<skill_overview> <purpose>Design predictable and ergonomic error handling in Rust</purpose> <triggers> <trigger>Designing error types</trigger> <trigger>Propagating errors with Result</trigger> <trigger>Mapping errors at API boundaries</trigger> <trigger>Choosing between panic and Result</trigger> </triggers> <sources> <source url="https://doc.rust-lang.org/book/ch09-00-error-handling.html">The Rust Book - Error Handling</source> </sources> </skill_overview> <panic_vs_result> <use_panic_when> <case>Unrecoverable, invariant-breaking failures</case> <case>Bug in the program (logic error)</case> </use_panic_when> <use_result_when> <case>Expected errors (not found, validation)</case> <case>I/O or external dependency failures</case> </use_result_when> </panic_vs_result> <error_types> <rules> <rule>Define a dedicated error enum for a module or service</rule> <rule>Implement Display and Error for custom errors</rule> <rule>Use From to convert lower-level errors</rule> </rules> <example> <code> enum ServiceError { NotFound, InvalidInput, Io(std::io::Error) } </code> </example> </error_types> <propagation> <rules> <rule>Use ? to propagate errors</rule> <rule>Add context with map_err or custom variants</rule> <rule>Preserve original error for debugging</rule> </rules> </propagation> <boundary_mapping> <rules> <rule>Map internal errors to public error codes/messages</rule> <rule>Do not leak sensitive details to clients</rule> </rules> </boundary_mapping> <anti_patterns> <avoid name="unwrap_in_flow">Avoid unwrap/expect for normal flow</avoid> <avoid name="string_errors">Avoid plain String errors without structure</avoid> <avoid name="panic_for_validation">Do not panic on validation failures</avoid> </anti_patterns>