Rust Error Handling

Rust Error Handling

Guide for effective error handling in Rust.

Quick Start

Use Result<T, E> for operations that can fail:

use std::fs;
use std::io;

fn read_username(path: &str) -> Result<String, io::Error> {
    let content = fs::read_to_string(path)?;
    Ok(content.trim().to_string())
}

The ? operator:

• Returns early on Err
• Automatically converts errors via From trait
• Only works in functions returning Result or Option

Error Propagation

Chain operations with ?:

use anyhow::Result;

fn load_config() -> Result<Config> {
    let path = find_config_file()?;
    let content = fs::read_to_string(path)?;
    let config = toml::from_str(&content)?;
    Ok(config)
}

Custom Error Types

For libraries - use thiserror:

use thiserror::Error;

#[derive(Error, Debug)]
pub enum ConfigError {
    #[error("config file not found: {0}")]
    NotFound(String),
    
    #[error("invalid config format")]
    InvalidFormat(#[from] toml::de::Error),
    
    #[error("io error")]
    Io(#[from] std::io::Error),
}

For applications - use anyhow:

use anyhow::{Context, Result};

fn process() -> Result<()> {
    let config = load_config()
        .context("failed to load config")?;
    
    let data = fetch_data(&config.url)
        .context("failed to fetch data")?;
    
    Ok(())
}

Error Context

Add context to errors for better debugging:

use anyhow::{Context, Result};

fn parse_port(s: &str) -> Result<u16> {
    s.parse()
        .with_context(|| format!("failed to parse port: {}", s))
}

Advanced Patterns

See references/error_patterns.md for:

• Error downcasting
• Multiple error types
• Error recovery strategies
• Testing error conditions

Use scripts/check_error_handling.sh to verify error handling coverage in your codebase.

Best Practices

1. Use Result for recoverable errors, panic! for bugs
2. Add context with .context() - Makes debugging easier
3. Define custom errors for libraries - Better API ergonomics
4. Use anyhow for applications - Simpler error handling
5. Test error paths - Don't just test the happy path

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