Fix serialization of structs with only text content as attributes

Changelog.md

@@ -16,6 +16,9 @@
 
 ### New Features
 
+- [#907]: Allow to serialize and deserialize structs which contains only one `$text` field
+  as an attribute value or a text content value.
+
 ### Bug Fixes
 
 ### Misc Changes
@@ -25,6 +28,7 @@
   of `NsReader`. Use `.resolver().bindings()` and `.resolver().resolve()` methods instead.
 - [#913]: `Attributes::has_nil` now accepts `NamespaceResolver` instead of `Reader<R>`.
 
+[#907]: https://github.com/tafia/quick-xml/pull/907
 [#908]: https://github.com/tafia/quick-xml/pull/908
 [#913]: https://github.com/tafia/quick-xml/pull/913
 

src/de/simple_type.rs

@@ -3,13 +3,13 @@
 //! [simple types]: https://www.w3schools.com/xml/el_simpletype.asp
 //! [as defined]: https://www.w3.org/TR/xmlschema11-1/#Simple_Type_Definition
 
-use crate::de::Text;
+use crate::de::{Text, TEXT_KEY};
 use crate::encoding::Decoder;
 use crate::errors::serialize::DeError;
 use crate::escape::unescape;
 use crate::utils::{trim_xml_spaces, CowRef};
 use memchr::memchr;
-use serde::de::value::UnitDeserializer;
+use serde::de::value::{MapDeserializer, SeqAccessDeserializer, UnitDeserializer};
 use serde::de::{
     DeserializeSeed, Deserializer, EnumAccess, IntoDeserializer, SeqAccess, VariantAccess, Visitor,
 };
@@ -477,6 +477,17 @@ impl<'de, 'a> SeqAccess<'de> for ListIter<'de, 'a> {
     }
 }
 
+/// serde<=1.0.213 does not implement `IntoDeserializer` for `SeqAccessDeserializer`,
+/// which is required for `MapDeserializer`, so implement it for our type
+impl<'de, 'a> IntoDeserializer<'de, DeError> for ListIter<'de, 'a> {
+    type Deserializer = SeqAccessDeserializer<Self>;
+
+    #[inline]
+    fn into_deserializer(self) -> Self::Deserializer {
+        SeqAccessDeserializer::new(self)
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 /// A deserializer for an xml probably escaped and encoded value of XSD [simple types].
@@ -605,6 +616,19 @@ impl<'de, 'a> SimpleTypeDeserializer<'de, 'a> {
             },
         })
     }
+
+    #[inline]
+    fn as_seq_access(&self) -> Result<ListIter<'de, '_>, DeError> {
+        let content = match self.decode()? {
+            CowRef::Input(s) => Content::Input(s),
+            CowRef::Slice(s) => Content::Slice(s),
+            CowRef::Owned(s) => Content::Owned(s, 0),
+        };
+        Ok(ListIter {
+            content: Some(content),
+            escaped: self.escaped,
+        })
+    }
 }
 
 impl<'de, 'a> Deserializer<'de> for SimpleTypeDeserializer<'de, 'a> {
@@ -685,15 +709,7 @@ impl<'de, 'a> Deserializer<'de> for SimpleTypeDeserializer<'de, 'a> {
     where
         V: Visitor<'de>,
     {
-        let content = match self.decode()? {
-            CowRef::Input(s) => Content::Input(s),
-            CowRef::Slice(s) => Content::Slice(s),
-            CowRef::Owned(s) => Content::Owned(s, 0),
-        };
-        visitor.visit_seq(ListIter {
-            content: Some(content),
-            escaped: self.escaped,
-        })
+        visitor.visit_seq(self.as_seq_access()?)
     }
 
     /// Representation of tuples the same as [sequences][Self::deserialize_seq].
@@ -720,7 +736,22 @@ impl<'de, 'a> Deserializer<'de> for SimpleTypeDeserializer<'de, 'a> {
     }
 
     unsupported!(deserialize_map);
-    unsupported!(deserialize_struct(&'static str, &'static [&'static str]));
+
+    fn deserialize_struct<V>(
+        self,
+        _name: &'static str,
+        fields: &'static [&'static str],
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        if fields == [TEXT_KEY] {
+            let seq = self.as_seq_access()?;
+            return visitor.visit_map(MapDeserializer::new(std::iter::once((TEXT_KEY, seq))));
+        }
+        self.deserialize_str(visitor)
+    }
 
     fn deserialize_enum<V>(
         self,

src/se/simple_type.rs

@@ -3,11 +3,13 @@
 //! [simple types]: https://www.w3schools.com/xml/el_simpletype.asp
 //! [as defined]: https://www.w3.org/TR/xmlschema11-1/#Simple_Type_Definition
 
