Function contains_ignore_ascii_case 

pub fn contains_ignore_ascii_case<T1, T2>(s: T1, sub: T2) -> Option<usize>
where
    T1: AsRef<[u8]>,
    T2: AsRef<[u8]>,

Finds the first occurrence of sub within s, using ASCII case insensitive comparison.

The returned index is a byte offset into s. If sub is empty, this returns Some(0).