Rust Error Handling

Rust Error Handling Skill

This skill guides proper error handling in Rust projects, enforcing strict standards for robustness and maintainability.

When to use this skill

• Creating new error types for a module or domain
• Handling Result<T, E> types
• Converting between error types
• Implementing error propagation
• Reviewing code for error handling issues

Core Principles

1. Never Use unwrap() or expect() in Production Code

Rule: unwrap() and expect() are forbidden in production code. They are only allowed in:

• Test code (#[cfg(test)])
• Example code
• Prototypes explicitly marked as such

Why: These methods cause panics, which are unrecoverable in production. All errors must be handled gracefully.

2. Use Domain-Specific Error Types

Rule: Every module or domain should have its own error type using thiserror.

Why:

• Clear error semantics
• Type-safe error handling
• Better error messages
• Easier debugging

Example:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum CodecError {
    #[error("Unsupported format: {0}")]
    UnsupportedFormat(String),
    
    #[error("Invalid key count: {0} (expected 1-18)")]
    InvalidKeyCount(u8),
    
    #[error("Decode failed: {0}")]
    DecodeFailed(String),
    
    #[error("IO error: {0}")]
    Io(#[from] std::io::Error),
}

3. Avoid anyhow in Library Code

Rule:

• Use thiserror for library/core logic
• anyhow is acceptable only for CLI tools and applications

Why: Libraries should expose typed errors for downstream consumers.

4. Provide Context with Error Messages

Rule: Error messages must include:

• What went wrong
• Relevant values (sanitized if sensitive)
• Context about the operation

Good:

return Err(CodecError::InvalidKeyCount(key_count));

Bad:

return Err(CodecError::Invalid); // No context!

Error Handling Patterns

Pattern 1: Early Return with ?

pub fn decode_chart(path: &Path) -> Result<RoxChart, CodecError> {
    let data = std::fs::read(path)?; // Auto-converts io::Error
    let chart = parse_data(&data)?;
    validate_chart(&chart)?;
    Ok(chart)
}

Pattern 2: Match for Multiple Error Paths

pub fn load_audio(path: &Path) -> Result<AudioData, AudioError> {
    match std::fs::read(path) {
        Ok(data) => {
            if data.is_empty() {
                Err(AudioError::EmptyFile)
            } else {
                Ok(AudioData::from_bytes(data))
            }
        }
        Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
            Err(AudioError::FileNotFound(path.to_path_buf()))
        }
        Err(e) => Err(AudioError::Io(e)),
    }
}

Pattern 3: Custom Error Conversion

impl From<rkyv::validation::CheckArchiveError<_, _>> for CodecError {
    fn from(err: rkyv::validation::CheckArchiveError<_, _>) -> Self {
        CodecError::DecodeFailed(format!("Archive validation failed: {}", err))
    }
}

Pattern 4: Result Chaining with and_then

pub fn process_chart(path: &Path) -> Result<ProcessedChart, CodecError> {
    load_chart(path)
        .and_then(|chart| validate_timing(&chart).map(|_| chart))
        .and_then(|chart| normalize_notes(chart))
}

Decision Tree

When creating a new error type

1. Is this a library module?

   • Yes → Use thiserror, create domain-specific enum
   • No (CLI/app) → Consider anyhow for simplicity

2. Do I need to convert from other error types?

   • Yes → Use #[from] attribute in thiserror
   • No → Manual From impl if needed

3. Do I need additional context?

   • Yes → Add fields to error variants
   • No → Simple message-only variants

When handling errors

1. Can I recover from this error?

   • Yes → Use match or if let to handle specific cases
   • No → Propagate with ?

2. Do I need to add context?

   • Yes → Use .map_err() to wrap or annotate
   • No → Direct propagation with ?

3. Is this a validation error?

   • Yes → Return early with descriptive error
   • No → Continue processing

Common Mistakes to Avoid

❌ Using unwrap() in production

// BAD
let chart = decode_chart(path).unwrap();

// GOOD
let chart = decode_chart(path)?;

❌ Generic error messages

// BAD
#[error("Error occurred")]
InvalidInput,

// GOOD
#[error("Invalid key count: {0} (expected 1-18)")]
InvalidKeyCount(u8),

❌ Swallowing errors silently

// BAD
if let Err(_) = process() {
    // Silent failure
}

// GOOD
if let Err(e) = process() {
    error!("Processing failed: {}", e);
    return Err(e.into());
}

❌ Using String for errors

// BAD
pub fn decode(data: &[u8]) -> Result<Chart, String> {
    // ...
}

// GOOD
pub fn decode(data: &[u8]) -> Result<Chart, CodecError> {
    // ...
}

Testing Error Paths

Always test error conditions:

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_invalid_key_count() {
        let result = validate_key_count(0);
        assert!(matches!(result, Err(CodecError::InvalidKeyCount(0))));
    }

    #[test]
    fn test_file_not_found() {
        let result = load_chart(Path::new("nonexistent.rox"));
        assert!(result.is_err());
    }
}

Integration with Observability

Errors should be logged with context:

use tracing::{error, warn};

pub fn process_batch(charts: &[Path]) -> Result<Vec<RoxChart>, CodecError> {
    let mut results = Vec::new();
    
    for path in charts {
        match decode_chart(path) {
            Ok(chart) => results.push(chart),
            Err(e) => {
                error!(path = ?path, error = %e, "Failed to decode chart");
                return Err(e);
            }
        }
    }
    
    Ok(results)
}

Checklist

When implementing error handling, ensure:

• No unwrap() or expect() in production code
• Domain-specific error type using thiserror
• All error variants have descriptive messages
• Error conversions use #[from] where appropriate
• Errors include relevant context (values, paths, etc.)
• Error paths are tested
• Errors are logged with appropriate levels
• Public API errors are documented

References

• User rule: rule-input.md (Input Validation & Trust Boundary)
• User rule: rust-strict-standards.md (Robustness & Errors section)
• thiserror documentation
• Rust Error Handling Book

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