src/tools/clippy/tests/ui/iter_nth_zero.rs

#![warn(clippy::iter_nth_zero)]
use std::collections::HashSet;

struct Foo;

impl Foo {
    fn nth(&self, index: usize) -> usize {
        index + 1
    }
}

fn main() {
    let f = Foo {};
    f.nth(0); // lint does not apply here

    let mut s = HashSet::new();
    s.insert(1);
    let _x = s.iter().nth(0);

    let mut s2 = HashSet::new();
    s2.insert(2);
    let mut iter = s2.iter();
    let _y = iter.nth(0);

    let mut s3 = HashSet::new();
    s3.insert(3);
    let mut iter2 = s3.iter();
    let _unwrapped = iter2.nth(0).unwrap();
}

struct Issue9820;

impl Iterator for Issue9820 {
    type Item = ();

    fn nth(&mut self, _n: usize) -> Option<Self::Item> {
        todo!()
    }

    // Don't lint in implementations of `next`, as calling `next` in `next` is incorrect
    fn next(&mut self) -> Option<Self::Item> {
        self.nth(0)
    }
}