`thiserror`

The thiserror crate provides macros to help avoid boilerplate when defining error types. It provides derive macros that assist in implementing From<T>, Display, and the Error trait.

rust,editable,compile_fail

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
use std::io::Read;
use std::{fs, io};
use thiserror::Error;

#[derive(Debug, Error)]
enum ReadUsernameError {
    #[error("I/O error: {0}")]
    IoError(#[from] io::Error),
    #[error("Found no username in {0}")]
    EmptyUsername(String),
}

fn read_username(path: &str) -> Result<String, ReadUsernameError> {
    let mut username = String::with_capacity(100);
    fs::File::open(path)?.read_to_string(&mut username)?;
    if username.is_empty() {
        return Err(ReadUsernameError::EmptyUsername(String::from(path)));
    }
    Ok(username)
}

fn main() {
    //fs::write("config.dat", "").unwrap();
    match read_username("config.dat") {
        Ok(username) => println!("Username: {username}"),
        Err(err) => println!("Error: {err}"),
    }
}

The Error derive macro is provided by thiserror, and has lots of useful attributes to help define error types in a compact way.
The message from #[error] is used to derive the Display trait.
Note that the (thiserror::)Error derive macro, while it has the effect of implementing the (std::error::)Error trait, is not the same this; traits and macros do not share a namespace.

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