I’d love to use Rust in the OS I work on, but integrating our build system with Cargo and dealing with external crates is a huge obstacle.

That's a challenging one. I'd like to have native support for C integration as well, but we'll have to balance that with the stability of the corresponding clang interfaces, and the requirement to ship extra libraries that the Rust toolchain doesn't currently ship.

Working on it, though.

> The rand and regex crates are two other examples.

rand I'd agree with, though it would need paring down to not have as many dependencies. For regex, there are multiple tradeoffs to be made; for instance, the most popular regex crate is fast and does most of what people want but doesn't have backreferences or lookahead/lookbehind assertions.

Ah, that's something I did not consider...

I would actually prefer if the Rust std would not include functionality that's not essential, for example sockets don't seem essential and the standard library does not implement them in a particularly complete and well thought out manner either IMHO.