Lint for Debug formatting in Display implementations

[clarfonthey]

This is similar to use_debug but should be a warn by default.

[cgm616]

I think this already exists! Unless I'm misunderstanding the lints tested by tests/ui/print.rs and tests/ui/print.stderr.

[clarfonthey]

Potentially. This was filed 3 years ago and it's entirely possible that this was implemented and someone didn't notice to close this ticket.

If you can confirm that this already exists we can definitely close this.

[ebroto]

@cgm616 that test is for use_debug which is allow-by-default and lints on any use of debug formatting.

The suggested lint would trigger only in Display impls and would be warn-by-default.