous challenger had failed.
No, Rust is not at the paretto frontier for your mentioned 1, 2, 3 and also 4 compilation speed. It could have all of those things and also just have faster compilation. Rust devs have talked about how they have some regrets about not optimizing more for compilation, but instead another attribute got optimized in terms of paretto efficiency - the language's development time. Arguably I think that is the single worst trait you can optimize for in language development, because by saving a little time developing the language you cost humanity, let's say hundreds of millions of hours of development time downstream from you, with millions of users being less productive.
Contrast this with zig, where compiler performance is so important that it has influenced the language design.
Zig and Go are similar on that front.
Its similar to C++ compilers, there is no one reason. It is a language that tries to optimize a lot, its a big language, it does safety checks, it uses llvm which is a bit slow, its a language that makes use of generics which generate extra code etc etc.
C++ compilers have it better, despite the fame, because on the C++ world, people like their binary libraries (not compiling the world from scratch on each git clone), there better support for incremental compilation, and incremental linking, there are interpreters and hot code reload tooling available.
> there better support for incremental compilation
What do you mean by this?
Some C++ compilers and linkers have better support for incremental compilation and linking, and do it more fine grained than Rust, e.g. VC++.
Some tooling ideas go all the way back to when C++ vendors started adopting ideas from Smalltalk and Lisp, e.g. Energize C++ or Visual Age for C++ v4.
Or build systems like ClearMake from ClearCase, where the object files and binary libraries are shared across everyone on the cluster with the same views (ClearMake speak for what files/branches are selected).
It's doing static analysis that many other languages don't do at compile time.
This is not actually the main reason, most of the time.
Generics/monomorphization and how iterators work results in a lot of compiler bytecode that has to be churned through. More bytecode = longer compilation. It increases the size of the (debug) binaries, the debuginfo in general, causes performance issues with debug binaries in some situations unless you bump the optimization level, causes more IO, etc.
If you don't use generics and monomorphization, then what explains it?
It's unlikely that many people are using Rust without using Option<T> or Result<T, U> a fair bit. Idiomatic Rust fundamentally uses a lot of generics. And a basic for loop expands into quite a lot of intermediate representation due to Iterator.
Other languages support generics, monomorphization, and iterators (e.g. Zig or D), but they're not as slow as Rust to compile, what's the reason for that?
It's likely easier to answer for specific languages
All three of those words can mean "just like Rust" but equally "Not at all like Rust" for different languages.
Two examples to contrast: In C++ the iterators are basically a pointer analog (in some cases they're just literally pointers) and that's a very difference "feature" but it's still definitely iterators. In Ginger Bill's Odin, the iterators are a function, possibly generic, which returns a pair, the next item and a boolean telling you whether the iterator was exhausted.
Zig has their own backend, monomorphization is afaik slow because LLVM itself.
On top the borrow checker and other features are non-existent in these languages.
AFAIK it's because of trait solving. And also not every language does monomorphization.
Trait solving isn't a bottleneck for Rust compilation unless you're doing some extremely cursed things like trying to implement Doom in the type system. At the end of the day it's still mostly that Rust just generates a ton of IR for LLVM to chew on (because of monomorphization), then LLVM takes a while to process all of it (because LLVM is designed primarily to produce high-performance code, often resulting in reasonable tradeoffs against compilation speed), and then the linker takes a while to connect it all together. With an alternative backend to LLVM (e.g. Cranelift) you could choose to design it with a greater emphasis on compilation speed (likely trading off generated code quality in the process). And with a more radical vertically-integrated model where Rust controls the linker you could do in-place linking and nearly completely skip the final step (at least for debug builds), but that's a big change compared to the classic C-style build model.
Not sure why you are being downvoted here.
I finally gave in to the rust hype and built a new project in it (or my agent fleet did). Almost instant regret. My poor machine with just 2TB HDD and 24 cores was almost immediately crippled by agents each working in their own sandbox, each one building and compiling
Most projects my fleet works on are in other languages (TS, Go, Elixir) and can comfortably handle 10+ agents working in parallel, but not rust. I had to cap the fleet to 5 workers and build a dedicated resource monitor to step in and tidy up every time the disk almost filled up
That and general progress on building is far slower, with far more time spent building and testing than any other language I use.
Ended up rebuilding in Go, the perf gains weren't worth it
You should have set up https://github.com/mozilla/sccache at minimum. Setting up compiler caching is routine for people coming from older compiled language backgrounds but it's often missed by people coming from languages like JS/TS who have never been exposed to these ideas.
There's a new project called kache https://github.com/kunobi-ninja/kache that takes this even further and handles more edge cases with worktrees, but it's newer.
If you were pulling the entire project fresh and rebuilding everything in every worktree all the time without any caching then it would be frustrating. You could have asked your agent to set up basic caching and solved most of your problem in minutes.
Shouldn’t this be part of cargo and enabled by default then ?
Cargo could definitely improve with some inter-project sharing by default, but simultaneously working with a bunch of workspaces of the same project is a very new phenomena...
this is the issue with vibing...you don't know what you don't know
Most of that time was probably compiling dependencies. If you give it another go, I would recommend a shared sccache.
https://github.com/mozilla/sccache
Even that doesn't fully avoid recompilation of the same crate, due to features changes across the dependency chain.
Changing features should be happening a lot less than building, you surely would go weeks between those after early dev.
You really shouldn't build software with rust if your workflow involves 10+ agents. Why aren't you using Python or Typescript for things like that?
