Attribute cold 

Hint to the compiler that a function is unlikely to be called.

Marking a function #[cold] tells the compiler that calls to it are rare, so it can optimize for the common case where the function is not called. It is only a hint: the compiler may ignore it, and it does not change the function’s behavior.

It is typically used on functions that handle uncommon cases, such as error or panic paths:

fn check(value: i32) {
    if value < 0 {
        report_error("value must be non-negative");
    }
    // ... the common case continues here ...
}

#[cold]
fn report_error(message: &str) {
    eprintln!("error: {message}");
}

For more information, see the Reference on the cold attribute.