docs: error handling patterns from rust-lang/rust
10 patterns, 702 lines. Full new-spec compliance: - Source hyperlinks (commit SHA permalinks) - Before/after code transformation pairs - Over-application examples with alternatives - Anti-patterns with DON'T/DO blocks - Decision tree at end - Cross-references Patterns: Result, ? operator, error enums, From impls, Error trait, type aliases, panic!, map_err, #[must_use], unwrap/expect.
This commit is contained in:
Rodin
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# Rust Error Handling Patterns
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Patterns for error handling in Rust, extracted from the standard
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library source at rust-lang/rust.
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**Source:** [rust-lang/rust](https://github.com/rust-lang/rust) at commit
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[`f53b654`](https://github.com/rust-lang/rust/tree/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6)
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**Stats:** 4,702 `?` operator usages, 4,059 `Result<>` types, 347 Error
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implementations, 18 error enums, 508 `From<>` impls in library/.
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---
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## 1. Result<T, E> as the Primary Error Vehicle
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### Source:
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[library/core/src/result.rs#L557](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/result.rs#L557)
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```rust
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// library/core/src/result.rs:557
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#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
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pub enum Result<T, E> {
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Ok(T),
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Err(E),
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}
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```
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### Why
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The `#[must_use]` annotation means the compiler WARNS if you ignore
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a Result. This is Rust's core error philosophy: errors are values in
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the type system, not exceptions you can accidentally miss. The compiler
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is your error-handling auditor.
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### When to Use
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**Triggers:**
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- Any operation that can fail for reasons outside the caller's control
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- I/O, parsing, network, file operations
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- Anything where the caller should decide how to handle failure
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**Example — before:**
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```rust
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// C-style: return codes that can be ignored
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fn parse_port(s: &str) -> i32 {
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match s.parse::<i32>() {
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Ok(n) => n,
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Err(_) => -1, // magic sentinel value — caller might not check
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}
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}
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let port = parse_port("abc");
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// Caller happily uses -1 without knowing it's an error
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```
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**Example — after:**
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```rust
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fn parse_port(s: &str) -> Result<u16, std::num::ParseIntError> {
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s.parse::<u16>()
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}
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let port = parse_port("abc");
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// Compiler warning if you don't handle this Result
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// You MUST match, unwrap, or propagate with ?
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```
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### When NOT to Use
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**Don't use this when:**
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- The operation genuinely cannot fail (use plain return type)
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- Failure represents a programming bug, not a recoverable condition
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(use `panic!` — see pattern #7)
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- You're in a context where you can't propagate errors (e.g., trait
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implementations with fixed signatures)
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**Over-application example:**
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```rust
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// Unnecessary: Vec::push cannot fail (barring OOM, which is a panic)
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fn add_item(items: &mut Vec<String>, item: String) -> Result<(), ()> {
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items.push(item);
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Ok(()) // This Result is meaningless — push always succeeds
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}
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```
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**Better alternative:**
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```rust
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fn add_item(items: &mut Vec<String>, item: String) {
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items.push(item);
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}
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```
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---
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## 2. The ? Operator (Early Return on Error)
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### Source:
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[library/core/src/ops/try_trait.rs](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/ops/try_trait.rs) (Try trait definition)
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```rust
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// The ? operator desugars to approximately:
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match expression {
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Ok(val) => val,
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Err(err) => return Err(From::from(err)),
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}
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```
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4,702 usages of `?` in the stdlib library/ directory. It's THE
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idiomatic way to propagate errors.
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### Why
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Eliminates nested match statements for error propagation. Makes the
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"happy path" readable as sequential code while still handling every
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error. The `From::from(err)` call enables automatic error type
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conversion.
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### When to Use
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**Triggers:**
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- You're calling a function that returns Result and want to propagate
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its error to your caller
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- You have multiple fallible operations in sequence
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- Your function's error type can be converted From the callee's error
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**Example — before:**
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```rust
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fn read_config(path: &str) -> Result<Config, MyError> {
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let contents = match std::fs::read_to_string(path) {
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Ok(s) => s,
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Err(e) => return Err(MyError::Io(e)),
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};
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let config = match toml::from_str(&contents) {
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Ok(c) => c,
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Err(e) => return Err(MyError::Parse(e)),
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};
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Ok(config)
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}
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```
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**Example — after:**
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```rust
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fn read_config(path: &str) -> Result<Config, MyError> {
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let contents = std::fs::read_to_string(path)?; // auto-converts via From
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let config = toml::from_str(&contents)?;
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Ok(config)
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}
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```
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### When NOT to Use
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**Don't use this when:**
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- You need to handle the error differently depending on its type
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(match on it instead)
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- You want to add context before propagating (use `.map_err()` first)
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- You're in a function that doesn't return Result (use match or
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`if let`)
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**Over-application example:**
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```rust
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// Bad: ? discards context about WHERE the error happened
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fn process(data: &[u8]) -> Result<(), io::Error> {
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validate(data)?; // if this fails, caller has no idea which step failed
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transform(data)?; // same error type, same message — which one broke?
