CRITICAL: Use for error handling. Triggers: Result, Option, Error, ?, unwrap, expect, panic, anyhow, thiserror, when to panic vs return Result, custom error, error propagation, 错误处理, Result 用法, 什么时候用 panic

Error Handling Layer 1: Language Mechanics Core Question Is this failure expected or a bug? Before choosing error handling strategy: Can this fail in normal operation? Who should handle this failure? What context does the caller need? Error → Design Question Pattern Don't Just Say Ask Instead unwrap panics "Use ?" Is None/Err actually possible here? Type mismatch on ? "Use anyhow" Are error types designed correctly? Lost error context "Add .context()" What does the caller need to know? Too many error variants "Use Box<dyn Error " Is error granularity right? Thinking Prompt Before handling an error: 1. What kind of failure is this? Expected → Result<T, E Absence normal → Option<T Bug/invariant → panic! Unrecoverable → panic! 2. Who handles this? Caller → propagate with ? Current function → match/if let User → friendly error message Programmer → panic with message 3. What context is needed? Type of error → thiserror variants Call chain → anyhow::Context Debug info → anyhow or tracing Trace Up ↑ When error strategy is unclear: Situation Trace To Question Too many unwraps m09 domain Is the data model right? Error context design m13 domain error What recovery is needed? Library vs app errors m11 ecosystem Who are the consumers? Trace Down ↓ From design to implementation: Quick Reference Pattern When Example Result<T, E Recoverable error fn read() Result<String, io::Error Option<T Absence is normal fn find() Option<&Item ? Propagate error let data = file.read()?; unwrap() Dev/test only config.get("key").unwrap() expect() Invariant holds env.get("HOME").expect("HOME set") panic! Unrecoverable panic!("critical failure") Library vs Application Context Error Crate Why Library thiserror Typed errors for consumers Application anyhow Ergonomic error handling Mixed Both thiserror at boundaries, anyhow internally Decision Flowchart Common Errors Error Cause Fix unwrap() panic Unhandled None/Err Use ? or match Type mismatch Different error types Use anyhow or From Lost context ? without context Add .context() cannot use ? Missing Result return Return Result<(), E Anti Patterns Anti Pattern Why Bad Better .unwrap() everywhere Panics in production .expect("reason") or ? Ignore errors silently Bugs hidden Handle or propagate panic! for expected errors Bad UX, no recovery Result Box<dyn Error everywhere Lost type info thiserror Related Skills When See Domain error strategy m13 domain error Crate boundaries m11 ecosystem Type safe errors m05 type driven Mental models m14 mental model

m06-error-handling