Cargo Features

flat_map = { version = "0.0.10", default-features = false, features = ["std", "serde1"] }
default = std

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

std default
serde1 = serde, serde_derive

Features from optional dependencies

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

serde serde1?
serde_derive serde1?