% 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:

```rust
#[cfg(foo)]
# fn foo() {}

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

They also have some helpers:

```rust
#[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:

```rust
#[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][features] of your `Cargo.toml`:

[features]: http://doc.crates.io/manifest.html#the-features-section

```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`:

```text
--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:

```rust
#[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`:

```rust
#[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!` [syntax extension][compilerplugins] lets you use these kinds of flags
elsewhere in your code, too:

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

[compilerplugins]: compiler-plugins.html

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