+use crate::de::TEXT_KEY;
 use crate::escape::escape_char;
+use crate::se::text::TextSerializer;
 use crate::se::{QuoteLevel, SeError};
 use crate::utils::CDataIterator;
 use serde::ser::{
-    Impossible, Serialize, SerializeSeq, SerializeTuple, SerializeTupleStruct,
+    Impossible, Serialize, SerializeSeq, SerializeStruct, SerializeTuple, SerializeTupleStruct,
     SerializeTupleVariant, Serializer,
 };
 use std::fmt::{self, Write};
@@ -480,7 +482,7 @@ impl<W: Write> Serializer for SimpleTypeSerializer<W> {
     type SerializeTupleStruct = SimpleSeq<W>;
     type SerializeTupleVariant = Impossible<Self::Ok, Self::Error>;
     type SerializeMap = Impossible<Self::Ok, Self::Error>;
-    type SerializeStruct = Impossible<Self::Ok, Self::Error>;
+    type SerializeStruct = SimpleSeq<W>;
     type SerializeStructVariant = Impossible<Self::Ok, Self::Error>;
 
     write_primitive!();
@@ -561,18 +563,18 @@ impl<W: Write> Serializer for SimpleTypeSerializer<W> {
         ))
     }
 
+    #[inline]
     fn serialize_struct(
         self,
-        name: &'static str,
+        _name: &'static str,
         _len: usize,
     ) -> Result<Self::SerializeStruct, Self::Error> {
-        Err(SeError::Unsupported(
-            format!(
-                "cannot serialize struct `{}` as an attribute or text content value",
-                name
-            )
-            .into(),
-        ))
+        Ok(SimpleSeq {
+            writer: self.writer,
+            target: self.target,
+            level: self.level,
+            is_empty: true,
+        })
     }
 
     fn serialize_struct_variant(
@@ -679,6 +681,37 @@ impl<W: Write> SerializeTupleVariant for SimpleSeq<W> {
     }
 }
 
+impl<W: Write> SerializeStruct for SimpleSeq<W> {
+    type Ok = W;
+    type Error = SeError;
+
+    fn serialize_field<T: ?Sized + Serialize>(
+        &mut self,
+        key: &'static str,
+        value: &T,
+    ) -> Result<(), Self::Error> {
+        if self.is_empty && key == TEXT_KEY {
+            let ser = TextSerializer(SimpleTypeSerializer {
+                writer: &mut self.writer,
+                target: self.target,
+                level: self.level,
+            });
+            value.serialize(ser)?;
+            self.is_empty = false;
+            Ok(())
+        } else {
+            Err(SeError::Unsupported(
+                format!("only struct which contains exactly one `{TEXT_KEY}` field may be serialized as an attribute or text content value").into(),
+            ))
+        }
+    }
+
+    #[inline]
+    fn end(self) -> Result<Self::Ok, Self::Error> {
+        SerializeSeq::end(self)
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////////////////////////
 
 #[cfg(test)]
@@ -1260,7 +1293,7 @@ mod tests {
         err!(map: BTreeMap::from([(1, 2), (3, 4)])
             => Unsupported("cannot serialize map as an attribute or text content value"));
         err!(struct_: Struct { key: "answer", val: 42 }
-            => Unsupported("cannot serialize struct `Struct` as an attribute or text content value"));
+            => Unsupported("only struct which contains exactly one `$text` field may be serialized as an attribute or text content value"));
         err!(enum_struct: Enum::Struct { key: "answer", val: 42 }
             => Unsupported("cannot serialize enum struct variant `Enum::Struct` as an attribute or text content value"));
     }

tests/serde-issues.rs

@@ -709,3 +709,50 @@ fn issue888() {
         }
     );
 }
+
+/// Regression test for https://github.com/tafia/quick-xml/issues/906.
+#[test]
+fn issue906() {
+    #[derive(Debug, PartialEq, Deserialize, Serialize)]
+    struct AsElement {
+        #[serde(rename = "a-list")]
+        a_list: TextContent,
+    }
+
+    #[derive(Debug, PartialEq, Deserialize, Serialize)]
+    struct AsAttribute {
+        #[serde(rename = "@a-list")]
+        a_list: TextContent,
+    }
+
+    #[derive(Debug, PartialEq, Deserialize, Serialize)]
+    struct TextContent {
+        #[serde(rename = "$text")]
+        content: Vec<String>,
+        #[serde(skip)]
+        ignored: (),
+    }
+
+    let foo = AsElement {
+        a_list: TextContent {
+            content: vec!["A".to_string(), "B".to_string()],
+            ignored: (),
+        },
+    };
+    let bar = AsAttribute {
+        a_list: TextContent {
+            content: vec!["A".to_string(), "B".to_string()],
+            ignored: (),
+        },
+    };
+
+    let buffer = to_string_with_root("test", &foo).unwrap();
+    assert_eq!(buffer, "<test><a-list>A B</a-list></test>");
+    let foo2: AsElement = from_str(&buffer).unwrap();
+    assert_eq!(foo2, foo);
+
+    let buffer = to_string_with_root("test", &bar).unwrap();
+    assert_eq!(buffer, "<test a-list=\"A B\"/>");
+    let bar2: AsAttribute = from_str(&buffer).unwrap();
+    assert_eq!(bar2, bar);
+}