fix: support all accumulators in enum forward deserialization

Previously limited to Option<T> only.

instant-xml-macros/src/de.rs

@@ -170,9 +170,9 @@ fn deserialize_forward_enum(
         let v_ident = &variant.ident;
         variants.extend(
             quote!(if <#no_lifetime_type as FromXml>::matches(id, None) {
-                let mut value = None;
+                let mut value = <#no_lifetime_type as FromXml>::Accumulator::default();
                 <#no_lifetime_type as FromXml>::deserialize(&mut value, #field_str, deserializer)?;
-                *into = value.map(#ident::#v_ident);
+                *into = ::instant_xml::Accumulate::try_done(value, #field_str).map(#ident::#v_ident).ok();
             }),
         );
     }

instant-xml/tests/forward-enum.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use similar_asserts::assert_eq;
 
 use instant_xml::{from_str, to_string, FromXml, ToXml};
@@ -26,3 +28,33 @@ fn wrapped_enum() {
     assert_eq!(xml, to_string(&v).unwrap());
     assert_eq!(v, from_str(xml).unwrap());
 }
+
+#[derive(Debug, FromXml, PartialEq, ToXml)]
+#[xml(forward)]
+enum FooCow<'a> {
+    Bar(Cow<'a, [BarBorrowed<'a>]>),
+    Baz(Cow<'a, [BazBorrowed<'a>]>),
+}
+
+#[derive(Clone, Debug, FromXml, PartialEq, ToXml)]
+#[xml(rename = "Bar")]
+struct BarBorrowed<'a> {
+    bar: Cow<'a, str>,
+}
+
+#[derive(Clone, Debug, FromXml, PartialEq, ToXml)]
+#[xml(rename = "Baz")]
+struct BazBorrowed<'a> {
+    baz: Cow<'a, str>,
+}
+
+#[test]
+fn with_cow_accumulator() {
+    let v = FooCow::Bar(Cow::Borrowed(&[BarBorrowed {
+        bar: Cow::Borrowed("test"),
+    }]));
+    let xml = r#"<Bar><bar>test</bar></Bar>"#;
+
+    assert_eq!(xml, to_string(&v).unwrap());
+    assert_eq!(v, from_str(xml).unwrap());
+}