rust `cfg` which is always true / false

The cfg attribute in Rust is used to conditionally compile code based on certain conditions. It is often used to include or exclude specific sections of code based on the target platform, operating system, or other compile-time conditions.

The cfg attribute takes a boolean expression as its argument. If the expression evaluates to true, the code within the cfg attribute will be compiled. If the expression evaluates to false, the code within the cfg attribute will be ignored by the compiler.

Here are a few examples of how the cfg attribute can be used:

#[cfg(target_os = "windows")]
fn main() {
    println!("This is a Windows-specific code");
}

#[cfg(not(target_os = "windows"))]
fn main() {
    println!("This is code for non-Windows platforms");
}

#[cfg(feature = "debug")]
fn main() {
    println!("This code is only included when the 'debug' feature is enabled");
}

In the first example, the cfg attribute is used to compile different code depending on the target operating system. The code within the cfg(target_os = "windows") attribute will only be compiled when the target operating system is Windows.

In the second example, the cfg attribute is used with the not keyword to exclude code when the target operating system is Windows. This code will be compiled for non-Windows platforms.

In the third example, the cfg attribute is used with the feature keyword to conditionally compile code based on whether a specific feature is enabled. This can be useful when working with Rust crates that have optional features.

Overall, the cfg attribute in Rust allows for conditional compilation of code based on various conditions, providing flexibility and platform-specific optimizations.