Function xmlTextReaderConstPrefix 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn xmlTextReaderConstPrefix(
    reader: *mut XmlTextReader,
) -> *const xmlChar

const xmlChar *xmlTextReaderConstPrefix(xmlTextReaderPtr reader).

UPSTREAM-PARITY: at an attribute position the prefix comes from the attribute; for a namespace declaration the prefix is reported as “xmlns” (and NULL for the default declaration) — an upstream quirk reproduced here.