Function core_invoice_diff 

#[unsafe(no_mangle)]
pub extern "C" fn core_invoice_diff(
    left: *const c_char,
    right: *const c_char,
    out: *mut c_char,
    out_len: usize,
    err: *mut c_char,
    err_len: usize,
) -> c_int

Semantic diff. 0 identical, 1 differ, 2 unreadable. Writes the report into out.