Add expand_empty_elements fn to Serializer

Changelog.md

@@ -15,6 +15,7 @@
 - [#609]: Added `Writer::write_serializable` to provide the capability to serialize
   arbitrary types using serde when using the lower-level `Writer` API.
 - [#615]: Added ability to set entity resolver when deserialize using borrowing reader.
+- [#617]: Added ability to enforce the expansion of empty elements.
 
 ### Bug Fixes
 
@@ -23,6 +24,7 @@
 
 [#609]: https://github.com/tafia/quick-xml/pull/609
 [#615]: https://github.com/tafia/quick-xml/pull/615
+[#617]: https://github.com/tafia/quick-xml/pull/617
 
 
 ## 0.29.0 -- 2023-06-13

src/se/content.rs

@@ -56,6 +56,9 @@ pub struct ContentSerializer<'w, 'i, W: Write> {
     /// If `true`, then current indent will be written before writing the content,
     /// but only if content is not empty.
     pub write_indent: bool,
+    // If `true`, then empty elements will be serialized as `<element></element>`
+    // instead of `<element/>`.
+    pub expand_empty_elements: bool,
     //TODO: add settings to disallow consequent serialization of primitives
 }
 
@@ -85,16 +88,25 @@ impl<'w, 'i, W: Write> ContentSerializer<'w, 'i, W> {
             level: self.level,
             indent: self.indent.borrow(),
             write_indent: self.write_indent,
+            expand_empty_elements: false,
         }
     }
 
     /// Writes `name` as self-closed tag
     #[inline]
     pub(super) fn write_empty(mut self, name: XmlName) -> Result<(), DeError> {
         self.write_indent()?;
-        self.writer.write_char('<')?;
-        self.writer.write_str(name.0)?;
-        self.writer.write_str("/>")?;
+        if self.expand_empty_elements {
+            self.writer.write_char('<')?;
+            self.writer.write_str(name.0)?;
+            self.writer.write_str("></")?;
+            self.writer.write_str(name.0)?;
+            self.writer.write_char('>')?;
+        } else {
+            self.writer.write_str("<")?;
+            self.writer.write_str(name.0)?;
+            self.writer.write_str("/>")?;
+        }
         Ok(())
     }
 
@@ -483,6 +495,7 @@ pub(super) mod tests {
                         level: QuoteLevel::Full,
                         indent: Indent::None,
                         write_indent: false,
+                        expand_empty_elements: false,
                     };
 
                     $data.serialize(ser).unwrap();
@@ -503,6 +516,7 @@ pub(super) mod tests {
                         level: QuoteLevel::Full,
                         indent: Indent::None,
                         write_indent: false,
+                        expand_empty_elements: false,
                     };
 
                     match $data.serialize(ser).unwrap_err() {
@@ -672,6 +686,7 @@ pub(super) mod tests {
                         level: QuoteLevel::Full,
                         indent: Indent::Owned(Indentation::new(b' ', 2)),
                         write_indent: false,
+                        expand_empty_elements: false,
                     };
 
                     $data.serialize(ser).unwrap();
@@ -692,6 +707,7 @@ pub(super) mod tests {
                         level: QuoteLevel::Full,
                         indent: Indent::Owned(Indentation::new(b' ', 2)),
                         write_indent: false,
+                        expand_empty_elements: false,
                     };
 
                     match $data.serialize(ser).unwrap_err() {

src/se/element.rs

@@ -397,6 +397,7 @@ impl<'w, 'k, W: Write> Struct<'w, 'k, W> {
             level: self.ser.ser.level,
             indent: self.ser.ser.indent.borrow(),
             write_indent: true,
+            expand_empty_elements: false,
         };
 
