Rust Error Handling

Rust-specific error handling patterns that extend the base error handling skill.

Rules

• Use thiserror for defining library error types
• Use anyhow for application-level error handling
• Implement std::error::Error trait for custom error types
• Use the ? operator for ergonomic error propagation
• Prefer Result<T, E> over panicking for recoverable errors
• Use unwrap() only in tests or when failure is impossible

Examples

// Define library errors with thiserror
use thiserror::Error;

#[derive(Error, Debug)]
pub enum ConfigError {
    #[error("failed to read config file: {0}")]
    Io(#[from] std::io::Error),

    #[error("invalid config format: {0}")]
    Parse(#[from] toml::de::Error),

    #[error("missing required field: {field}")]
    MissingField { field: String },
}

// Application-level error handling with anyhow
use anyhow::{Context, Result};

fn load_config(path: &str) -> Result<Config> {
    let content = std::fs::read_to_string(path)
        .with_context(|| format!("failed to read config from {}", path))?;

    let config: Config = toml::from_str(&content)
        .context("failed to parse config TOML")?;

    Ok(config)
}

Checklist

• [ ] Custom error types use thiserror derive
• [ ] Error context is added with anyhow::Context
• [ ] No unwrap() calls in production code paths
• [ ] Result is used instead of panic! for recoverable errors