New lint suggestion: detect `HashMap/Set::iter()` followed by `take_while()/skip_while()/map_while()`.

[conradoplg]

What it does

Detects usage of HashMap/Set::iter() (or other iterator) followed by take_while()/skip_while()/map_while().

Categories (optional)

• Kind: suspicious, maybe correctness?

What is the advantage of the recommended code over the original code

iter() in HashMap/Set iterates in arbitrary order, and _while() methods usually make sense where is there some kind of order in the iterator. This would catch using a e.g. HashMap where a sorted structure would be required.

Drawbacks

False positives, but it seems unlikely.

Example

Note sure if anything useful can be suggested other than detecting the issue.

[teor2345]

Iterator::last, nth, position, rposition, rev, and step_by are also suspicious with HashMap and HashSet, because element positions are arbitrary.

(A possible substitute for last and nth is next, because they all get an arbitrary element.)

[teor2345]

rustc has a similar internal-only lint: rust-lang/rust#89558

[emilk]

I would be very interested in a clippy against any call to .iter() or .drain() on HashMap/Set, no matter what you do with the result. The arbitrary iteration order has been the source of many bugs. Maybe I should make that its own ticket?