`never_loop`: Also check all known iterator reductions.

[Jarcho]

Description

never_loop currently only checks loop bodies (loop, while and for), but iterator reduction functions that always diverge are just as likely to be bugs.

fn f(x: i32) -> ! {
    panic!();
}

[0, 1].into_iter().for_each(|x| f(x)); // Only calls `f` once

If not looping is actually intended using if let Some(x) = _.next() is much clearer that we do something if any item exists. This suggestion should not be MachineApplicable since looping was likely intended.

fn f(x: i32) -> ! {
    panic!();
}

if let Some(x) = [0, 1].into_iter().next() {
    f(x); 
}

See #16054 for an example of a loop that always diverged without it being immediately obvious. If that were written using for_each instead of a for loop it would still be a bug.

Version

clippy 0.1.93 (6647be9364 2025-11-09)

[cuongleqq]

Hey @Jarcho , this looks like a nice starter task. If you’re not actively on it right now, would you mind if I take a stab at it?