Function encode_as_url_query_string 

pub fn encode_as_url_query_string<'a>(
    string: &'a str,
    encoding: &'static Encoding,
) -> Cow<'a, [u8]>

This is equivalent to Encoding::encode, except nonmappable code points are handled according to the url specification, which expects nonmappable code points to be wrapped in %26%23 and %3B (see percent encode after encoding).