Crate universe isolated makes splicing 4 times slower at least

[havasd]

After moving to rules_rust 0.57.1 version the splicing of the workspace is at least 4 times slower compared when running with isolated = True. When I set the isolated = False the runtime is comparable to the previous one.

We use from_specs with a pretty large monorepo which contains roughly ~500 external rust dependencies.

EDIT: clarified the issue

[UebelAndre]

Interesting, I would have expected isolated = False to speed things up as multiple crate_universe modules would be able to share the user's CARGO_HOME. The way crate_universe currently runs in bzlmod is somewhat flawed in that it's bottle-necked by evaluating each module serially. A nice improvement could be to avoid calling cargo-bazel for the repin check, splicing, and generation separately, and instead have the starlark batch all these into a single call to cargo-bazel where cargo-bazel is able to parallelize processing each module. This is something I thought of after migrating //examples/crate_universe to bzlmod which is now incredibly painful to run. Help would be greatly appreciated here!

[havasd]

Sorry, my phrasing was bad. When I set isolated = False it is 4 times faster.
To put into context on our repo it runs for 40+ seconds in isolated = True and 10+ seconds with isolated = False

[UebelAndre]

Yeah, that’s expected. Instead of reusing an already downloaded registry, cargo will download a fresh one which is not super fast.

[havasd]

Where did it re-use previously? Did it use the user's cargo cache?

[UebelAndre]

Yup! Whatever you set as CARGO_HOME or whatever cargo would identify as the default. It’s typically ~/.cargo