How to Use Feature Flags for Optional Functionality

Use the #![feature] attribute at the top of your crate root to enable unstable features, which requires compiling with the nightly toolchain. Add the specific feature flag name inside the attribute and ensure you are using rustup to switch to the nightly channel before building.

#![feature(your_feature_name)]

fn main() {
    // Code using the unstable feature
}

Run the following command to switch to the nightly toolchain and compile your code:

rustup run nightly cargo build

Feature flags are switches that let you turn on experimental tools in Rust before they are officially ready for everyone. You use them when you need a new capability that hasn't been released yet, but you must use a special 'nightly' version of the compiler to make them work. Think of it like a beta test mode for your code that gives you access to future features.