Rust Impl Error Handling

rust-impl-error-handling

Recoverable errors are values: Result<T, E> flows through the call graph, the ? operator early-returns on Err with an automatic From conversion, and a well-designed error type implements Display + std::error::Error (or core::error::Error in no_std). Panic is reserved for programmer bugs, not for expected failure modes.

Cross-references: [[rust-errors-thiserror-anyhow]] (library vs application error-crate choice), [[rust-errors-runtime]] (panic mechanics, unwind vs abort), [[rust-errors-build-link]] (compile-time error decoding), [[rust-syntax-pattern-matching]] (matching on Result and Option).

When to use this skill

• User writes .unwrap() or .expect() in library code.
• User asks "should this return Result or Option".
• User asks "why does ? not compile" or gets E0277: the trait From<X> for Y is not implemented.
• User designs a new error type and chooses between enum and struct.
• User asks about core::error::Error vs std::error::Error.
• User wants to chain errors with a source (cause) pointer.
• User asks "should I panic here, or return Result".
• User looks for Result combinators (map, map_err, and_then, or_else, ok_or).

Result vs Option: the rule

ALWAYS return Result<T, E> when the caller needs to know why the operation failed. NEVER return Option<T> to mask an error: the caller loses the cause and cannot recover intelligently.

Convert between them when bridging APIs:

let v: Option<i32> = Some(7);
let r: Result<i32, &str> = v.ok_or("missing");          // Option -> Result
let r2: Result<i32, &str> = v.ok_or_else(|| "missing"); // lazy variant

let r: Result<i32, &str> = Ok(7);
let o: Option<i32> = r.ok();                            // Result -> Option (discards Err)

NEVER discard Err via .ok() in library code. Either propagate via ? or transform via .map_err(...).

The ? operator: desugaring and the From chain

expr? on a Result<T, E> desugars (roughly) to:

match expr {
    Ok(v)  => v,
    Err(e) => return Err(From::from(e)),
}

The enclosing function MUST return a Result<_, E2> where E2: From<E>. If the conversion is not available the compiler reports E0277: ? couldn't convert the error: the trait From<E> for E2 is not implemented.

use std::fs;
use std::io;
use std::num::ParseIntError;

#[derive(Debug)]
enum MyError {
    Io(io::Error),
    Parse(ParseIntError),
}

impl From<io::Error> for MyError {
    fn from(e: io::Error) -> Self { MyError::Io(e) }
}
impl From<ParseIntError> for MyError {
    fn from(e: ParseIntError) -> Self { MyError::Parse(e) }
}

fn parse_number_from_file(path: &str) -> Result<i32, MyError> {
    let s = fs::read_to_string(path)?;   // io::Error -> MyError via From
    let n: i32 = s.trim().parse()?;      // ParseIntError -> MyError via From
    Ok(n)
}

ALWAYS provide From<InnerError> for every error type that callers may propagate through ?. NEVER write a match expr { Ok(v)=>v, Err(e)=>return Err(MyError::X(e)) } chain by hand: that is exactly what ? does, and the manual form rots when error variants grow.

The same desugaring applies to Option<T> (None propagates), to ControlFlow, and to any type that implements the unstable Try trait. For stable code, mentally treat ? as Result-or-Option early return + From conversion.

The Error trait: Display + source chain

The std::error::Error trait is the contract for errors:

pub trait Error: Debug + Display {
    fn source(&self) -> Option<&(dyn Error + 'static)> { None }
    // (provided method; default returns None)
}

Any type that wants to participate in the wider error ecosystem (anyhow, eyre, ? to Box<dyn Error>, error reporters) MUST implement:

1. Debug (derive it).
2. Display (write the user-facing message, NO trailing newline, NO leading "Error:" prefix).
3. Error (override source() to return the underlying cause when wrapping another error).

use std::fmt;

#[derive(Debug)]
struct ConfigError {
    path: String,
    source: std::io::Error,
}

impl fmt::Display for ConfigError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "failed to load config {}", self.path)
    }
}

impl std::error::Error for ConfigError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        Some(&self.source)
    }
}

ALWAYS implement source() when your error wraps another error. Downstream tools (anyhow report, eyre's report, tracing::error!) walk the chain via source() to print the full cause.