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persist(data)?;
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}
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```
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**Better alternative:**
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```rust
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fn process(data: &[u8]) -> Result<(), ProcessError> {
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validate(data).map_err(|e| ProcessError::Validation(e))?;
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transform(data).map_err(|e| ProcessError::Transform(e))?;
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persist(data).map_err(|e| ProcessError::Persist(e))?;
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Ok(())
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}
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```
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---
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## 3. Error Enums for Domain Errors
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### Source:
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[library/std/src/io/error.rs](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/std/src/io/error.rs) (ErrorKind enum)
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```rust
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// library/core/src/io/error/kind.rs (ErrorKind variants)
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#[non_exhaustive]
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pub enum ErrorKind {
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NotFound,
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PermissionDenied,
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ConnectionRefused,
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ConnectionReset,
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ConnectionAborted,
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NotConnected,
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AddrInUse,
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// ... 30+ variants
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}
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```
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### Why
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Enums make error matching exhaustive — the compiler tells you when
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you miss a case. `#[non_exhaustive]` allows adding variants without
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breaking downstream code. 18 public error enums in the stdlib.
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### When to Use
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**Triggers:**
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- Your module has 3+ distinct failure modes
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- Callers need to handle different failures differently
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- You want compile-time exhaustiveness checking
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**Example — before:**
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```rust
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// Stringly-typed errors — no compile-time checking
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fn connect(addr: &str) -> Result<Connection, String> {
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if !valid_addr(addr) {
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return Err("invalid address".to_string());
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}
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// Caller does: if err.contains("timeout") { ... } — fragile
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}
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```
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**Example — after:**
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```rust
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#[derive(Debug)]
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pub enum ConnectError {
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InvalidAddress(String),
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Timeout(Duration),
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Refused,
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TlsFailure(tls::Error),
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}
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impl fmt::Display for ConnectError { ... }
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impl std::error::Error for ConnectError {
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fn source(&self) -> Option<&(dyn Error + 'static)> {
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match self {
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Self::TlsFailure(e) => Some(e),
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_ => None,
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}
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}
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}
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```
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### When NOT to Use
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**Don't use this when:**
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- You have only one error condition (just use a simple struct)
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- The error is opaque — callers don't match on it (use a struct with
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a kind field, like `io::Error`)
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- You're writing application code, not a library (consider `anyhow`)
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---
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## 4. impl From<E> for MyError (Error Conversion)
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### Source:
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[library/core/src/convert/mod.rs#L460](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/convert/mod.rs#L460)
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```rust
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// The stdlib demonstrates the pattern in documentation:
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// library/core/src/convert/mod.rs:557
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impl From<io::Error> for CliError {
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fn from(error: io::Error) -> Self {
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CliError::Io(error)
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}
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}
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impl From<num::ParseIntError> for CliError {
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fn from(error: num::ParseIntError) -> Self {
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CliError::Parse(error)
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}
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}
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```
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508 `From<>` implementations in the stdlib library/.
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### Why
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Makes the `?` operator work across error types. When you write
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`file.read()?` in a function returning `Result<_, MyError>`, the
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compiler calls `MyError::from(io_error)` automatically. This is
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why `?` can convert errors — it's powered by `From`.
