env: different error output with `cannot unset '=': Invalid argument` OSS-Fuzz issue 384591131

[oss-fuzz-robot]

OSS-Fuzz has found a bug in this project. Please see https://oss-fuzz.com/testcase?key=6592863400427520 for details and reproducers.

This issue is mirrored from https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=384591131 and will auto-close if the status changes there.

If you have trouble accessing this report, please file an issue at https://github.com/google/oss-fuzz/issues/new.

[sylvestre]

	Discrepancy detected: stderr differs
	Test completed with discrepancies for env: ["-0", "--chdir=/tmp"]
	Running test ["env", "-u="]
	Test Type: env
	Input: ["-u="]
	Rust stderr: cannot unset '': Invalid argument
	GNU stderr: cannot unset '=': Invalid argument
	Diff=
	-cannot unset '': Invalid argument
	+cannot unset '=': Invalid argument
	

[jovielarue]

Hi @sylvestre, I'd like to try to tackle this issue if that's alright!

[cakebaker]

@jovielarue sure, go ahead :)

[jovielarue]

It looks like this issue is a duplicate of #6165 and #6166 and has had attempted to be fixed in #6172 and #6711; however it doesn't look like either of those fixes were complete.

It seems as though this issue is a result of clap parsing differing from the GNU/POSIX getopt standards. There was an issue on the clap repository attempting to foster a conversation with the clap developers to change this, but it has been denied as they don't want to introduce breaking changes.

Would it be beneficial for me to attempt to finish #6172 or #6711?

[sylvestre]

Sure, sounds good, thanks :)
and well done for the investigation!