Cargo Features

s3-algo = { version = "0.6.1", default-features = false, features = ["serde1"] }
default = serde1

The serde1 feature is set by default whenever s3-algo is added without default-features = false somewhere in the dependency tree.

serde1 default = serde

Features from optional dependencies

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

serde serde1

With derive and default (std)