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Comment by ReleaseCandidat | Hacker News Reader

> 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.

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> (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

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.

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.

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