[SimonSapin]

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`.