Cargo Features

zmq = { version = "0.10.0", default-features = false, features = ["zmq_has"] }
default = zmq_has

The zmq_has feature is set by default whenever zmq is added without default-features = false somewhere in the dependency tree.

zmq_has default

zmq_has was added in zeromq 4.1. As we now require 4.1 or newer,
this feature is a no-op and only present for backward-compatibility;
it will be removed in the next API-breaking release.