Allow direct String fields to match the empty string

Empty is not the same as missing, and the empty string
crops up a lot in upnp/dlna.

instant-xml-macros/src/de.rs

@@ -234,6 +234,7 @@ fn deserialize_struct(
     let mut declare_values = TokenStream::new();
     let mut return_val = TokenStream::new();
     let mut direct = TokenStream::new();
+    let mut after_loop = TokenStream::new();
 
     let mut borrowed = BTreeSet::new();
     for (index, field) in fields.named.iter().enumerate() {
@@ -263,6 +264,7 @@ fn deserialize_struct(
             field_meta,
             &input.ident,
             &container_meta,
+            &mut after_loop,
         );
 
         if let Err(err) = result {
@@ -361,6 +363,7 @@ fn deserialize_struct(
                         node => return Err(Error::UnexpectedNode(format!("{:?} in {}", node, #ident_str))),
                     }
                 }
+                #after_loop
 
                 *into = Some(Self { #return_val });
                 Ok(())
@@ -401,6 +404,7 @@ fn deserialize_inline_struct(
     let mut declare_values = TokenStream::new();
     let mut return_val = TokenStream::new();
     let mut direct = TokenStream::new();
+    let mut after_loop = TokenStream::new();
 
     let mut borrowed = BTreeSet::new();
     let mut matches = TokenStream::new();
@@ -433,6 +437,7 @@ fn deserialize_inline_struct(
             field_meta,
             &input.ident,
             &meta,
+            &mut after_loop,
         );
 
         let data = match result {
@@ -548,6 +553,7 @@ fn named_field<'a>(
     mut field_meta: FieldMeta,
     type_name: &Ident,
     container_meta: &ContainerMeta,
+    after_loop: &mut TokenStream,
 ) -> Result<FieldData<'a>, syn::Error> {
     let field_name = field.ident.as_ref().unwrap();
     let field_tag = field_meta.tag;
@@ -598,6 +604,15 @@ fn named_field<'a>(
         let mut #val_name = <#no_lifetime_type as FromXml>::Accumulator::default();
     ));
 
+    if field_meta.direct {
+        declare_values.extend(quote!(
+            // We will synthesize an empty text node when processing a direct
+            // element. Without this, we can miscategorize an empty tag as
+            // a missing tag
+            let mut seen_direct = false;
+        ));
+    }
+
     let deserialize_with = field_meta
         .deserialize_with
         .map(|with| {
@@ -630,10 +645,21 @@ fn named_field<'a>(
         } else if field_meta.direct {
             direct.extend(quote!(
                 Node::Text(text) => {
+                    seen_direct = true;
                     let mut nested = deserializer.for_node(Node::Text(text));
                     <#no_lifetime_type>::deserialize(&mut #val_name, #field_str, &mut nested)?;
                 }
             ));
+            // We can only enter this FromXml impl if the caller found the opening
+            // tag, so if we don't see the text node before the closing tag that is
+            // implied by terminating the loop, we need to populate the
+            // direct field with the implied empty text node.
+            after_loop.extend(quote!(
+                if !seen_direct {
+                    let mut nested = deserializer.for_node(Node::Text("".into()));
+                    <#no_lifetime_type>::deserialize(&mut #val_name, #field_str, &mut nested)?;
+                }
+            ));
         } else {
             tokens.r#match.extend(quote!(
                 __Elements::#enum_name => match <#no_lifetime_type as FromXml>::KIND {

instant-xml/src/impls.rs

@@ -270,10 +270,10 @@ impl<'xml> FromXml<'xml> for String {
             return Err(Error::DuplicateValue(field));
         }
 
-        match deserializer.take_str()? {
-            Some(value) => *into = Some(value.into_owned()),
-            None => return Ok(()),
-        }
+        *into = Some(match deserializer.take_str()? {
+            Some(value) => value.into_owned(),
+            None => String::new(),
+        });
 
         Ok(())
     }
@@ -300,12 +300,11 @@ impl<'xml, 'a> FromXml<'xml> for Cow<'a, str> {
             return Err(Error::DuplicateValue(field));
         }
 
-        let value = match deserializer.take_str()? {
-            Some(value) => value,
-            None => return Ok(()),
-        };
+        into.inner = Some(match deserializer.take_str()? {
+            Some(value) => value.into_owned().into(),
+            None => "".into(),
+        });
 
-        into.inner = Some(value.into_owned().into());
         Ok(())
     }
 

instant-xml/tests/de-direct.rs

@@ -1,3 +1,5 @@
+use std::borrow::Cow;
+
 use similar_asserts::assert_eq;
 
 use instant_xml::{from_str, Error, FromXml};
@@ -33,3 +35,70 @@ fn direct_namespaces() {
         Err::<StructDirectNamespace, _>(Error::MissingValue("StructDirectNamespace::flag"))
     );
 }
+
+#[derive(Debug, Eq, PartialEq, FromXml)]
+struct DirectString {
+    s: String,
+}
+
+#[test]
+fn direct_string() {
+    assert_eq!(
+        from_str("<DirectString><s>hello</s></DirectString>"),
+        Ok(DirectString {
+            s: "hello".to_string()
+        })
+    );
+}
+
+#[derive(Debug, Eq, PartialEq, FromXml)]
+struct DirectStr<'a> {
+    s: Cow<'a, str>,
+}
+
+#[test]
+fn direct_empty_str() {
+    assert_eq!(
+        from_str("<DirectStr><s></s></DirectStr>"),
+        Ok(DirectStr { s: "".into() })
+    );
+}
+
+#[test]
+fn direct_missing_string() {
+    assert_eq!(
+        from_str("<DirectString></DirectString>"),
+        Err::<DirectString, _>(Error::MissingValue("DirectString::s"))
+    );
+}
+
+#[derive(Debug, PartialEq, FromXml)]
+struct ArtUri {
+    #[xml(direct)]
+    uri: String,
+}
+
+#[derive(Debug, PartialEq, FromXml)]
+struct Container {
+    art: Option<ArtUri>,
+}
+
+#[test]
+fn container_empty_string() {
+    assert_eq!(
+        from_str("<Container><ArtUri></ArtUri></Container>"),
+        Ok(Container {
+            art: Some(ArtUri {
+                uri: "".to_string()
+            })
+        })
+    );
+    assert_eq!(
+        from_str("<Container><ArtUri/></Container>"),
+        Ok(Container {
+            art: Some(ArtUri {
+                uri: "".to_string()
+            })
+        })
+    );
+}