Add suggestions to extra_unused_type_parameters

[mkrasnitski]

Change the extra_unused_type_parameters lint to provide machine applicable suggestions rather than just help messages. Exception to this are cases when any unused type parameters appear bounded in where clauses - for now I've deemed these cases unfixable and separated them out. Future work might be able to provide suggestions in these cases.

Also, added a test case for the avoid_breaking_exported_api config option.

r? @flip1995

changelog: [extra_unused_type_parameters]: Now provides fixable suggestions.

[mkrasnitski]

Would be great if this could get reviewed (and hopefully merged) before next week's sync.

cc-ing @Jarcho who has helped with review on this lint in the past

[flip1995]

This looks like it was more work than I anticipated. Thanks for doing this. Making this suggestion auto-applicable is really awesome!

[flip1995]

@bors r+

Sorry for taking so long.

[bors]

📌 Commit 50d92d0 has been approved by flip1995

It is now in the queue for this repository.

[bors]

⌛ Testing commit 50d92d0 with merge 2998706...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: flip1995
Pushing 2998706 to master...

[mkrasnitski]

This looks like it was more work than I anticipated. Thanks for doing this. Making this suggestion auto-applicable is really awesome!

I was able to take inspiration from the unnecessary_lifetimes lint, which does something similar. In fact, I was able to provide better suggestions in the case where multiple parameters at the end of the list should be removed (that's what the whole iterating-in-reverse and chain-extension logic is for). It was definitely a fun problem to solve!

Sorry for taking so long.

No worries! Thanks for the review!