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Comment by dastbe | Hacker News Reader

How does Cargo handle indirect dependency visibility? This article talks about visibility in terms of "do I need to manually include indirect dependencies? No!" but not in terms of "can I accidentally write code against an indirect dependency?" which isn't sufficiently answered.

If the answer is that indirect dependencies are still visible, I'd be interested in knowing if rust-lang/cargo plan to change that, similar to JDK9's module system for re-export.

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Short answer: `extern crate foo;` doesn’t work if `foo` is an indirect dependency. So you cannot accidentally use something without declaring it in `Cargo.toml`.

Longer answer: when running rustc, Cargo passes individual `--extern bar=/path/to/bar.rlib` options for each direct dependency, not just a path to a directory with everything. You can see the exact command with `cargo build --verbose` or `cargo build -v`.

As the sibling answer says, they are not visible.

However, crates can re-export symbols they have imported from their dependency, which couples their semver to that dependency's in a way that people have often not realized. This has been the source of some upgrade headaches so far, but its less of a problem than it has been with languages in my experience.

It's also relevant that rust can use multiple direct/indirect dependencies of different versions, due to the way that symbols are exported.

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