Cargo Features

num_parser has no features set by default.

[dependencies]
num_parser = { version = "1.0.2", features = ["serde_support"] }
serde_support = serde

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.

serde serde_support?