Cargo Features

tun-tap = { version = "0.1.4", default-features = false, features = ["tokio"] }
default = tokio

The tokio feature is set by default whenever tun-tap is added without default-features = false somewhere in the dependency tree.

tokio default = futures, libc, mio, tokio-core

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features.

futures tokio

Enables futures ~0.1

libc tokio
mio tokio

Enables mio ~0.6

tokio-core tokio