How to Use Conditional Dependencies in Cargo

Use the optional flag in your Cargo.toml dependencies and enable them via features or environment variables. Add optional = true to the dependency definition and reference it in a feature or conditional compilation block.

[dependencies]
serde = { version = "1.0", optional = true }

[features]
default = []
with-serde = ["serde"]

Enable the dependency by running cargo build --features with-serde or by setting RUSTFLAGS="--cfg feature=\"with-serde\"" for environment-based control.

Conditional dependencies let you include extra libraries only when needed, keeping your project small and fast by default. Think of it like a car with optional accessories; you only pay for and install the sunroof if you specifically ask for it. This is useful when a feature is only required for certain platforms or advanced use cases.