unnecessary_lazy_evaluations gives wrong suggestions for then

[danieledapo]

Summary

In the attached scenario unnecessary_lazy_evaluations is triggered, but it gives a wrong suggestion because the lazy expression relies on the fact that's called only when the number is greater than 0 because otherwise always subtracting 1 would result in an underflow.

Lint Name

unnecessary_lazy_evaluations

Reproducer

I tried this code:

pub fn sub(n: u8) -> Option<u8> {
    (n > 0).then(|| n - 1)
}

I saw this happen:

    Checking playground v0.0.1 (/playground)
warning: unnecessary closure used with `bool::then`
 --> src/lib.rs:4:5
  |
4 |     (n > 0).then(|| n - 1)
  |     ^^^^^^^^--------------
  |             |
  |             help: use `then_some(..)` instead: `then_some(n - 1)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations
  = note: `#[warn(clippy::unnecessary_lazy_evaluations)]` on by default

warning: `playground` (lib) generated 1 warning (run `cargo clippy --fix --lib -p playground` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 0.61s

I expected to see this happen:

No lint should be emitted

Version

rustc 1.73.0 (cc66ad468 2023-10-03)
binary: rustc
commit-hash: cc66ad468955717ab92600c770da8c1601a4ff33
commit-date: 2023-10-03
host: x86_64-unknown-linux-gnu
release: 1.73.0
LLVM version: 17.0.2

Additional Labels

No response

[y21]

Duplicate of #9814, will be fixed by #11002