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Comment by darksaints | Hacker News Reader

I know a lot of people will disagree with me, and I know that the rust compiler has room for improvement, but compile time isn't really that bad for rust when you consider what it actually does for you.

Assuming no unsafe blocks, rust's type system eliminates entire classes of memory safety errors. It fundamentally eliminates data races. It eliminates large swaths of logical errors, due to its use of algebraic data types, and strong type enforcement. No null pointers, explicit error handling, and no undefined behavior.

It boggles my mind that people will balk at compile times for rust, but then not even bat an eye at their C++ testkit that runs for 8+ hours, primarily checking for errors that rust would never let you make in the first place.

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Part of what makes rustc's slow compile times so frustrating is that the slow compile times are mostly unrelated to all of the fancy compile-time analysis you listed. You can see this in action if you run `cargo check`. That will run all the special static analysis (typeck, borrowck, dropck, etc.) and complete in a very reasonable amount of time.

To a first approximation, rustc is slow because it generates terrible LLVM IR, and relies on a number of slow LLVM optimization passes to smooth that IR into something reasonable. As far as I can tell this is entirely a result of not having the necessary engineering resources to throw at the problem; Mozilla has a limited budget for Rust, and needs to spread that budget across not just the compiler toolchain, but the language and ecosystem.

Brian Anderson's been working on a blog post series that covers Rust's slow compile times in more detail, if you're curious: https://pingcap.com/blog/tag/Rust

Yeah, I realize that Rust has a lot of room for improvement. I'm just saying that compared to the alternative, you get a relatively inefficient compiler that, despite its inefficiency, still gives you a more efficient development process. You get instant feedback, which you can't get from a testkit, and you can eliminate massive amounts of testing.

If the focus is entirely on reducing compile times, it can very quickly lead down a path that limits runtime performance. There is so much potential for increased runtime performance in strongly typed languages, due to the guarantees made by the type system. It allows for analysis and reasoning that not just enables micro-optimizations but also macro-optimizations across entire codebases. Entire theses have been written describing optimizations that rely on immutability alone, and we haven't even touched so many other possibilities that rust's design enables (such as the ownership system).

I'm fairly familiar with the project to work on the LLVM-IR code emitter. If the focus is on reducing the complexity and cruft of the LLVM-IR emitted, that's a win all around (because you get faster code and faster compile times). I just hope that reducing compile times doesn't become the core objective, because that eliminates a lot of future potential.

Note that Rust doesn't really generate worse LLVM IR than clang does, given the same input. It's just that as a language, Rust needs MIR optimizations more than C and C++ do, given the increased abstraction (largely necessary for safety).

I think your and OP's perspectives are not exclusive:

A. Yes, as you say, taken from a distance and considering the whole development lifecycle from idea to shipping, "compile time isn't really that bad for rust when you consider what it actually does for you".

B. However, -during- development, you want compile speed, regardless of all the benefits. I don't know about you, but I have a hard time rationalizing "it's okay, rustc is doing many things" at that moment. I just want it to be faster, and as OP mentioned, I hope there's a future where companies / sponsors get to "hiring about 25 people" to crunch and improve things noticeably (and I will do more than hope, I already donate money to Ferrous Systems to improve rust-analyzer, and will donate if a "make rust faster" fund asks for it). Your argument sounds to me a bit like "who cares about making Python faster? It's just glue". To which I answer: maybe (though not always), but even then but I'll take faster glue anytime :) .

To sum up: rust being already awesome and being "fast given all the thing it does" doesn't mean it shouldn't be faster, and doesn't mean speaking about it is futile.

I'm not opposed to faster compile times, and there is plenty of room for improvement, as I have already stated.

Generally speaking, if you are using incremental compilation, Rust is fast enough for development. It could be better, especially on large projects, but I haven't found it to be an issue generally. There are a bunch of improvements that could still be made. The LLVMIR issue mentioned above is one of them. But also, why do we have to rely on a code emitter at all? In development mode, the type checker should be incremental, synchronous, and fast...but the code emitter could be relatively slow as long as it is asynchronously coupled to the type checker.

But relentlessly pursuing the path of compiler performance can be a bad route to take, because it invariably leads to stagnation in compiler capabilities. I'd gladly accept a 10x increase in `opt-level=3` compile times if it meant 10% faster runtime performance. I'd gladly accept a 10x increase in `opt-level='z'` compile times if it meant a 50% decrease in binary sizes. We can't get to those capabilities if those possibilities are squashed by concerns about compile times.

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