I'd say rustdoc is pretty rough. It's very hard to navigate the rust stdlib vs, say, Go, and when you do find what you want, it's often specified in some very cryptic manner that seems to require a PL PhD.

It is a really big barrier to adoption because it's very hard to find out what functionality is or is not offered. Here's an example: I want to iterate over a set. Easy, right?

No.

http://static.rust-lang.org/doc/master/std/collections/hash_...

Look at the signature for Cycle. It's really hard to know what this does even though it's trivial:

fn cycle(self) -> Cycle<Self> where Self: Clone

When you click through to the real documentation, it describes a useful use case for this. But that's not how programmers think. They think "I need to do X right now so I'll find something that looks right" not "I'm going to click through the documentation... oh X looks interesting I'll remember that later". There's no way to infer what Cycle really does without having actually clicked through or used it.