If you only ever need to build Rust code then cargo is all you should need. However if you need to build Rust, C, C++, python, and JavaScript all together in one build then Cargo is inevitably going to make you sad. That is when you reach for something like Bazel.

I may be missing something, but it’s quite similar to pkgconfig+(cmake|autoconf). If you include a header, your build system probably needs to know where on the filesystem that header exists.