labels on the closure body block can be disappearing

[anatawa12]

Fixes #6465

This PR also adds many test cases for labels on the closure body block.

[anatawa12]

@rustbot ready

[rustbot]

Error: The feature shortcut is not enabled in this repository.
To enable it add its section in the triagebot.toml in the root of the repository.

Please file an issue on GitHub at triagebot if there's a problem with this bot, or reach out on #t-infra on Zulip.

[ytmimi]

Thanks for applying the feedback. I've got one last ask before we merge this.

[ytmimi]

Thanks for reporting this issue and helping to fix it.

[ytmimi]

Running one last CI check

Edit: Job passed ✅

[anatawa12]

Today I found that there is copy of rustfmt in rust-lang/rust tree. is it better to send pull request to rust-lang/rust tree?