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Why is the Rust compiler so slow?

(sharnoff.io)
302 points Bogdanp | 1 comments | 26 Jun 25 19:29 UTC | HN request time: 0s | source
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taylorallred ◴[26 Jun 25 20:29 UTC] No.44390996[source]▶
So there's this guy you may have heard of called Ryan Fleury who makes the RAD debugger for Epic. The whole thing is made with 278k lines of C and is built as a unity build (all the code is included into one file that is compiled as a single translation unit). On a decent windows machine it takes 1.5 seconds to do a clean compile. This seems like a clear case-study that compilation can be incredibly fast and makes me wonder why other languages like Rust and Swift can't just do something similar to achieve similar speeds.
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js2 ◴[26 Jun 25 20:35 UTC] No.44391046[source]▶
"Just". Probably because there's a lot of complexity you're waving away. Almost nothing is ever simple as "just".
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taylorallred ◴[26 Jun 25 20:51 UTC] No.44391231[source]▶
That "just" was too flippant. My bad. What I meant to convey is "hey, there's some fast compiling going on here and it wasn't that hard to pull off. Can we at least take a look at why that is and maybe do the same thing?".
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steveklabnik ◴[26 Jun 25 20:53 UTC] No.44391246[source]▶
> "hey, there's some fast compiling going on here and it wasn't that hard to pull off. Can we at least take a look at why that is and maybe do the same thing?".

Do you really believe that nobody over the course of Rust's lifetime has ever taken a look at C compilers and thought about if techniques they use could apply to the Rust compiler?

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taylorallred ◴[26 Jun 25 20:57 UTC] No.44391276[source]▶
Of course not. But it wouldn't surprise me if nobody thought to use a unity build. (Maybe they did. Idk. I'm curious).
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1. steveklabnik ◴[26 Jun 25 21:06 UTC] No.44391368[source]▶
Rust and C have differences around compilation units: Rust's already tend to be much larger than C on average, because the entire crate (aka tree of modules) is the compilation unit in Rust, as opposed to the file-based (okay not if you're on some weird architecture) compilation unit of C.

Unity builds are useful for C programs because they tend to reduce header processing overhead, whereas Rust does not have the preprocessor or header files at all.

They also can help with reducing the number of object files (down to one from many), so that the linker has less work to do, this is already sort of done (though not to literally one) due to what I mentioned above.

In general, the conventional advice is to do the exact opposite: breaking large Rust projects into more, smaller compilation units can help do less "spurious" rebuilding, so smaller changes have less overall impact.

Basically, Rust's compile time issues lie elsewhere.