Cargo Features

[dependencies]
circle = { version = "0.1.9", default-features = false, features = ["f32", "f64", "serde"] }
default = f32

The f32 feature is set by default whenever circle is added without default-features = false somewhere in the dependency tree.

f32 default
f64

Features from optional dependencies

serde implicit feature

Enables serde

serde:

A generic serialization/deserialization framework