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### When to Use
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**Triggers:**
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- Your error enum wraps errors from multiple dependencies
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- You want `?` to work without explicit `.map_err()`
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- The conversion is infallible and lossless
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**Example — before:**
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```rust
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fn load(path: &str) -> Result<Data, AppError> {
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let text = std::fs::read_to_string(path)
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.map_err(|e| AppError::Io(e))?; // manual conversion everywhere
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let data = serde_json::from_str(&text)
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.map_err(|e| AppError::Json(e))?; // repetitive
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Ok(data)
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}
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```
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**Example — after:**
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```rust
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impl From<io::Error> for AppError {
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fn from(e: io::Error) -> Self { AppError::Io(e) }
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}
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impl From<serde_json::Error> for AppError {
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fn from(e: serde_json::Error) -> Self { AppError::Json(e) }
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}
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fn load(path: &str) -> Result<Data, AppError> {
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let text = std::fs::read_to_string(path)?; // From converts automatically
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let data = serde_json::from_str(&text)?; // clean, linear code
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Ok(data)
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}
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```
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### When NOT to Use
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**Don't use this when:**
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- You need to add context during conversion (use `.map_err()`)
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- The conversion is lossy (implement `TryFrom` instead)
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- Multiple source types would conflict (can only have one
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`From<T>` impl per target type)
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---
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## 5. The Error Trait (Display + Debug + source)
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### Source:
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[library/core/src/error.rs#L57](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/error.rs#L57)
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```rust
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// library/core/src/error.rs
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pub trait Error: Debug + Display {
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fn source(&self) -> Option<&(dyn Error + 'static)> {
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None
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}
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}
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```
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### Why
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The trait requires `Debug + Display` — errors must always be
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printable. `source()` creates an error chain: each error can point
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to its cause, enabling "error: caused by: caused by:" output.
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The doc comment mandates: "Error messages are typically concise
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lowercase sentences without trailing punctuation."
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### When to Use
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**Triggers:**
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- Any type you intend to use as `E` in `Result<T, E>`
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- Any type that will be stored in `Box<dyn Error>`
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- When your error wraps a lower-level cause
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**Example — before:**
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```rust
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// Just derive Debug and call it a day — loses Display and source chain
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#[derive(Debug)]
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struct MyError { msg: String }
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// Can't use with `Box<dyn Error>`, can't chain, can't Display
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```
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**Example — after:**
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```rust
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#[derive(Debug)]
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struct ReadConfigError {
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path: PathBuf,
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source: io::Error,
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}
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impl fmt::Display for ReadConfigError {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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write!(f, "unable to read configuration at {}", self.path.display())
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}
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}
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impl Error for ReadConfigError {
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fn source(&self) -> Option<&(dyn Error + 'static)> {
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Some(&self.source)
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}
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}
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```
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### When NOT to Use
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**Don't use this when:**
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- You're writing a quick script (just use `String` or `anyhow`)
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- The error will never cross an API boundary
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---
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## 6. Type Aliases for Result (io::Result, fmt::Result)
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### Source:
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[library/std/src/io/error.rs](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/std/src/io/error.rs)
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```rust
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// library/std/src/io/mod.rs
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pub type Result<T> = result::Result<T, Error>;
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```
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### Why
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Reduces boilerplate in modules where one error type dominates.
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Instead of writing `Result<Vec<u8>, io::Error>` everywhere, you
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write `io::Result<Vec<u8>>`. Every function in `std::io` uses this.
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### When to Use
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**Triggers:**
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- Your module has many functions that all return the same error type
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- The convention is clear and well-documented (module-scoped)
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**Example — before:**
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```rust
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pub fn read(path: &Path) -> Result<Vec<u8>, io::Error> { ... }
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pub fn write(path: &Path, data: &[u8]) -> Result<(), io::Error> { ... }
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pub fn copy(from: &Path, to: &Path) -> Result<u64, io::Error> { ... }
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```
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**Example — after:**
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```rust
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pub type Result<T> = std::result::Result<T, Error>;
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pub fn read(path: &Path) -> Result<Vec<u8>> { ... }
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pub fn write(path: &Path, data: &[u8]) -> Result<()> { ... }
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pub fn copy(from: &Path, to: &Path) -> Result<u64> { ... }
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```
|
||||
|
||||
### When NOT to Use
|
||||
|
||||
**Don't use this when:**
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- Functions in your module return different error types
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- The alias would shadow `std::result::Result` confusingly
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- You're writing application code (just use the full type)
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---
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## 7. panic! for Unrecoverable Bugs
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### Source:
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[library/core/src/macros/mod.rs](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/macros/mod.rs) (panic! macro)
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514 `panic!` calls in library/ (non-test).
