I can't speak for other distros, but at least in Fedora what happens is that library code is distributed in various devel packages, where the base package for, say, "futures-io" contains the actual code. So that's the "rust-futures-io-devel.noarch". After this, you get various "subpackages" for each feature. These are mostly there so that you can declare in a package that you need certain features, these packages are fully virtual, it seems, even though they all claim ownership of the relevant Cargo.toml in the local registry.

So to be fair, I was incorrect about it being a combinatorial explosion, since I was under the impression that each combination of features would be a package, but this makes a lot more sense. It's still a quite foreign way of packaging software, though. Although I'm glad that at least Cargo can be operated offline and from official repos.