Function xmlInputSetEncodingHandler 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn xmlInputSetEncodingHandler(
    input: *mut _xmlParserInput,
    handler: *mut c_void,
) -> c_int

xmlInputSetEncodingHandler — attach an encoding handler to an input (upstream parserInternals.c; handler closed on error / UTF-8 pass).