unused_labels should be reported as warnings

[BartMassey]

A long while back there was Issue #50751 which pointed out that the compiler was not linting for unused break labels. This resulted after a lot of fantastic hard work in PR #50763 which added the unused_labels lint.

At the time, the lint was set to allow by default. The consensus was that there would need to be a review and a crater run before it was changed to warn. Issue #50751 was closed without this happening, and this is where we stand today.

I'd love to see this changed to warn by default. Any chance that could happen?

[Centril]

Seems reasonable; want to file a PR? cc @rust-lang/lang

[BartMassey]

Working…

[BartMassey]

OK, PR #66325 is in the queue. Let me know if I need to do something different. Thanks!