Document pub requirement for new_without_default lint

[nsunderland1]

fixes #8415

Also adds some UI tests that ensure that pub is required on both the struct and the field. The only thing I'm not sure about is that the lint actually checks if new is reachable, not public. To the best of my understanding, both the struct and the method need to be public for the method to be reachable for external crates (I certainly didn't manage to craft a counterexample).

changelog: Document pub requirement for [`new_without_default`] lint.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) soon.

Please see the contribution instructions for more information.

[llogiq]

Thank you! I wouldn't worry with the distinction between public and exported here. You could in theory have a struct X; pub use X, but I don't know anyone coding that way.

@bors r+

[bors]

📌 Commit 78c2e0b has been approved by llogiq

[bors]

⌛ Testing commit 78c2e0b with merge 0836970...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 0836970 to master...