[steveklabnik]

Cargo does allow multiple versions of transitive dependencies. It tries to unify them as much as possible, but if it can't, it will bring in two different versions.

What it _won't_ let you do is pass a v1.0 type to something that requires a v2.0 type. This will result in a compilation error.

There's been some discussion around increasing Cargo's capacity in this regard (being able to say that a dependency is purely internal vs not), we'll see.