Centralize serializer prefix logic in one place

instant-xml-macros/src/ser.rs

@@ -49,14 +49,14 @@ pub fn to_xml(input: &syn::DeriveInput) -> proc_macro2::TokenStream {
                 serializer: &mut instant_xml::Serializer<W>,
             ) -> Result<(), instant_xml::Error> {
                 // Start tag
-                match serializer.default_ns() == #default_namespace {
-                    true => serializer.write_start(None, #root_name, None)?,
-                    false => serializer.write_start(None, #root_name, Some(#default_namespace))?,
-                }
+                let prefix = serializer.write_start(#root_name, #default_namespace, false)?;
+                debug_assert_eq!(prefix, None);
 
+                // Set up element context, this will also emit namespace declarations
                 #context
                 let old = serializer.push(new)?;
 
+                // Finalize start element
                 #attributes
                 serializer.end_start()?;
 
@@ -110,15 +110,7 @@ fn process_named_field(
                 self.#field_value.serialize(serializer)?;
             }
             ::instant_xml::Kind::Scalar => {
-                let (prefix, ns) = match serializer.default_ns() == #ns {
-                    true => (None, None),
-                    false => match serializer.prefix(#ns) {
-                        Some(prefix) => (Some(prefix), None),
-                        None => (None, Some(#ns)),
-                    },
-                };
-
-                serializer.write_start(prefix, #name, ns)?;
+                let prefix = serializer.write_start(#name, #ns, true)?;
                 serializer.end_start()?;
                 self.#field_value.serialize(serializer)?;
                 serializer.write_close(prefix, #name)?;

instant-xml/src/ser.rs

@@ -30,26 +30,37 @@ impl<'xml, W: fmt::Write + ?Sized> Serializer<'xml, W> {
 
     pub fn write_start(
         &mut self,
-        prefix: Option<&str>,
         name: &str,
-        ns: Option<&str>,
-    ) -> Result<(), Error> {
+        ns: &str,
+        scalar: bool,
+    ) -> Result<Option<&'static str>, Error> {
         if self.state != State::Element {
             return Err(Error::UnexpectedState);
         }
 
-        match prefix {
-            Some(prefix) => self.output.write_fmt(format_args!("<{prefix}:{name}"))?,
-            None => match ns {
-                Some(ns) => self
-                    .output
-                    .write_fmt(format_args!("<{name} xmlns=\"{ns}\""))?,
-                None => self.output.write_fmt(format_args!("<{name}"))?,
+        let prefix = match ns == self.default_ns {
+            true => {
+                self.output.write_fmt(format_args!("<{name}"))?;
+                None
+            }
+            // In the case of scalars, we can use the prefix instead of setting a
+            // default namespace (since there are no child elements that will
+            // expect the default namespace to be set).
+            false => match (scalar, self.prefixes.get(ns)) {
+                (false, _) | (true, None) => {
+                    self.output
+                        .write_fmt(format_args!("<{name} xmlns=\"{ns}\""))?;
+                    None
+                }
+                (true, Some(&prefix)) => {
+                    self.output.write_fmt(format_args!("<{prefix}:{name}"))?;
+                    Some(prefix)
+                }
             },
-        }
+        };
 
         self.state = State::Attribute;
-        Ok(())
+        Ok(prefix)
     }
 
     pub fn write_attr<V: ToXml + ?Sized>(&mut self, name: &str, value: &V) -> Result<(), Error> {