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 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.

> 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.