Q: How to find the kth smallest element in an unsorted array or its segment? Does Rust have a function to find the kth smallest element in an array or a segment of an array? 
(Similar to std::nth_element in C++)

A: Yes (since rust 1.49)
select_nth_unstable

A: I don't think there is such a function in std.
However, you can use the crate order_stat, which offers a kth function.

A: Hope this helps.
fn find_smallest_int(arr: &[i32]) -> i32 {
    let mut small_number=arr.to_vec();
    small_number.sort();
    small_number[0]
}

