Only lint manual_non_exhaustive for exported types

[Alexendoo]

For types that are not exported the attribute doesn't make a difference, but the manual pattern can still be used to achieve module level non exhaustiveness

Fixes #10301
Fixes #12106

changelog: none

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[y21]

As for the second linked issue, isn't that still an issue even with this change of restricting the lint to only warn exported types?

As in, someone could have a type that happens to be exported but would still only want it constructed through a specific constructor and not anywhere else in the same crate, which isn't something that #[non_exhaustive] helps with.

But then again, the lint requires that there's a public field and exactly one private field that is the unit type, so I don't see why someone would want that behavior. The field clearly doesn't have invariants that need to be upheld in the constructor if it's public, because it can just be reassigned a new value anywhere.

the manual pattern can still be used to achieve module level non exhaustiveness

Out of curiosity, are there use cases for wanting to restrict exhaustiveness to specific modules within the same crate?

(To be clear, this PR makes sense to me, just mentioning this because the description mentions closing that one, but not sure it fixes it)

[Alexendoo]

#12106 mentions that it's about a private module, but yeah the PR doesn't handle the case of an exported struct with a module private constructor

[y21]

Ah oops sorry, I missed the word "private" while reading through them. In that case, looks all good to me :)

@bors r+

[bors]

📌 Commit f7f5505 has been approved by y21

It is now in the queue for this repository.

[bors]

⌛ Testing commit f7f5505 with merge c41be9e...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: y21
Pushing c41be9e to master...