Function xml_read_io 

pub fn xml_read_io<F>(
    read: F,
    url: Option<&str>,
    encoding: Option<&str>,
    options: i32,
) -> Result<XmlDoc, XmlError>
where
    F: FnMut(&mut [u8]) -> Result<usize, Error>,

xmlReadIO — caller-supplied read callback, no network.