←back to thread

Why is the Rust compiler so slow?

(sharnoff.io)
302 points Bogdanp | 8 comments | 26 Jun 25 19:29 UTC | HN request time: 0.031s | source | bottom
Show context
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.
replies(18): >>44391046 #>>44391066 #>>44391100 #>>44391170 #>>44391214 #>>44391359 #>>44391671 #>>44391740 #>>44393057 #>>44393294 #>>44393629 #>>44394710 #>>44395044 #>>44395135 #>>44395226 #>>44395485 #>>44396044 #>>44401496 #
1. ceronman ◴[26 Jun 25 21:05 UTC] No.44391359[source]▶
I bet that if you take those 278k lines of code and rewrite them in simple Rust, without using generics, or macros, and using a single crate, without dependencies, you could achieve very similar compile times. The Rust compiler can be very fast if the code is simple. It's when you have dependencies and heavy abstractions (macros, generics, traits, deep dependency trees) that things become slow.
replies(2): >>44391384 #>>44392083 #
2. 90s_dev ◴[26 Jun 25 21:08 UTC] No.44391384[source]▶
I can't help but think the borrow checker alone would slow this down by at least 1 or 2 orders of magnitude.
replies(3): >>44391473 #>>44391548 #>>44391760 #
3. steveklabnik ◴[26 Jun 25 21:15 UTC] No.44391473[source]▶
Your intuition would be wrong: the borrow checker does not take much time at all.
4. FridgeSeal ◴[26 Jun 25 21:21 UTC] No.44391548[source]▶
Again, as this been often repeated, and backed up with data, the borrow-checker is a tiny fraction of a Rust apps build time, the biggest chunk of time is spent in LLVM.
5. tomjakubowski ◴[26 Jun 25 21:42 UTC] No.44391760[source]▶
The borrow checker is really not that expensive. On a random example, a release build of the regex crate, I see <1% of time spent in borrowck. >80% is spent in codegen and LLVM.
6. taylorallred ◴[26 Jun 25 22:24 UTC] No.44392083[source]▶
I'm curious about that point you made about dependencies. This Rust project (https://github.com/microsoft/edit) is made with essentially no dependencies, is 17,426 lines of code, and on an M4 Max it compiles in 1.83s debug and 5.40s release. The code seems pretty simple as well. Edit: Note also that this is 10k more lines than the OP's project. This certainly makes those deps suspicious.
replies(1): >>44392867 #
7. MindSpunk ◴[27 Jun 25 00:43 UTC] No.44392867[source]▶
The 'essentially no dependencies' isn't entirely true. It depends on the 'windows' crate, which is Microsoft's auto-generated Win32 bindings. The 'windows' crate is huge, and would be leading to hundreds of thousands of LoC being pulled in.

There's some other dependencies in there that are only used when building for test/benchmarking like serde, zstd, and criterion. You would need to be certain you're building only the library and not the test harness to be sure those aren't being built too.

replies(1): >>44396190 #
8. ◴[27 Jun 25 12:15 UTC] No.44396190{3}[source]▶