Add Termination support for the E side of Result<_, E>

[SUPERCILEX]

🌟 What is the purpose of this PR?

The fact that Report implements Termination and is in the Err variant of Result is very confusing because the stdlib completely ignores the Err variant's termination, leading to the exit code being dropped. This PR adds a new type Exit that will return the ExitCode provided from the Report Termination. Without this PR, one must write:

        match result {
            Ok(()) => ExitCode::SUCCESS,
            Err(err) => {
                // Ignore error if the write fails, for example because stderr is
                // already closed. There is not much point panicking at this point.
                drop(writeln!(io::stderr(), "Error: {err:?}"));
                err.report()
            }
        }

🔗 Related links

rust-lang/rust#48711 (comment)

TODO

I need to write docs if this moves forward.

[CLAassistant]

All committers have signed the CLA.

[Alfred-Mountfield]

Thanks for the PR!
This is quite an intriguing one and we're not sure how to handle it if I'm honest. We definitely share the frustration, it greatly affects the ergonomics (and indeed even the value of Termination for error-stack). It seems that this is much more of an upstream problem though, and patching it here feels.. hacky.

We're not entirely against the proposal, but at the moment I think we'd lean towards not including it.

How would you feel about opening a GitHub discussion, and we can gauge if this is something other people feel strongly about, and in the mean time I think we'd close this PR off.

[SUPERCILEX]

Yeah I just thought of this which seems better than having every library add a hack: rust-lang/rust#48711 (comment)