If you need to use (for the sake of the example) Rusoto, since it is based on tokio, you'll need to set up the Tokio executor or at least add a macro to your main for this to be done for you. I believe Rusoto actually would take care of this for you if you haven't done it yourself however friction arises when you were already using a different version of tokio and Rusoto is built against another.

Basically, it isn't entirely opaque to you how it is handled.