Feature flags in Rust

Rust allows to define feature flags to compile certain parts of a binary optionally. The Cargo Book has a section on them.

Cargo.toml file can have a [features] section. In this section, features can be defined, and they can allow other features.

[features]
my-feature = []
another-feature = ["my-feature"]

These markers are used in the code like:

#[cfg(feature = "my-feature")] 
pub mod my_module;

The default features are listed under default of [features] section. If it’s not defined, it’s considered blank, so cargo build supplies nothing as feature to the build system.

If you don’t want default features added automatically, you can use --no-default-features option in cargo.

Starting from Rust 1.60, dependencies can be tied to features. When you define a dependency in [dependencies] section, add optional = true to its options, and in [features] , use dep:package syntax.

[dependencies]

pack = {version = "1.0", optional = true }

[features]

my-feature = ["dep:pack"]

This way, cargo doesn’t compile pack if my-feature is not enabled.

An caveat to use features is that they should be additive. There shouldn’t be a no-std feature to remove stdlib dependency, instead you should define a std feature and enable it by default. The reason behind this is that cargo makes sure there is a single copy of a package with only the necessary features enabled for all the binary. If you have a no-x feature, this breaks the whole logic behind this operation.