Implement the manual_non_exhaustive lint

[ebroto]

Some implementation notes:

• Not providing automatic fixups because additional changes may be needed in other parts of the code, e.g. when constructing a struct.
• Even though the attribute is valid on enum variants, it's not possible to use the manual implementation of the pattern because the visibility is always public, so the lint ignores enum variants.
• Unit structs are also ignored, it's not possible to implement the pattern manually without fields.
• The attribute is not accepted in unions, so those are ignored too.
• Even though the original issue did not mention it, tuple structs are also linted because it's possible to apply the pattern manually.

changelog: Added the manual non-exhaustive implementation lint

Closes #2017

[ebroto]

I've realized that we should keep linting if the attribute was added but the private field/marker variant was not removed. I'll add a follow-up commit tonight.

Also, the RFC proposes a clippy lint to check when matching non-exhaustive enums that all patterns are covered, to help downstream crates notice that new variants were added. Would it make sense to add it?

[flip1995]

I've realized that we should keep linting if the attribute was added but the private field/marker variant was not removed.

I should have read your comment before reviewing. Just removed my comment about this 😄

Would it make sense to add it?

This would be a good allow-by-default lint. So I would separate it from this lint and add it as a pedantic lint.

[flip1995]

One small thing left.

[flip1995]

@bors r+

[bors]

📌 Commit 350c17d has been approved by flip1995

[bors]

⌛ Testing commit 350c17d with merge d9894c1...

[bors]

💥 Test timed out

[flip1995]

@bors retry

[bors]

⌛ Testing commit 350c17d with merge cc9088f...

[bors]

💔 Test failed - checks-action_test

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing cc9088f to master...