unnecessary_lazy_evaluations suggests wrong fix

[barafael]

Summary

unnecessary_lazy_evaluations is normally a great lint, but in this example it produces a fix that does not compile.

The realistic example comes from embassy/embassy-usb.

Reproducer

I tried this code (see playground):

fn main() {
    let data = vec![1, 2, 3];
    let len = 3;
    let max_packet_size = 2;
    let need_zlp = true;
    let chunks = data[0..len]
        .chunks(max_packet_size)
        .chain(need_zlp.then(|| -> &[u8] { &[] }));
        //.chain(need_zlp.then_some({ &[] }));
    dbg!(chunks);
}

I expected to see this happen: No lint or a valid correction

Instead, this happened:

warning: unnecessary closure used with `bool::then`
 --> src/main.rs:8:16
  |
8 |         .chain(need_zlp.then(|| -> &[u8] { &[] }));
  |                ^^^^^^^^^-------------------------
  |                         |
  |                         help: use `then_some(..)` instead: `then_some({ &[] })`

Version

rustc 1.75.0-nightly (e0d7ed1f4 2023-10-01)
binary: rustc
commit-hash: e0d7ed1f453fb54578cc96dfea859b0e7be15016
commit-date: 2023-10-01
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.2

Additional Labels

@rustbot label +I-suggestion-causes-error

[Jarcho]

This wasn't actually fixed, it just won't be changed by clippy fix.