new rule `unused_extract_if`

What it does

Many collections have the extract_if (previous named drain_filter) method. Sometimes the returned iterator ExtractIf are dropped directly. In such case, retain/retain_mut have better performance and readability.

Advantage

• Improve the performance
• Improve the readability

Drawbacks

No

Example

let mut v = vec![1, 2, 3, 4, 5];
let _ = v.extract_if(|&x| x % 2 == 0)

Could be written as:

let mut v = vec![1, 2, 3, 4, 5];
let _ = v.retain(|&x| x % 2 != 0)

Start by locating the existing handling or lint tests for Rust's extract_if and retain methods. Implement the requested lint so discarded extract_if iterators are identified and the equivalent retain or retain_mut approach is suggested, then verify the example behavior with focused tests.