add lint against unit tests in doctests

[llogiq]

During RustLab, Alice Ryhl brought to my attention that the Andoid team stumbled over the fact that if one attempts to write a unit test within a doctest, it will be summarily ignored. So this lint should help people wondering why their tests won't run.

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changelog: New lint: [test_attr_in_doctest]
#11872

[rustbot]

r? @xFrednet

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[llogiq]

@rustbot author

I want to extend the span to also cover the signature (otherwise it will only show the attr) and avoid limiting on no_run and compile_fail tests.

[llogiq]

@rustbot review

I've now decided to limit the span to the item ident, which ensures it'll be in the message without potentially taking lots of lines that the declaration may take. I'm also thinking about renaming the doc::needless_doctest_main submodule because it is no longer only for that lint.

[xFrednet]

LGTM, two small nits and then it should be good :D

[llogiq]

@xFrednet thanks for the quick review! Your nits should be gone now.

[xFrednet]

LGTM, thank you! Nice new lint!

@bors r+

[bors]

📌 Commit 0ba9bf9 has been approved by xFrednet

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0ba9bf9 with merge 665fd52...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: xFrednet
Pushing 665fd52 to master...