Rust is what you use when you want to maximize on the runtime performance. It will build software that requires significantly less memory and maximize the CPU that will save time and energy. It's a system language for writing software meant to access the hardware.
Why would that imply that you wouldn't use a lot of agents?
Make a cargo wrapper (or ask your clanker) with a global build lock.
Cargo run/test/nextest/run only, check/fmt etc should be excluded.
This avoids contention/oversubscription of the CPU which makes builds up to 300% slower from what I measured.
first time i've seen rust's compile performance actually act as a filter haha
Biggest win for me was splitting one fat crate into three, parallelism got real.
Yes this seems like the main way to go. With dependency injection using "dyn" traits on top of that, it seems that this mostly solves the compilation issues with Rust. It might be better for tests and agents to split code in small chunks too.
I am experimenting with this in my project right now. I haven't reached a verdict yet, but in theory it sounds good. I think you're still paying the linking cost though?
Yes.
Splitting out a project into multiple crates reduces what needs to be recompiled during development. I tend to break out areas of my application that aren't going to change regularly so I'm only having to rebuild the main application.
In a production scenario where you're probably building your application in some CI environment, unless you cache the release build artifacts to re-use when applicable, the entire project will be rebuilt every run.
It’s honestly my biggest issue right now during development. I’m refactoring into a “vertical slice” architecture and then will try crating by feature… fingers crossed.
The parallel frontend is inching closer to stability too, with an open PR to enable it by default on nightly.
But the real speedup will happen when TPDE is enabled: https://goals.rust-lang.org/2026/tpde.html
I think there are a few more. Especially cranelift, but I would not use it for production stuff and it is missing a lot of llvm intrinsics that will just trap if you are trying to use them. But for most it seems to be just fine for regular dev builds with a HUGE speedup.
I recall a talk about makepad.dev, I think it was by Rik Arends, that explained how they achieved outstanding compilation times in rust for makepad.
I can't find that talk again, but it was quite interesting: the rust compiler is fast, but often times it has to perform a lot of unnecessary checks because crates contain more stuff than needed. By stripping unnecessary work, the makepad team made building pretty fast.
Incremental seems to keep winning the easy wins, so the interesting part is whether the remaining compile-time still lives in the same places as last year.
Oh man just miseed it, October already.
How does the Rust compiler's speed compare to the speed of the C compiler GCC
Is it slower
It’s not compiling C code, so that’s not really possible to answer. C is uniquely fast to compile, especially compared with more featureful languages like C++ and Rust.
In practice, C++ and Rust feel pretty similar, mostly due to the kind of code people tend to write in both languages (favoring static dispatch, generics/templates, etc.).
Rephrased question: How does the Rust compiler's speed compare to the C compiler GCC's speed with respect the language each is compiling, i.e., rust and C, respectively
I don't understand why developers are interested in Rust.
- Raw performance doesn't actually matter for the vast majority of use cases. Performance does not equate scalability. Moreover, differences usually disappear in practice once you've actually developed the software because time and storage complexity of operations usually dwarf constant resource costs. Furthermore, do you realize how much more computational resources AI uses compared to a classic piece of software? Trying to optimize tools in the age of AI is like putting lipstick on a pig.
- It's more verbose than most other languages; waste of tokens, context window, time and AI reasoning capacity. The strain most devs feel when reading Rust, also impacts LLMs. It obfuscates essential logic and replaces it with ceremony.
- The Rust training set is far smaller than other popular programming languages. It's also quite different from other programming languages so it probably doesn't benefit as much from cross-language patterns; in fact, could be problematic.
- You have to wait for a compiler; more wasted time which the AI agent could have been using to write code.
It's like TypeScript. Devs became obsessed with TypeScript. Everyone was sure that it would be better for AI. Not the case. LLMs essentially never make type errors in JavaScript (or TypeScript). Static typing is completely redundant. These are the most trivial kinds of errors to avoid. Seriously, try Claude with vanilla JavaScript on Node.js. TypeScript is worse with AI because people who used it tended to over-engineer, also a lot of TypeScript is ceremonial and performative complexity intended for a bureaucratic audience; those are the main patterns which AIs picked up from their training set.
Seriously, try working with a TypeScript codebase and notice how Claude invents all these contrived abstractions, spread out over a huge number of files with unclear separation of concerns and full of lengthy, highly contrived comments.
It's the same issue with all these over-engineered languages which were developed for people who were not good at coding and needed a whole bunch of extra safety features in order to produce functioning software. It's like training wheels on a bike; great to learn in the early stages but if you want to learn how the pros do it, you've got to look at the people who compete in the Olympics; and note that there's a reason none of them still have training wheels on their bikes!
Gonna have to name my next project Penelope Hammertime, or at least Pineapple Häagen-Dazs.
The EverInitializedPlaces example really stands out. Going from ~1.5M to ~90K apply_effects_in_block calls by changing the CFG traversal is a reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
It also seems like the new Polonius/trait-solver work is pushing compiler performance toward a more interesting problem: doing expensive analysis only when it is actually needed.
4.57% mean wall-time reduction across 629 benchmarks in two months is pretty remarkable. Great progress.
> reminder that the biggest compiler optimizations often come from changing the algorithm, not optimizing the hot loop itself.
Isn't this true for most optimizations, not just in compilers? My usual goto process for optimizing is "Find stuff we're doing that we don't have to do, re-evaluate what data structures we use and then re-evaluate what algorithms we use" basically, with minor changes depending on the results. Served me well so far, and haven't (intentionally) written any compilers.
This isn't a human you are asking your question to.