Dependencies for all platforms are downloaded.

[kevincox]

When building for a single platform naersk downloads the dependencies for all platforms. Ideally it would only download for the current target. For example when building a project that depends on tokio on linux it downloads all of the winapi crates.

One downside is that the crates derivation will be different for different OSes. I don't think this is an issue because right now the checksum is done per-crate so that shouldn't be an issue. Maybe slightly wasted space if we build up different dependency trees for different platforms? But symlinking or hardlinking would likely resolve that.

[Patryk27]

Hmm, but that's a chicken-egg problem, no?

We have to download / checkout crate even if to read its Cargo.toml (to see the targets), and if we do that, then we might as well just keep the downloaded crate around.

Is there some magic Cargo.lock-per-platform I'm not aware of? 😅

[Patryk27]

Ah, cargo metadata returns dependencies[].target - that could come useful 👀

Edit: even better, cargo metadata supports --filter-platform.

[kevincox]

Yup, the dependant crate has the dependency conditions in its Cargo.toml. If you look at Tokio's dependencies you will see that lots of them are conditional on the platform.