Cargo Features

[dependencies]
decimate = { version = "0.2.2", default-features = false, features = ["rustc-serialize", "serde"] }
default = serde

The serde feature is set by default whenever decimate 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.

rustc-serialize implicit feature
serde default