`filter_map_identity` FN on `&Option` and other non-identity functions

[Centri3]

Summary

While it's technically not an identity function, |&x| x and ||x| x.as_ref()| should be linted for the same reasons as |x| x

Lint Name

clippy::filter_map_identity

Reproducer

I tried this code:

struct NonCopy;

fn main() {
    _ = vec![Some(()), None, Some(())].iter().filter_map(|&x| x);
    _ = vec![Some(NonCopy), None, Some(NonCopy)].iter().filter_map(|x| x.as_ref());
    _ = vec![Some(()), None, Some(())].iter_mut().filter_map(|&mut x| x);
    _ = vec![Some(NonCopy), None, Some(NonCopy)].iter_mut().filter_map(|x| x.as_mut());
}

I expected to see this happen: 4 warnings

Instead, this happened: clippy is silent

Version

cargo 1.78.0-nightly (2fe739fcf 2024-03-15)
release: 1.78.0-nightly
commit-hash: 2fe739fcf16c5bf8c2064ab9d357f4a0e6c8539b
commit-date: 2024-03-15
host: x86_64-pc-windows-msvc
libgit2: 1.7.2 (sys:0.18.2 vendored)
libcurl: 8.6.0-DEV (sys:0.4.72+curl-8.6.0 vendored ssl:Schannel)
os: Windows 10.0.19045 (Windows 10 Home) [64-bit]

[Centri3]

rustc suggestions when the & or as_ref is missing are really bad, too, it suggests Some(x) which is useless. An issue should be opened about that too

[PartiallyUntyped]

@rustbot claim

[PartiallyUntyped]

Speaking of, maybe we should also lint .filter_map(|x| Some(x)) if we don't already do that.

[Centri3]

We do that already yeah, but the suggestion suggests map instead which is pretty bad. We could change that too if it's Option<Option<T>>