Rust Conditional Compilation: #[cfg] & cfg! for Cross-Platform Code

Rust's conditional compilation allows selectively including or excluding code during compilation based on different conditions (such as target platform, feature flags, etc.). Mainly implemented through the #[cfg] attribute and cfg! macro.

1. #[cfg(...)] Attribute

Different functions will be compiled for different operating systems, with non-matching code being completely removed:

#[cfg(target_os = "linux")]
fn get_os_info(){
	println!("linux");
}
 
#[cfg(target_os = "windows")]
fn get_os_info() {
    println!("windows");
}
 
fn main() {
    get_os_info();
}

You can add combined conditions:

• all() means all conditions must be met.
• any() means any one condition being met is sufficient.
• not() means condition negation.

#[cfg(not(target_os = "linux"))]
fn get_os_info() {
    println!("not linux");
}

This function will be compiled on non-Linux systems.

2. cfg! Macro

The cfg! macro evaluates conditions during compilation and returns a bool value:

fn main() {
 
    if cfg!(target_os = "linux") {
        println!("linux");
    } else {
        println!("not linux");
    }
}

Which method to use depends on the specific situation.