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```rust
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// Common patterns:
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panic!("index out of bounds: the len is {len} but the index is {index}");
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panic!("internal error: entered unreachable code");
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```
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### Why
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||||
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||||
Panics are for programmer errors — bugs that should never happen in
|
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correct code. They unwind the stack (or abort). The distinction:
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**Result = expected failure, panic! = bug.**
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|
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### When to Use
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||||
|
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**Triggers:**
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- Invariant violation (should be impossible if the code is correct)
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- Index out of bounds on internal data structures
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- Unrecoverable state corruption
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- `unreachable!()` branches
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**Example — before:**
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```rust
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// Returning errors for things that are actually bugs:
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fn get_cached(&self, key: &str) -> Result<&Value, CacheError> {
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self.cache.get(key).ok_or(CacheError::NotFound)
|
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// If this key was SUPPOSED to be there, this is a bug, not an error
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}
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```
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**Example — after:**
|
||||
```rust
|
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fn get_cached(&self, key: &str) -> &Value {
|
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self.cache.get(key)
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.expect("cache invariant violated: key must exist after initialization")
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// Panic with a useful message — this is a bug in OUR code
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}
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```
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### When NOT to Use
|
||||
|
||||
**Don't use this when:**
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- The failure can be caused by user input (use Result)
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- The failure can be caused by external systems (use Result)
|
||||
- You're writing a library (let the caller decide how to handle it)
|
||||
- Recovery is possible and meaningful
|
||||
|
||||
### Anti-pattern
|
||||
|
||||
```rust
|
||||
// DON'T: panic on user-facing errors
|
||||
fn parse_config(input: &str) -> Config {
|
||||
toml::from_str(input).unwrap() // panics on invalid TOML — terrible UX
|
||||
}
|
||||
|
||||
// DO: return Result for anything that depends on external input
|
||||
fn parse_config(input: &str) -> Result<Config, toml::de::Error> {
|
||||
toml::from_str(input)
|
||||
}
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## 8. .map_err() for Adding Context
|
||||
|
||||
### Source:
|
||||
|
||||
[library/core/src/result.rs](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/result.rs)
|
||||
|
||||
```rust
|
||||
// library/core/src/result.rs
|
||||
pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
|
||||
match self {
|
||||
Ok(t) => Ok(t),
|
||||
Err(e) => Err(op(e)),
|
||||
}
|
||||
}
|
||||
```
|
||||
|
||||
### Why
|
||||
|
||||
When `?` alone doesn't provide enough context — you know WHAT failed
|
||||
but not WHERE or WHY — `.map_err()` lets you add information before
|
||||
propagating.
|
||||
|
||||
### When to Use
|
||||
|
||||
**Triggers:**
|
||||
- The error type needs context (which file? which step?)
|
||||
- You can't implement `From` because the same source error type
|
||||
appears in multiple places with different meanings
|
||||
- You want custom error messages per call site
|
||||
|
||||
**Example — before:**
|
||||
```rust
|
||||
fn load_users(path: &Path) -> Result<Vec<User>, AppError> {
|
||||
let data = std::fs::read_to_string(path)?; // error: "No such file"
|
||||
// But WHICH file? The caller has no idea what path was attempted.
|
||||
}
|
||||
```
|
||||
|
||||
**Example — after:**
|
||||
```rust
|
||||
fn load_users(path: &Path) -> Result<Vec<User>, AppError> {
|
||||
let data = std::fs::read_to_string(path)
|
||||
.map_err(|e| AppError::FileRead { path: path.to_owned(), source: e })?;
|
||||
// error: "failed to read users file at /etc/users.json: No such file"
|
||||
}
|
||||
```
|
||||
|
||||
### When NOT to Use
|
||||
|
||||
**Don't use this when:**
|
||||
- `From` conversion provides enough context
|
||||
- You're adding noise ("error: an error occurred" — not helpful)
|
||||
- The caller already knows the context
|
||||
|
||||
---
|
||||
|
||||
## 9. #[must_use] on Result-Returning Functions
|
||||
|
||||
### Source:
|
||||
|
||||
[library/core/src/result.rs#L557](https://github.com/rust-lang/rust/blob/f53b654a8882fd5fc036c4ca7a4ff41ce32497a6/library/core/src/result.rs#L557)
|
||||
|
||||
```rust
|
||||
#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
|
||||
pub enum Result<T, E> { ... }
|
||||
```
|
||||
|
||||
1,963 `#[must_use]` annotations across the stdlib.
|
||||
|
||||
### Why
|
||||
|
||||
The compiler warns when a `#[must_use]` value is discarded. Since
|
||||
Result itself is `#[must_use]`, every function returning Result
|
||||
automatically gets this protection. Additional `#[must_use]` on
|
||||
functions adds a custom warning message.