NEVER cram the cause into the Display message (e.g. format!("config failed: {}", inner)). Report frameworks walk source() and would print the cause twice.

core::error::Error (stabilized 1.81)

Since Rust 1.81 (2024-09-05) the Error trait is also available as core::error::Error, identical signature, usable in no_std. In std crates std::error::Error is a re-export of core::error::Error, so existing code is unaffected.

ALWAYS use core::error::Error if your crate is no_std. NEVER use std::error::Error in a no_std crate: the import will fail to resolve.

#![no_std]
extern crate alloc;
use core::error::Error;
use core::fmt;

#[derive(Debug)]
pub struct ProtocolError;

impl fmt::Display for ProtocolError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("protocol violation")
    }
}

impl Error for ProtocolError {}

Custom error type design: enum vs struct

// Enum: one variant per kind
#[derive(Debug)]
pub enum LoadError {
    Io { path: String, source: std::io::Error },
    Parse { line: usize, source: serde_json::Error },
    SchemaViolation(String),
}

// Struct: single kind + context
#[derive(Debug)]
pub struct DbError {
    pub code: i32,
    pub query: String,
    pub source: Option<Box<dyn std::error::Error + Send + Sync + 'static>>,
}

Design rules:

• ALWAYS keep error types Send + Sync + 'static if they may cross thread or async boundaries. Box trait-object causes as Box<dyn Error + Send + Sync + 'static>.
• ALWAYS keep the error type small (one machine word per variant payload, boxing large payloads). Result<T, E> is returned by value; a fat E makes the Ok path slower.
• NEVER expose String as your only error type in a library API: callers cannot match on it. Strings are fine for application binaries.
• NEVER return Box<dyn Error> from a library function: callers lose the ability to match on specific variants. Use a concrete enum (typically via thiserror) and reserve Box<dyn Error> for application glue.

For the derive-macro route (thiserror for libraries, anyhow for binaries) see [[rust-errors-thiserror-anyhow]].

Panic vs Result: the decision

Rust Book Ch 9.3 ("To panic! or Not to panic!") rule: panic only when continuing would violate an invariant or risk corruption. Everything else is Result.

// Library API: return Result for expected failures.
pub fn load_user(id: u64) -> Result<User, LoadError> { /* ... */ }

// Programmer bug: panic.
fn assert_sorted(v: &[i32]) {
    debug_assert!(v.windows(2).all(|w| w[0] <= w[1]), "input must be sorted");
}

// Application top-level: unwrap with a message documenting the invariant.
fn main() {
    let cfg = load_config().expect("config.toml is required at startup");
}

NEVER use .unwrap() in library code. It turns an expected error into an unrecoverable panic and the caller cannot intercept it. Use ? and let the caller decide.

NEVER use .expect("...") to log a recoverable error: the message goes to stderr only on panic, and the application aborts.

ALWAYS use expect(msg) over unwrap() when you do panic intentionally: the message documents which invariant you are asserting, making the panic transcript actionable.

ALWAYS use assert!, debug_assert!, or unreachable!() for invariant checks. Match arms that should be impossible: unreachable!("variant X cannot occur after step Y").

Result combinators: avoid the match ladder

Result has a rich combinator API. Prefer combinators over multi-line match blocks when the logic is one step:

let r: Result<i32, String> = Ok(7);

let doubled:  Result<i32, String> = r.map(|x| x * 2);                  // transform Ok
let labeled:  Result<i32, String> = r.map_err(|e| format!("bad: {e}"));// transform Err
let chained:  Result<i32, String> = r.and_then(|x| Ok(x + 1));         // chain Result-returning fn
let recover:  Result<i32, String> = r.or_else(|_e| Ok(0));             // recover from Err

let val:  i32 = r.unwrap_or(0);                                        // default on Err
let val2: i32 = r.unwrap_or_else(|_e| 0);                              // lazy default
let val3: i32 = r.unwrap_or_default();                                 // T: Default

For Option<T> <-> Result<T, E>:

let o: Option<i32> = Some(7);
let r: Result<i32, &str> = o.ok_or("missing");
let r2: Result<i32, &str> = o.ok_or_else(|| "missing");

ALWAYS use map_err to convert an error into your function's error type before ?. Either From (automatic) or map_err (explicit) MUST bridge the inner error type to the outer.

