Cargo Features

happy-eyeballs = { version = "0.2.1", default-features = false, features = ["std-net", "async-std", "tokio"] }
default = std-net

The std-net feature is set by default whenever happy-eyeballs is added without default-features = false somewhere in the dependency tree.

std-net default = libc, polling, socket2

Features from optional dependencies

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

libc std-net

With default (std)

polling std-net

Enables default (std) of polling ^2

async-std implicit feature

With default features

Affects happy-eyeballs::async_std

tokio implicit feature

With io-util, macros, net, rt and time

Affects happy-eyeballs::tokio

socket2 std-net