I've seen this error a lot when working with two different versions of the same library. Specifically, you can directly include version A, but a different library depends on a version B, with some of that exposed in the public API.
I think the compiler will now give you a hint that they may be from different versions. If not, next time you see it you should open up a bug in rustc.
To anyone who gets a cryptic error message - that's a bug, report it.
