Read and use deprecated configuration (as well as emitting a warning)

[bossmc]

Original change written by @flip1995 I've simply rebased to master and fixed up the formatting/tests. This change teaches the configuration parser which config key replaced a deprecated key and attempts to populate the latter from the former. If both keys are provided this fails with a duplicate key error (rather than attempting to guess which the user intended).

Currently this on affects cyclomatic-complexity-threshold -> cognitive-complexity-threshold but will also be used in #8974 to handle blacklisted-names -> disallowed-names.

changelog: deprecated configuration keys are still applied as if they were provided as their non-deprecated name.

• cargo test passes locally
• Run cargo dev fmt

[rust-highfive]

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

Please see the contribution instructions for more information.

[flip1995]

I only vaguely remember writing this 😅 Thanks for seeing it to an end!

[Jarcho]

One small issue. This won't warn about duplicate fields when the old name appears after the new name. It's not a big deal as you still get the warning about the deprecated field.

Otherwise LGTM.

[bossmc]

I've added a check to handle duplicates in either order and added a test for duplicates. I also extended the error message to be clear what's happening if one of the duplicates is using the deprecated name (though this only works on one direction).

[Jarcho]

Thank you. That should be good enough. The deprecation warning mentions both names in either case.

@bors r+

[bors]

📌 Commit ea25ef1 has been approved by Jarcho

It is now in the queue for this repository.

[bors]

⌛ Testing commit ea25ef1 with merge 53a09d4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Jarcho
Pushing 53a09d4 to master...