`flat_map_identity` misses de- and re-structuring

[TomFryersMidsummer]

Summary

If the identity function passed to flat_map_identity de- and re-builds the object (e.g. |[x, y]| [x, y]), then the lint doesn't fire.

Lint Name

flat_map_identity

Reproducer

I tried this code:

fn main() {
    let i = std::iter::empty::<[f32; 2]>();
    for e in i.flat_map(|[x, y]| [x, y]) {
        println!("{e:?}");
}

I expected to see this happen:

use of `flat_map` with an identity function

as happens with .flat_map(|e| e).

Instead, this happened:

[no error]

Version

rustc 1.90.0-nightly (71e4c005c 2025-07-01)
binary: rustc
commit-hash: 71e4c005caa812a16fcb08d0bf1e6f1eda7c8381
commit-date: 2025-07-01
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

[ada4a]

let me give this a try

@rustbot claim