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Comment by liftm | Hacker News Reader

Ironic. Rust is saving others from slow compilation speed, but can't save itself.

> but can't save itself.

Sort of. The Rust compiler has gotten much faster in the last few years. On most common crates it's become 25-30% faster (https://arewefastyet.pages.dev). This page isn't up to date with the latest releases, which have also become faster.

During development the most typical configuration would be debug-incremental builds. Those are pretty damn fast. And personally, I don't even use rustc during development to figure out errors. rust-analyzer does a great job of giving all errors in line. I think most others who use one of the IDE options might not be waiting on rustc much.

Honestly, "rust is slow" is a bit of a meme at this point. There are engineers working on triaging compiler performance regressions every week, as well as engineers working on improving performance. But there's no concerted effort to communicate these efforts to the community. Which leads to this meme being perpetuated. I guarantee that even if the compiler improves by a further 50%, there will still be comments saying it's too slow, simply because commenters aren't aware of changes over time.

> Honestly, "rust is slow" is a bit of a meme at this point.

But it _is_ slow. Same as C++ and Haskell compilers are slow. And you cannot get it to really compile faster - like OCaml speed - because the language (macros and traits I guess are the worst) prohibits that. Of course, if all somebody knows are C++, Haskell and Rust the compilation doesn't seem slow to them.

> (macros and traits I guess are the worst)

This is a common assumption but I've rarely see cargo check, which both expands macros and solves traits as well as runs the borrow checker take more than a few seconds on my machine, even when full compilation takes minutes. Most of the slowness is really in the parts that come afterwards, I suspect especially once you hit the generics explosion in later passes.

Ironically C++ can be faster than Rust in practice, because the ecosystem embraces binary libraries, so unless one is doing crazy stuff with Boost, C++ manages to have clean builds that are faster than Rust's clean builds.

Each crate is a separate build unit, so after the first build your dependencies won’t recompile unless you change the optimization-level or something

You know what is better? Not having the first build at all.

Microsoft packages a full set of binary libraries with a build.rs script for Rust/WinRT, otherwise no one would bother to wait for the build.

https://github.com/microsoft/windows-rs/tree/master/.windows

Also seeing the same crate consumed in multiple version being compiled multiple times isn't fun.

For example, the image crate.

I was under the impression that must of Rust's compile time comes down to LLVM

It's less LLVM itself and more the amount of work that rustc gives to LLVM. Formerly for example, rustc did not do any kind of constant propagation itself and relied fully on LLVM to propagate them and say, remove any unused branches in the final expanded code. This wasted a lot of work and propagating them earlier was able to reduce compile times significantly.

So while most of the compile time is indeed usually in LLVM, there are still a lot of things rustc can do to give LLVM an easier time.

I guess using LLVM doesn't help performance, but the real 'problem' still is the language itself. Just look at C++ and it's compilers for comparison.

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