Nix uses `cargo vendor` to literally vendor the source of all dependencies as a separate derivation. Maybe cargo keeps a global cache of the source as well when

Nix uses `cargo vendor` to literally vendor the source of all dependencies as a separate derivation. Maybe cargo keeps a global cache of the source as well when doing `cargo vendor`, I don't know, but Nix does construct a derivation that contains just the vendored dependency sources and then provides that as input when building the full crate.