Add new lint while_float

[yusufraji]

This PR adds a nursery lint that checks for while loops comparing floating point values.

changelog:

changelog: [`while_float`]: Checks for while loops comparing floating point values.

Fixes #758

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @xFrednet (or someone else) some time within the next two weeks.

Please see the contribution instructions for more information. Namely, in order to ensure the minimum review times lag, PR authors and assigned reviewers should ensure that the review label (S-waiting-on-review and S-waiting-on-author) stays updated, invoking these commands when appropriate:

• @rustbot author: the review is finished, PR author should check the comments and take action accordingly
• @rustbot review: the author is ready for a review, this PR will be queued again in the reviewer's queue

[yusufraji]

r? @llogiq

[llogiq]

Thank you for making clippy better! This is a good starting point. We might want to extend the lint to look into the loop variant and check if the increment is integer, but as this is a nursery lint, we might well do that in a follow-up PR.

So I just left a small doc suggestion, otherwise this looks good to merge.

[llogiq]

Ok, let's merge this and see how it fares.

@bors r+

If you'd like to keep working on this lint, here are some ideas for possible future directions:

1. Try to extract a constant from the comparison arguments and check if it is integral. If not, avoid linting.
2. Try to extract the loop variable and walk the loop body to find the increment(s), then also check that those are integral.
3. (once 2. is in place), visit the whole method looking for uses of that variable and check if all can be converted to integer math. If so, add a multi-suggestion changing all found uses. Note: This is pretty advanced stuff already.

[bors]

📌 Commit cb3fcbb has been approved by llogiq

It is now in the queue for this repository.

[bors]

⌛ Testing commit cb3fcbb with merge 2efebd2...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 2efebd2 to master...