Lint .cloned().filter() to suggest reverse

[GnomedDev]

What it does

In an iterator chain, .cloned().filter() will lead to cloning items that are filtered out. This is wasteful and should be the other way around.

Advantage

Improves performance by cloning less.

Drawbacks

None to my knowledge.

Example

fn process_items<'a>(items: impl Iterator<Item = &Item>) -> ProcessedItems {
    items.cloned().filter(filter_item).map(process_item).collect()
}

Could be written as:

fn process_items<'a>(items: impl Iterator<Item = &Item>) -> ProcessedItems {
    items.filter(filter_item).cloned().map(process_item).collect()
}

[GnomedDev]

Note: This is split from #3302

[y21]

This lint already exists: https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned

warning: unnecessarily eager cloning of iterator items
 --> src/lib.rs:8:5
  |
8 |     items.cloned().filter(filter_item).map(process_item).collect()
  |     ^^^^^-----------------------------
  |          |
  |          help: try: `.filter(|&x| filter_item(x)).cloned()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#iter_overeager_cloned
  = note: `#[warn(clippy::iter_overeager_cloned)]` on by default

[GnomedDev]

Got it.