Add let_underscore_untyped

[JirkaVebr]

Fixes #6842

This adds a new pedantic let_underscore_untyped lint which checks for let _ = <expr>, and suggests to either provide a type annotation, or to remove the let keyword. That way the author is forced to specify the type they intended to ignore, and thus get forced to re-visit the decision should the type of <expr> change. Alternatively, they can drop the let keyword to truly just ignore the value no matter what.

r? @llogiq

changelog: New lint: [let_underscore_untyped]

[rustbot]

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

Please see the contribution instructions for more information.

[llogiq]

Thank you!

@bors r+

[bors]

📌 Commit 0b1ae20 has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0b1ae20 with merge 99d4ea4...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 99d4ea4 to master...