What Is the Difference Between thiserror and anyhow?

Use thiserror to define custom error types that implement std::error::Error for your public APIs, and use anyhow to handle errors internally without defining custom types. thiserror generates the Error trait implementation via a derive macro, while anyhow provides a flexible Result type that can hold any error.

use thiserror::Error;
use anyhow::Result;

#[derive(Error, Debug)]
#[error("Failed to parse: {0}")]
pub struct ParseError(String);

fn internal_logic() -> Result<()> {
    // Any error type works here
    if false { Err(anyhow::anyhow!("oops")) } else { Ok(()) }
}

fn public_api() -> Result<(), ParseError> {
    // Must return the specific custom error
    internal_logic().map_err(|e| ParseError(e.to_string()))
}

Think of thiserror as a custom box you build to hold specific problems you want to show to users, while anyhow is a universal catch-all bag for problems you just want to fix internally. You use thiserror when you need to define exactly what can go wrong in your library's public interface. You use anyhow when you are writing an application and just want to handle errors quickly without creating a new type for every possible failure.