Cargo Features

[dependencies]
threefish = { version = "0.5.2", default-features = false, features = ["zeroize", "cipher"] }
default = cipher

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

Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features. These features may have been created by mistake, and this functionality may be removed in the future.

zeroize implicit feature
cipher default