expect-exit: display an error message and exit without a panic

The expect-exit library defines the Expected, ExpectedWithError, and ExpectedResult traits and implements them for the standard Result and Option types as appropriate. This allows a program to display an error message and exit with a non-zero exit code without invoking a Rust panic, yet optionally unwinding the stack so that various objects may perform some clean-up actions.

The methods with an _e suffix append an appropriate error message to the supplied one. The methods with an _f suffix allow the caller to only build the error message if an error is to be reported, similar to the Option.or_else method.

Example

use expect_exit::{Expected, ExpectedResult};

{
    env::var(name).or_exit_f(|| format!("{} not specified in the environment", name))

    fs::read_to_string(path).or_exit_e_f(|| format!("Could not read {:?}", path))

    tx.send(result).await.or_exit_e("Could not tell the main thread");

    let config = parse().expect_result("Could not parse the config")?;
    Ok(config.value + 1)
}