cfg attribute for conditional compilation

The #[cfg(...)] attribute enables conditional compilation based on features, target OS, or build profiles. I use #[cfg(test)] for test-only code, #[cfg(target_os = \"linux\")] for platform-specific logic, and #[cfg(feature = \"...\")] for optional features. The compiler excludes disabled code entirely, so there's no runtime cost. For inline conditions, use cfg!(...) which evaluates to a boolean. I also use #[cfg(debug_assertions)] for debug-only checks. Conditional compilation is powerful for cross-platform libraries, feature flags, and test utilities. It keeps the codebase clean and binaries small.