Cargo Features

approx = { version = "0.5.1", default-features = false, features = ["std", "num-complex"] }
default = std

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

std default

Features from optional dependencies

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

num-complex implicit feature

With default (std)