Skip None attributes

instant-xml-macros/src/ser.rs

@@ -298,7 +298,9 @@ fn named_field(
 
         attributes.extend(quote!(
             #error
+            if self.#field_name.present() {
                 serializer.write_attr(#tag, #ns, &self.#field_name)?;
+            }
         ));
         return Ok(());
     }

instant-xml/src/impls.rs

@@ -453,6 +453,10 @@ impl<T: ToXml> ToXml for Option<T> {
             None => Ok(()),
         }
     }
+
+    fn present(&self) -> bool {
+        self.is_some()
+    }
 }
 
 fn encode(input: &str) -> Result<Cow<'_, str>, Error> {

instant-xml/src/lib.rs

@@ -20,6 +20,10 @@ pub trait ToXml {
         field: Option<Id<'_>>,
         serializer: &mut Serializer<W>,
     ) -> Result<(), Error>;
+
+    fn present(&self) -> bool {
+        true
+    }
 }
 
 impl<'a, T: ToXml + ?Sized> ToXml for &'a T {

instant-xml/tests/option.rs

@@ -17,3 +17,24 @@ fn option_vec() {
     assert_eq!(xml, to_string(&v).unwrap());
     assert_eq!(v, from_str(xml).unwrap());
 }
+
+#[derive(Debug, Eq, FromXml, PartialEq, ToXml)]
+struct Bar<'a> {
+    #[xml(attribute, borrow)]
+    maybe: Option<&'a str>,
+}
+
+#[test]
+fn option_borrow() {
+    let v = Bar { maybe: Some("a") };
+    let xml = r#"<Bar maybe="a"></Bar>"#;
+
+    assert_eq!(xml, to_string(&v).unwrap());
+    assert_eq!(v, from_str(xml).unwrap());
+
+    let v = Bar { maybe: None };
+    let xml = r#"<Bar></Bar>"#;
+
+    assert_eq!(xml, to_string(&v).unwrap());
+    assert_eq!(v, from_str(xml).unwrap());
+}