I had similar issues but I was able go largely solve them.

Break down your project in crates in a cargo workspace. My reload time went from 10/15s to ~4.

I followed the instructions here: https://benw.is/posts/how-i-improved-my-rust-compile-times-b... . Specifically the 2nd part of the post

I've tried following that same guide, and I believe I've tried every thing on that blog post except the mold linker, because I'm on MacOS, and the MacOS version of the mold repo says "use the default linker if you have XCode 15 or higher."

And I do think I have a pretty granular crate system (would be _very_ happy to hear otherwise, because that would mean there's low hanging compile time fruit!): https://github.com/dnaaun/heimisch

My current _incremental_ compilation time swings anywhere between 15 seconds and 3 minutes (no, I'm not kidding). And I work on an M3 max macbook pro.

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Things that I suspect are making my compile times worse:

  1. The fact that I am doing SSR, which means my frontend code is included in the backend code as well.
  2. I _think_ Rust is unnecessarily recompiling dependencies on incremental builds?  (I don't understand how incremental compilation times can be so bad otherwise). But I'm clueless about how to go about debugging that.

I also had an issue where Rust was recompiling dependencies unnecessarily. It turned out to be because rust-analyzer has one more level of `Bash` than VSCode's integrated terminal. My `.bashrc` was loading something unconditionally which meant if you nested bash sessions the PKG_CONFIG_PATH changed (duplicate entries).

I had OpenSSL as a dependency, and if `PKG_CONFIG_PATH` changes it rebuilds it (this is correct), which means if you make an edit to a file and save it, rust-analyzer would blow away the cache, then you build it on the command line and it blows it away again.

To test:

1. If you quit VSCode and do an incremental build is it still slow? 2. Try `CARGO_LOG=cargo::core::compiler::fingerprint=info cargo build` (took me a while to find that; there is a bug open to make it less stupidly hard to find).

That will print a load of info about why Cargo is rebuilding stuff. Note that it isn't really in a sensible order - the first message isn't necessarily the cause. The message about the environment variable changing was somewhere in the middle for me, so read the whole log.

Thanks for that very helpful tip! According to the output of that, no unnecessary recompilation is taking place.

I actually figured out the issue! It turns out, I had unquestioningly transplanted the following from the blog post in discussion, and that is the cause of all my issues:

    [profile.dev]
    opt-level = 1

    [profile.dev.package."*"]
    opt-level = 3

Removing that seems to give me an order (or two) of magnitude reduction in compile times.

(Worth noting: it was passing RUSTFLAGS=-Ztime-passes that made me look here because, apparently, `llvm_passes` was taking the majority of the time).

Just skimmed through it.

One thing that popped out is that you might want to try to define dependencies at the workspace level and then in the sub-crates point to those instead.

Otherwise you are correct, your setup is already very granular. Furthermore, I have no clue about why it takes so long on your end :/ . I am running on Linux, so I wonder if there's something in the MacOs stack that messes things up and so you end up recompiling dependencies over and over.

Another thing I noticed in my setup is that one of the biggest offenders was sqlx if the macro feature was enabled since the queries were ran at each incremental compilation.

I so deeply appreciate your time, thank you!

I figured out the issue: I transplanted the bit about setting optimization levels from the blogpost into my setup (which sets opt-level = 3 for all dependencies), and when I removed that, my compile times went down by a factor of 5-10!

Np! And glad you managed to also solve the issue! Sadly with niche things (leptos still is IMHO) hiccups are expected:)

> I _think_ Rust is unnecessarily recompiling dependencies on incremental builds? (I don't understand how incremental compilation times can be so bad otherwise). But I'm clueless about how to go about debugging that.

Make sure that your ide/vscode is using the exact same rust toolchain as you to build/check/run. Otherwise you don't get incremental compiling but recompilation a lot. I think there's an environment variable to force it.

It turns out this wasn't my issue (more on this on other threads here), but I genuinely appreciate your time and help.

Yeah compile times were an issue back when i was trying Dioxus out too. Their `hot reload` was noticably not terribly hot for me. Stark contrast to what I'm used to in the node.js world.

This version fixes a lot of inconsistencies in our hot-reload. We can't reload all Rust code, but we can reload some simple Rust expressions and way more RSX.