Conditional Compilation - The Rust Programming Language

# 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:

    # #![allow(unused_variables)]
    #fn main() {
    #[cfg(foo)]
    # fn foo() {}

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

    #}

They also have some helpers:

    # #![allow(unused_variables)]
    #fn main() {
    #[cfg(any(unix, windows))]
    # fn foo() {}

    #[cfg(all(unix, target_pointer_width = "32"))]
    # fn bar() {}

    #[cfg(not(foo))]
    # fn not_foo() {}

    #}

These can nest arbitrarily:

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

    #}

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`:

    [features]
    # no features by default
    default = []

    # Add feature "foo" here, then you can use it.
    # Our "foo" feature depends on nothing else.
    foo = []

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:

    # #![allow(unused_variables)]
    #fn main() {
    #[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.

# cfg_attr

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

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

    #}

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

# cfg!

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

    # #![allow(unused_variables)]
    #fn main() {
    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.