xml: Some restructuring

crates/xml/src/se.rs

@@ -1,9 +1,10 @@
-use std::collections::HashMap;
-
-use quick_xml::{events::attributes::Attribute, name::Namespace};
-pub use xml_derive::XmlSerialize;
-
 use crate::XmlRootTag;
+use quick_xml::{
+    events::{attributes::Attribute, BytesStart, Event},
+    name::{Namespace, QName},
+};
+use std::collections::HashMap;
+pub use xml_derive::XmlSerialize;
 
 pub trait XmlSerialize {
     fn serialize<W: std::io::Write>(
@@ -54,3 +55,42 @@ impl<T: XmlSerialize + XmlRootTag> XmlSerializeRoot for T {
         self.serialize(Self::root_ns(), Some(Self::root_tag()), &namespaces, writer)
     }
 }
+
+impl XmlSerialize for () {
+    fn serialize<W: std::io::Write>(
+        &self,
+        ns: Option<Namespace>,
+        tag: Option<&[u8]>,
+        namespaces: &HashMap<Namespace, &[u8]>,
+        writer: &mut quick_xml::Writer<W>,
+    ) -> std::io::Result<()> {
+        let prefix = ns
+            .map(|ns| namespaces.get(&ns))
+            .unwrap_or(None)
+            .map(|prefix| {
+                if !prefix.is_empty() {
+                    [*prefix, b":"].concat()
+                } else {
+                    Vec::new()
+                }
+            });
+        let has_prefix = prefix.is_some();
+        let tagname = tag.map(|tag| [&prefix.unwrap_or_default(), tag].concat());
+        let qname = tagname.as_ref().map(|tagname| QName(tagname));
+        if let Some(qname) = &qname {
+            let mut bytes_start = BytesStart::from(qname.to_owned());
+            if !has_prefix {
+                if let Some(ns) = &ns {
+                    bytes_start.push_attribute((b"xmlns".as_ref(), ns.as_ref()));
+                }
+            }
+            writer.write_event(Event::Empty(bytes_start))?;
+        }
+        Ok(())
+    }
+
+    #[allow(refining_impl_trait)]
+    fn attributes<'a>(&self) -> Option<Vec<quick_xml::events::attributes::Attribute<'a>>> {
+        None
+    }
+}