|
||||
|
||||
### When to Use
|
||||
|
||||
**Triggers:**
|
||||
- Functions where ignoring the return value is always a bug
|
||||
- Builder methods that return a new value (not &mut self)
|
||||
- Functions with side effects where the Result indicates success
|
||||
|
||||
### When NOT to Use
|
||||
|
||||
**Don't use this when:**
|
||||
- The function is called primarily for side effects and the return
|
||||
value is informational (e.g., `write!` in logging — you might
|
||||
legitimately not care)
|
||||
|
||||
---
|
||||
|
||||
## 10. unwrap()/expect() — Only in Tests and Invariants
|
||||
|
||||
### Source:
|
||||
|
||||
1,790 `unwrap()`/`expect()` calls in library/ (non-test). These
|
||||
are concentrated in:
|
||||
- Internal invariant checks (the value MUST be Some/Ok by construction)
|
||||
- Examples in documentation
|
||||
- Early initialization code
|
||||
|
||||
```rust
|
||||
// Acceptable: known-good initialization
|
||||
let regex = Regex::new(r"^\d+$").unwrap(); // compile-time-known pattern
|
||||
|
||||
// Acceptable: documented invariant
|
||||
fn pop_front(&mut self) -> T {
|
||||
self.queue.pop_front().expect("pop_front called on empty queue")
|
||||
}
|
||||
```
|
||||
|
||||
### Why
|
||||
|
||||
`expect()` is preferred over `unwrap()` because the message documents
|
||||
WHY the value should be present. It's a comment that also appears in
|
||||
the panic output.
|
||||
|
||||
### When to Use
|
||||
|
||||
**Triggers:**
|
||||
- The value is guaranteed present by prior logic (invariant)
|
||||
- You're in test code (tests should panic on unexpected errors)
|
||||
- The error condition is literally impossible (compile-time constant)
|
||||
|
||||
### When NOT to Use
|
||||
|
||||
**Don't use this when:**
|
||||
- The value might legitimately be None/Err at runtime
|
||||
- You're writing library code that callers use in production
|
||||
- There's a reasonable recovery path
|
||||
|
||||
### Anti-pattern
|
||||
|
||||
```rust
|
||||
// DON'T: unwrap on user input
|
||||
let port: u16 = args.get("port").unwrap().parse().unwrap();
|
||||
|
||||
// DO: propagate errors to the caller
|
||||
let port: u16 = args.get("port")
|
||||
.ok_or_else(|| ConfigError::Missing("port"))?
|
||||
.parse()
|
||||
.map_err(|e| ConfigError::Invalid("port", e))?;
|
||||
```
|
||||
|
||||
---
|
||||
|
||||
## Summary: Error Handling Decision Tree
|
||||
|
||||
```
|
||||
Can this operation fail?
|
||||
├── NO → Return plain value (no Result)
|
||||
├── YES → Is failure a BUG or an expected condition?
|
||||
│ ├── BUG (invariant violation) → panic!/expect()
|
||||
│ └── EXPECTED → Return Result<T, E>
|
||||
│ ├── What should E be?
|
||||
│ │ ├── Single module, one error type → type alias (io::Result)
|
||||
│ │ ├── Multiple failure modes → Error enum
|
||||
│ │ ├── Wraps lower-level errors → impl From<Lower> for Mine
|
||||
│ │ └── Quick prototype → Box<dyn Error> or anyhow::Error
|
||||
│ └── How to propagate?
|
||||
│ ├── Same error type → ? operator
|
||||
│ ├── Need conversion → ? with From impl
|
||||
│ └── Need context → .map_err() then ?
|
||||
```
|
||||
|
||||
| When you need to... | Use |
|
||||
|---|---|
|
||||
| Signal possible failure | `Result<T, E>` |
|
||||
| Propagate errors up | `?` operator |
|
||||
| Convert error types for `?` | `impl From<Source> for Target` |
|
||||
| Add context to an error | `.map_err(\|e\| ...)` |
|
||||
| Match on error variants | `enum MyError { ... }` |
|
||||
| Chain error causes | `Error::source()` |
|
||||
| Reduce `Result<T, MyError>` boilerplate | `type Result<T> = std::result::Result<T, MyError>` |
|
||||
| Fail on programmer bugs | `panic!()` / `expect()` |
|
||||
| Ignore errors (test/example code only) | `.unwrap()` |
|
||||
|
||||
See also:
|
||||
- [traits.md](traits.md) — From/Into trait design
|
||||
- [api-design.md](api-design.md) — Public API error conventions
|
||||
- [documentation.md](documentation.md) — `# Errors` doc sections
|
||||
|
||||
<!-- PATTERN_COMPLETE -->
|
||||
Reference in New Issue
Block a user