Add new `mixed_attributes_style` lint

[GuillaumeGomez]

Add a new lint to detect cases where both inner and outer attributes are used on a same item.

r? @llogiq

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changelog: Add new [mixed_attributes_style] lint

[GuillaumeGomez]

Note for future self: run cargo dev fmt AFTER uibless.

[GuillaumeGomez]

And also dogfood...

[GuillaumeGomez]

Ci passed. \o/

[llogiq]

Thank you!

@bors r+

[bors]

:pushpin: Commit 1631ee335fefe05b0ab97c58c2a711e652af1739 has been approved by llogiq

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit 1631ee335fefe05b0ab97c58c2a711e652af1739 with merge 586e7916d6bde160b07454738d4a2791829993cb...

[bors]

:broken_heart: Test failed - checks-action_test

[GuillaumeGomez]

Seems completely unrelated. Retry? :3

[llogiq]

@bors retry

[bors]

:hourglass: Testing commit 1631ee335fefe05b0ab97c58c2a711e652af1739 with merge cb4d3f4588ca1ad036d2494637745d959a9d93b5...

[bors]

:broken_heart: Test failed - checks-action_test

[GuillaumeGomez]

Rebased and allowed this lint in empty_docs lint.

[GuillaumeGomez]

Rebased again.

[llogiq]

@bors r+

[bors]

:pushpin: Commit f30138623bd9a078698328a32785780cee379e8d has been approved by llogiq

It is now in the queue for this repository.

[bors]

:hourglass: Testing commit f30138623bd9a078698328a32785780cee379e8d with merge 00ff8c92d395d08325301c1a1462fc38abd82053...

[bors]

:sunny: Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test Approved by: llogiq Pushing 00ff8c92d395d08325301c1a1462fc38abd82053 to master...