Conditional Compilation

Rust has a special attribute, #[cfg], which allows you to compile code based on a flag passed to the compiler. It has two forms:

fn main() { #[cfg(foo)] fn foo() {} #[cfg(bar = "baz")] fn bar() {} }

#[cfg(bar = "baz")]

They also have some helpers:

fn main() { #[cfg(any(unix, windows))] fn foo() {} #[cfg(all(unix, target_pointer_width = "32"))] fn bar() {} #[cfg(not(foo))] fn not_foo() {} }
#[cfg(any(unix, windows))]

#[cfg(all(unix, target_pointer_width = "32"))]


These can nest arbitrarily:

fn main() { #[cfg(any(not(unix), all(target_os="macos", target_arch = "powerpc")))] fn foo() {} }
#[cfg(any(not(unix), all(target_os="macos", target_arch = "powerpc")))]

As for how to enable or disable these switches, if you’re using Cargo, they get set in the [features] section of your Cargo.toml:

# no features by default
default = []

# The “secure-password” feature depends on the bcrypt package.
secure-password = ["bcrypt"]

When you do this, Cargo passes along a flag to rustc:

--cfg feature="${feature_name}"

The sum of these cfg flags will determine which ones get activated, and therefore, which code gets compiled. Let’s take this code:

fn main() { #[cfg(feature = "foo")] mod foo { } }
#[cfg(feature = "foo")]
mod foo {

If we compile it with cargo build --features "foo", it will send the --cfg feature="foo" flag to rustc, and the output will have the mod foo in it. If we compile it with a regular cargo build, no extra flags get passed on, and so, no foo module will exist.


You can also set another attribute based on a cfg variable with cfg_attr:

fn main() { #[cfg_attr(a, b)] fn foo() {} }
#[cfg_attr(a, b)]

Will be the same as #[b] if a is set by cfg attribute, and nothing otherwise.


The cfg! syntax extension lets you use these kinds of flags elsewhere in your code, too:

fn main() { if cfg!(target_os = "macos") || cfg!(target_os = "ios") { println!("Think Different!"); } }
if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
    println!("Think Different!");

These will be replaced by a true or false at compile-time, depending on the configuration settings.