Design code to work correctly with any combination of feature flags. When implementing conditional compilation with feature flags: 1. Use conditional imports for dependencies that are only needed when certain features are enabled: ```rust // Good: Only import when feature is enabled use polars_core::POOL; #[cfg(feature = "new_streaming")] use polars_core::StringCacheHolder; // Bad: Requiring dependencies unconditionally use polars_core::{POOL, StringCacheHolder}; // Breaks when "new_streaming" is disabled ``` 2. Consider user scenarios with different feature combinations. Test both with and without features enabled. 3. Ensure error messages reference correct feature names: ```rust // Good: Reference the correct feature name polars_bail!( ComputeError: "consider compiling with polars-bigidx feature, or set 'streaming'" ) // Bad: Referencing incorrect feature name polars_bail!( ComputeError: "consider compiling with polars-u64-idx feature, or set 'streaming'" ) ``` 4. When adding code conditionally compiled with feature flags, check that the code still compiles and functions correctly when those features are disabled. 5. Consider organizing feature-dependent code to minimize duplication while maintaining compatibility with all feature combinations.