`filter_map_identity` is arguably overzealous

Description

lint: https://rust-lang.github.io/rust-clippy/master/index.html#filter_map_identity

This was originally added as being like flat_map_identity (https://github.com/rust-lang/rust-clippy/pull/6685), but they're not quite analogous. .flatten() is unambiguously better than .flat_map(identity), but the same isn't true for .filter_map(identity).

The problem is that (as I recently mentioned in https://github.com/rust-lang/rust/pull/99230#discussion_r955364588), FlatMap (including Flatten) is a fundamentally harder problem than FilterMap, because it needs to deal in iterators that might have more than one item. And thus, for example,

let flatten_it = a.iter().copied().flatten();
let filter_map_it = a.iter().copied().filter_map(|x| x);
dbg!([size_of_val(&flatten_it), size_of_val(&filter_map_it)]); // 32, 16
dbg!([flatten_it.size_hint(), filter_map_it.size_hint()]); // (0, None), (0, Some(3))

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=f1ccc42f95540d435e6d7caa58d29ed8

So perhaps this shouldn't be warn-by-default when it would also be plausible to have a performance-category lint to say to do exactly the opposite.

Version

(Not really version-specific; inherent to the definition of the lint.)

Additional Labels

@rustbot label +I-false-positive

Start with the filter_map_identity lint documentation linked in the issue and compare its rationale with the supplied size and size_hint examples. No source file or test is named, so locate the lint implementation and existing tests before deciding whether the default warning behavior is appropriate. Done means the lint's behavior and rationale clearly reflect the decision.