fix(derive): Support serialization of enums with no arms. (#113)

Author: lukasfri

xmlity-derive/src/ser/serialize/none.rs

@@ -150,6 +150,12 @@ impl SerializeBuilder for DeriveEnum<'_> {
             });
         }
 
+        if variants.is_empty() {
+            return Ok(parse_quote! {
+                unreachable!("Enum {} has no variants", stringify!(#enum_ident))
+            });
+        }
+
         let variants = variants
             .iter()
             .map::<Result<Arm, DeriveError>, _>(|variant| {

xmlity-quick-xml/tests/other/enum_with_no_arm.rs

@@ -0,0 +1,8 @@
+#[derive(
+    ::core::fmt::Debug,
+    ::xmlity::Serialize,
+    ::xmlity::Deserialize,
+    ::core::cmp::PartialEq,
+    ::core::clone::Clone,
+)]
+pub enum NamespaceList {}

xmlity-quick-xml/tests/other/mod.rs

@@ -1,5 +1,6 @@
 pub mod combined;
 pub mod empty;
 pub mod empty_variant;
+pub mod enum_with_no_arm;
 pub mod variant;
 pub mod xml_value;