NEVER write .map(|x| Ok(x)) followed by a flatten: that is and_then for Result and is built-in.

NEVER write if let Err(e) = result { return Err(e.into()); } else { result.unwrap() }: that is ?, written badly.

A full method table lives in references/methods.md.

Source chain: walking the cause

When errors wrap other errors, callers walk the source() chain to print the full context. The pattern:

fn print_chain(e: &dyn std::error::Error) {
    eprintln!("error: {e}");
    let mut cur = e.source();
    while let Some(src) = cur {
        eprintln!("caused by: {src}");
        cur = src.source();
    }
}

Inside the error type, return the wrapped error from source():

impl std::error::Error for ConfigError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
        Some(&self.source)   // <- the wrapped error
    }
}

In application code (binary crate) use anyhow::Context::context("loading config") to push a layer on the chain. See [[rust-errors-thiserror-anyhow]].

ALWAYS provide a source() impl when your error variant carries an inner error. NEVER mash the inner error into the Display text: report tools will then duplicate the cause when they walk the chain.

Decision tree: error-handling strategy

Operation can fail?
|
+-- No (only programmer bugs): use assert!/debug_assert!/unreachable!.
|
+-- Yes:
    |
    +-- Library crate?
    |   |
    |   +-- Return Result<T, E> with a concrete `enum` error type (often via thiserror).
    |   +-- Implement Display + Error (+ source() if wrapping inner errors).
    |   +-- Implement From<InnerError> for every error you propagate via ?.
    |   +-- NEVER use anyhow::Error in public API. NEVER unwrap/expect.
    |
    +-- Binary / application crate?
        |
        +-- Use anyhow::Result<T> at boundaries; .context("...") at each layer.
        +-- main() -> anyhow::Result<()> prints the chain on Err.
        +-- At the very top: expect("invariant X") is acceptable for startup-only requirements.

See [[rust-errors-thiserror-anyhow]] for the derive-macro details.

Quick reference table

Avoid these mistakes

(Full list with WHYs in references/anti-patterns.md.)

• .unwrap() or .expect() in library code: turns a recoverable Err into a process-killing panic. Use ? and let the caller decide.
• Defining an error type without Display + Error impls: it cannot compose with ?, anyhow, eyre, or Box<dyn Error>. Always implement both.
• Forgetting From<InnerError> for MyError: ? fails to compile with E0277 and you fall back to match ladders. Always add From per inner error.
• Returning Box<dyn Error> from a library function: callers lose the ability to match on variants. Return a concrete enum; reserve Box<dyn Error> for application glue.
• Result<(), ()>: lossy, the Err carries no information. Use a real error type, even a unit struct with a meaningful name.
• panic! for expected failure modes (parse error, missing file, network down): callers cannot recover. Return Result.
• Stuffing the inner error into the Display message AND returning it from source(): report tools print the cause twice. Pick one (always source()).
• Using std::error::Error in a no_std crate (since 1.81): the import does not resolve. Use core::error::Error.
• .ok() to discard an Err in library code: silently swallows failures. Either propagate via ? or convert via map_err.
• Mixing anyhow::Error into a library's public API: downstream cannot match on variants, your error type is forever opaque. anyhow belongs in binaries.

Reference links

• std::result: full Result API.
• std::option: full Option API and the conversions to Result.
• std::error::Error: the trait, source(), downcasting rules.
• core::error::Error: identical trait, no_std-friendly (stable since 1.81).
• Rust Book Ch 9: Result, ?, panic vs Result.
• Rust Book Ch 9.3: "To panic! or Not to panic!" decision criteria.
• Rust 1.81.0 release: announcement of core::error::Error stabilization.
• RFC 0243: the ? operator design.

For deeper drill-downs see:

• references/methods.md: complete signatures for Result, Option, Error, and the From/? desugaring.
• references/examples.md: working code for custom enum errors, source chain, no_std errors, combinator chains.
• references/anti-patterns.md: 10+ documented anti-patterns with WHY and FIX.