From e1866848243ab0d8b3292bf2d4456d07037fedcc Mon Sep 17 00:00:00 2001
From: Dirkjan Ochtman <dirkjan@ochtman.nl>
Date: Fri, 2 Dec 2022 14:41:07 +0100
Subject: [PATCH] Skip None attributes

---
 instant-xml-macros/src/ser.rs |  4 +++-
 instant-xml/src/impls.rs      |  4 ++++
 instant-xml/src/lib.rs        |  4 ++++
 instant-xml/tests/option.rs   | 21 +++++++++++++++++++++
 4 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/instant-xml-macros/src/ser.rs b/instant-xml-macros/src/ser.rs
index b036a18..7ea8e51 100644
--- a/instant-xml-macros/src/ser.rs
+++ b/instant-xml-macros/src/ser.rs
@@ -298,7 +298,9 @@ fn named_field(
 
         attributes.extend(quote!(
             #error
-            serializer.write_attr(#tag, #ns, &self.#field_name)?;
+            if self.#field_name.present() {
+                serializer.write_attr(#tag, #ns, &self.#field_name)?;
+            }
         ));
         return Ok(());
     }
diff --git a/instant-xml/src/impls.rs b/instant-xml/src/impls.rs
index 5b90b36..ea5e237 100644
--- a/instant-xml/src/impls.rs
+++ b/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> {
diff --git a/instant-xml/src/lib.rs b/instant-xml/src/lib.rs
index 8c52888..5de63c4 100644
--- a/instant-xml/src/lib.rs
+++ b/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 {
diff --git a/instant-xml/tests/option.rs b/instant-xml/tests/option.rs
index e776a93..17dcc47 100644
--- a/instant-xml/tests/option.rs
+++ b/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());
+}