Write comprehensive tests that cover all relevant cases and variations while maintaining proper organization and structure. Key practices: 1. Place tests in appropriate locations (e.g., core tests in `library/coretests`) 2. Include FileCheck annotations for validation 3. Test both positive and negative cases separately 4. Cover analogous behaviors and backwards compatibility 5. Structure tests within proper test functions Example: ```rust // In library/coretests/your_module.rs #[test] fn test_feature_behavior() { // Test primary functionality let result = do_something(); assert_eq!(result.primary(), expected); // Test analogous case let alt_result = do_something_similar(); assert_eq!(alt_result.similar(), expected); // Test backwards compatibility assert_eq!(result.legacy_method(), old_expected); assert_eq!(result.new_method(), new_expected); } // Separate file for failure cases // In tests/ui/your_module_failures.rs //@ build-fail //@ ignore-pass fn test_invalid_case() { // Test expected failure let result = invalid_operation(); //~^ ERROR expected error message } ```