Module error Copy item path

All utils return exit with an exit code. Usually, the following scheme is used:

• 0: succeeded
• 1: minor problems
• 2: major problems

This module provides types to reconcile these exit codes with idiomatic Rust error handling. This has a couple advantages over manually using std::process::exit:

1. It enables the use of ?, map_err, unwrap_or, etc. in uumain.
2. It encourages the use of UResult/Result in functions in the utils.
3. The error messages are largely standardized across utils.
4. Standardized error messages can be created from external result types (i.e. std::io::Result & clap::ClapResult).
5. set_exit_code takes away the burden of manually tracking exit codes for non-fatal errors.

§Usage

The signature of a typical util should be:

fn uumain(args: impl uucore::Args) -> UResult<()> {
    ...
}

UResult is a simple wrapper around Result with a custom error trait: UError. The most important difference with types implementing std::error::Error is that UErrors can specify the exit code of the program when they are returned from uumain:

• When Ok is returned, the code set with set_exit_code is used as exit code. If set_exit_code was not used, then 0 is used.
• When Err is returned, the code corresponding with the error is used as exit code and the error message is displayed.

Additionally, the errors can be displayed manually with the crate::show and crate::show_if_err macros:

let res = Err(USimpleError::new(1, "Error!!"));
show_if_err!(res);
// or
if let Err(e) = res {
   show!(e);
}

Note: The crate::show and crate::show_if_err macros set the exit code of the program using set_exit_code. See the documentation on that function for more information.

§Guidelines

• Use error types from uucore where possible.
• Add error types to uucore if an error appears in multiple utils.
• Prefer proper custom error types over ExitCode and USimpleError.
• USimpleError may be used in small utils with simple error handling.
• Using ExitCode is not recommended but can be useful for converting utils to use UResult.