Function CSTL_u16string_replace_char_at 

pub unsafe extern "C" fn CSTL_u16string_replace_char_at(
    instance: CSTL_UTF16StringRef,
    off: usize,
    count: usize,
    count2: usize,
    ch: char16_t,
    alloc: *mut CSTL_Alloc,
) -> bool

Replaces the substring at offset off in instance with the length given by count with count copies of the character ch.

If off is outside of the range [instance, instance + CSTL_u16string_size(instance)] or if the length of the resulting string is greater than CSTL_u16string_max_size() this function has no effect and returns false, otherwise it returns true.