Each rust crate is compiled on their own. Means if a library you pull in, needs the same crate as a another dependency, even with incompatible versions, it doesn’t matter. Then cargo understands the difference between test and build dependencies next to the dependencies for the actual lib/binaries.

The fact that each library and its dependencies gets compiled separately adds quite a lot in build time depending how many crates you reference. But you usually don‘t fight with dependency version issues. The only thing which is not expressed in crates is the minimum rust version needed. This is a pain point when you want or need to stay on a specific toolchain version. Because transient dependencies are defined by default like „major.minor.patch“ without any locking, cargo will pull the latest compatible version during an update or fresh checkout (means anything that is still compatible in terms of semantic versioning; e.g 1.1.0 means resolve a version that is >= 1.1.0 && < 2.0.0) And because toolchain version updates usually happen as minor updates it happens that a build suddenly fails after an update. Hope this makes sense.