So this is really exciting for anyone looking to privately develop or distribute Rust crates (packaged libraries), or to mirror some portion of crates.io for other reasons (e.g. availability, isolation, modification of public crates, etc.).
[1]: Rust 1.34: https://blog.rust-lang.org/2019/04/11/Rust-1.34.0.html
Edit: Someone else got there before me. Dupe of: https://news.ycombinator.com/item?id=19798413