Add iter::successors and iter::repeat_with to the list of possibly infinite iterators #17719

use std::iter::{repeat_with, successors};

fn infinity() {
    // this will be infinite, and might hang in runtime (or even panic, if we invoke count method!)
    //      v
    let _ = successors(Some(1i8), |&x| Some(x)).collect::<Vec<_>>();

    // this may be infinite, since some returned values are Some
    //      v
    let _ = successors(Some(1), |&x| if x != 12 { Some(x + 1) } else { None }).collect::<Vec<_>>();
    // this will be finite (empty actually), since the initial value is None
    //      v
    let _ = successors(None, |&x: &i32| Some(x)).count();
    // we consider this as maybe infinite, since we don't operate with cfg
    // even though the generator will return None
    //      v
    let _ = successors(Some(1), dummy_func).count();

    // we consider this as always finite
    // although this makes no sense tbh
    //      v
    let _ = successors(Some(1), |_| None).count();

    // same rules apply to repeat_with
    // this will panic!
    //      v
    let _ = repeat_with(|| 1).count();

    // and the rest is similar to iter::repeat(x) function
}

fn dummy_func(val: &i32) -> Option<i32> {
    None
}