Function CSTL_wstring_append_substr 

pub unsafe extern "C" fn CSTL_wstring_append_substr(
    instance: CSTL_WideStringRef,
    other: CSTL_WideStringCRef,
    other_off: usize,
    count: usize,
    alloc: *mut CSTL_Alloc,
) -> bool

Appends the substring at offset other_off in other with the length given by count to instance.

If the length of the resulting string is greater than CSTL_wstring_max_size() this function has no effect and returns false, otherwise it returns true.