xso: only take reference in transform

This avoids the need for an expensive clone. Since we switched to AsXml
instead of IntoXml, we don't necessarily have to clone the data when
building new elements, only when it's absolutely necessary. The clones
then happen implicitly in the ItemToEvent iterator used internally.

This mostly fixes #86, with the caveat that there's no absolutely cheap
test: On success, the entire element will be copied, while on failure,
you learn about it rather quickly.

xso/src/lib.rs

@@ -381,7 +381,7 @@ impl UnknownChildPolicy {
 
 /// Attempt to transform a type implementing [`AsXml`] into another
 /// type which implements [`FromXml`].
-pub fn transform<T: FromXml, F: AsXml>(from: F) -> Result<T, self::error::Error> {
+pub fn transform<T: FromXml, F: AsXml>(from: &F) -> Result<T, self::error::Error> {
     let mut iter = self::rxml_util::ItemToEvent::new(from.as_xml_iter()?);
     let (qname, attrs) = match iter.next() {
         Some(Ok(rxml::Event::StartElement(_, qname, attrs))) => (qname, attrs),

xso/src/minidom_compat.rs

@@ -475,7 +475,7 @@ mod tests {
     #[test]
     fn transform_element_is_equivalent() {
         let el: Element = "<foo xmlns='urn:a' a='b' c='d'><child a='x'/><child a='y'>some text</child><child xmlns='urn:b'><nested-child/></child></foo>".parse().unwrap();
-        let transformed: Element = crate::transform(el.clone()).unwrap();
+        let transformed: Element = crate::transform(&el).unwrap();
         assert_eq!(el, transformed);
     }
 }