Report for loops using iter() and iter_mut() unnecesarily

[jdm]

Rust allows using for foo in &bar instead of for foo in bar.iter(), and for foo in &mut bar instead of for foo in bar.iter_mut(). Let's add a check to check_rust that warns about instances of .iter() and .iter_mut() that could be replaced. There are a lot of existing instances of this pattern that will need replacing, too.

Note: patterns like for foo in bar.iter().enumerate() (ie. places where the return value of iter() is further refined) cannot be replaced and shouldn't be reported.

Code: python/tidy/servo_tidy/tidy.py
Tests: add a test to python/tidy/servo_tidy_tests/test_tidy.py and run it via ./mach test-tidy --self-test

[aeischeid]

Since I was touching the check_rust method earlier I think I have the sense for how to do this.

"places where the return value of iter() is further refined..." - I am only a little bit familiar with Rust syntax. Are there cases where the return val might be further refined but .iter() is followed by a space character?

[jdm]

That shouldn't be possible, no.

[aeischeid]

think I may be close on defining the rule and the test cases, but stuck on why the tests are failing when I don't expect them to. see commit in my branch aeischeid@57f5483

[wafflespeanut]

I'm removing the easy label, since this can't be fixed straightaway (given the conclusion we've arrived at #11116). Maybe we should have a todo list in the issue mentioning which types need the Iterator trait so that we can go about fixing this.

[nox]

We have some types that have an iter method but don't even implement IntoIterator. I think that's a job better left to Clippy. Closing.