Using crates.io is entirely optional, you can download a library's source code and specify the path to it in your cargo config file. (Which is not uncommon in production)
For that matter, using cargo is optional, you can compile rust code using GNU make or shell scripts if you want to. (That's what the Linux kernel does)
For that matter, using cargo is optional, you can compile rust code using GNU make or shell scripts if you want to. (That's what the Linux kernel does)