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Comment by almostgotcaught | Hacker News Reader

You're wrong it's been debunked that the borrow checker is any appreciable part of the compile time - Steve Klabnik actually verified it on here somewhere.

Edit: found it

https://news.ycombinator.com/item?id=44391240

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I don't see that debunking it. Instead, it says "usually". That means that it depends on the project.

There is definitely Rust code that takes exponential time to compile, borrow checker or not.

https://play.rust-lang.org/?version=stable&mode=release&edit...

https://github.com/rust-lang/rust/issues/75992

Some people used async in ways that surfaced these problems. Upgraded rustc, then the project took forever to compile.

I say “usually” because of course sometimes bugs happen and of course you can conduct degenerate stress tests. But outside of those edge cases, it’s not an issue. If it were, blog posts that talk about lowering compile times would be discussing avoiding the borrow checker to get better times, but they never do. It’s always other things.

Is there any tool for Rust that does profiling that detects what part of compilation time is caused by what? Like, a tool that reports:

- Parsing: x ms

- Type checking: y ms

- LLVM IR generation: z ms

And have there been any statistics done on that across open-source projects, like mean, median, percentiles and so on?

I am asking because it should depend a lot on each project what is costly in compile time, making it more difficult to analyse. And I am also curious about how many projects are covered by "edge cases", if it is 1%, 0.1%, 0.01%, and so on.

The post my original comment is on discusses doing this at length.

> And have there been any statistics done on that across open-source projects, like mean, median, percentiles and so on?

I am not aware of any. But in all the posts on this topic over the years, codegen always ends up being half the time. It’s why cargo check is built the way it is, and why it’s always faster than a full build. If non-codegen factors were significant with any regularity, you’d be seeing reports of check being super slow compared to build.

I have actually seen a few posts here and there of 'cargo check' being slow. I have also heard of complaints of rust-analyzer being slow, though rust-analyzer may be doing more than just 'cargo check'.

https://www.reddit.com/r/rust/comments/1daip72/rust_checkrun...

May not be indicative, not sure what crate the author was using.

cargo check can be slow but what I mean is relative to a full build. Large projects are going to be slow to build by virtue of being large.

That project had 'cargo check' take 15-20 minutes, though it might not have been indicative, the submitter posted an update about how it was fixed.

This may be a better example.

https://github.com/rust-lang/rust/issues/132064

>cargo check with 1.82: 6m 19s

>cargo check with 1.81: 1m 22s

It may be difficult to fix.

https://github.com/rust-lang/rust/issues/132064#issuecomment...

>Triage notes (AFAIUI): #132625 is merged, but the compile time is not fully clawed back as #132625 is a compromise between (full) soundness and performance in favor of a full revert (full revert would bring back more soundness problems AFAICT)

Update: They fixed it, but another issue surfaced, possibly related, as I read the comments.

Yes, there's a -Ztime-passes, but it's nightly only (or stable with the bootstrap env var set)

My understanding is that -Ztime-passes isn't very accurate anymore, as it's not integrated into the query system well, or something.

Thanks, I'm pretty sure I was thinking of this whole discussion when I made my original comment :)

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