cc is kinda weird - it handles some of the linker flag shenanigans for you.
i thought i understood what was going on too, but i was wrong.
to add some flavour to this: if you add this to `foo/lib.rs`

```rust
pub fn call_foo_rust() {
    extern "C" {
        fn call_foo();
    }
    unsafe {
        call_foo();
    }
}
```

then in bar/lib.rs

```rust

fn _call_foo_rust() {
    foo::call_foo_rust();
}
```

even though no-one calls this function (and it's not public) baz links just fine 🙃