Function utf8proc_decompose_custom 

pub unsafe extern "C" fn utf8proc_decompose_custom(
    str_: *const utf8proc_uint8_t,
    strlen: utf8proc_ssize_t,
    buffer: *mut utf8proc_int32_t,
    bufsize: utf8proc_ssize_t,
    options: utf8proc_option_t,
    custom_func: utf8proc_custom_func,
    custom_data: *mut c_void,
) -> utf8proc_ssize_t

The same as utf8proc_decompose(), but also takes a custom_func mapping function that is called on each codepoint in str before any other transformations (along with a custom_data pointer that is passed through to custom_func). The custom_func argument is ignored if it is NULL. See also utf8proc_map_custom().