Code shared from the Rust Playground

playground.rs

#![allow(dead_code)]
#![allow(unused_variables)]

mod real_impl {
    pub fn find<El>(haystack: impl IntoIterator<Item = El>, needle: El) -> Option<usize>
    where
        El: core::cmp::PartialEq,
    {
        haystack
            .into_iter()
            .enumerate()
            .find(|(_, item)| item == &needle)
            .map(|(index, _)| index)
    }
}

mod naive {

    fn foo(x: &[i32], y: i32) -> Option<usize> {
        // 10000 строк нечитаемого кода
        super::real_impl::find(x, &y)
    }

    #[test]
    fn test() {
        assert_eq!(foo(&[1, 2, 3], 2), Some(1));
        assert_eq!(foo(&[1, 2, 3], 4), None);
    }
}