Ensure complete test coverage

// 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
}