The positive side is that Debian isn’t the packager of record, so to speak: simplifying quite a bit, it’s pretending to be crates.io, just with a subset of the contents. That means that the developer doesn’t need to worry about it and can continue using Cargo and crates.io like normal, and the Debian packager, if there is one, just needs to translate Cargo.toml’s [dependencies] section into whatever Debian uses, and Debian-package any theretofore-unpackaged dependencies.

All up, this makes it genuinely practicable. I’m still not convinced it’s worthwhile (and it does make life a bit harder in other ways, especially in ways that I’m sure will become more apparent over time as the system gets used increasingly), but it’s not as unreasonable as it may initially sound.

Really, what’s being presented is Rust without crates.io (the domain name and content host), but with crates.io (the packages).