         if key == TEXT_KEY {
@@ -574,6 +575,7 @@ mod tests {
                             level: QuoteLevel::Full,
                             indent: Indent::None,
                             write_indent: false,
+                            expand_empty_elements: false,
                         },
                         key: XmlName("root"),
                     };
@@ -597,6 +599,7 @@ mod tests {
                             level: QuoteLevel::Full,
                             indent: Indent::None,
                             write_indent: false,
+                            expand_empty_elements: false,
                         },
                         key: XmlName("root"),
                     };
@@ -1533,6 +1536,7 @@ mod tests {
                             level: QuoteLevel::Full,
                             indent: Indent::Owned(Indentation::new(b' ', 2)),
                             write_indent: false,
+                            expand_empty_elements: false,
                         },
                         key: XmlName("root"),
                     };
@@ -1556,6 +1560,7 @@ mod tests {
                             level: QuoteLevel::Full,
                             indent: Indent::Owned(Indentation::new(b' ', 2)),
                             write_indent: false,
+                            expand_empty_elements: false,
                         },
                         key: XmlName("root"),
                     };
@@ -2533,4 +2538,76 @@ mod tests {
                     </root>");
         }
     }
+
+    mod expand_empty_elements {
+        use super::*;
+        use pretty_assertions::assert_eq;
+
+        /// Checks that given `$data` successfully serialized as `$expected`
+        macro_rules! serialize_as {
+            ($name:ident: $data:expr => $expected:expr) => {
+                #[test]
+                fn $name() {
+                    let mut buffer = String::new();
+                    let ser = ElementSerializer {
+                        ser: ContentSerializer {
+                            writer: &mut buffer,
+                            level: QuoteLevel::Full,
+                            indent: Indent::None,
+                            write_indent: false,
+                            expand_empty_elements: true,
+                        },
+                        key: XmlName("root"),
+                    };
+
+                    $data.serialize(ser).unwrap();
+                    assert_eq!(buffer, $expected);
+                }
+            };
+        }
+
+        /// Checks that attempt to serialize given `$data` results to a
+        /// serialization error `$kind` with `$reason`
+        macro_rules! err {
+            ($name:ident: $data:expr => $kind:ident($reason:literal)) => {
+                #[test]
+                fn $name() {
+                    let mut buffer = String::new();
+                    let ser = ElementSerializer {
+                        ser: ContentSerializer {
+                            writer: &mut buffer,
+                            level: QuoteLevel::Full,
+                            indent: Indent::None,
+                            write_indent: false,
+                            expand_empty_elements: false,
+                        },
+                        key: XmlName("root"),
+                    };
+
+                    match $data.serialize(ser).unwrap_err() {
+                        DeError::$kind(e) => assert_eq!(e, $reason),
+                        e => panic!(
+                            "Expected `{}({})`, found `{:?}`",
+                            stringify!($kind),
+                            $reason,
+                            e
+                        ),
+                    }
+                    // We can write something before fail
+                    // assert_eq!(buffer, "");
+                }
+            };
+        }
+
+        serialize_as!(option_some_empty: Some("") => "<root></root>");
+        serialize_as!(option_some_empty_str: Some("") => "<root></root>");
+
+        serialize_as!(unit: () => "<root></root>");
+        serialize_as!(unit_struct: Unit => "<root></root>");
+        serialize_as!(unit_struct_escaped: UnitEscaped => "<root></root>");
+
+        serialize_as!(enum_unit: Enum::Unit => "<Unit></Unit>");
+        err!(enum_unit_escaped: Enum::UnitEscaped
+            => Unsupported("character `<` is not allowed at the start of an XML name `<\"&'>`"));
+    }
 }

src/se/mod.rs

@@ -458,6 +458,7 @@ impl<'w, 'r, W: Write> Serializer<'w, 'r, W> {
                 level: QuoteLevel::Full,
                 indent: Indent::None,
                 write_indent: false,
+                expand_empty_elements: false,
             },
             root_tag: None,
         }
@@ -522,11 +523,45 @@ impl<'w, 'r, W: Write> Serializer<'w, 'r, W> {
                 level: QuoteLevel::Full,
                 indent: Indent::None,
                 write_indent: false,
+                expand_empty_elements: false,
             },
             root_tag: root_tag.map(|tag| XmlName::try_from(tag)).transpose()?,
         })
     }
 
+    /// Enable or disable expansion of empty elements. Defaults to `false`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use pretty_assertions::assert_eq;
+    /// # use serde::Serialize;
+    /// # use quick_xml::se::Serializer;
+    ///
+    /// #[derive(Debug, PartialEq, Serialize)]
+    /// struct Struct {
+    ///     question: Option<String>,
+    /// }
+    ///
+    /// let mut buffer = String::new();
+    /// let mut ser = Serializer::new(&mut buffer);
+    /// ser.expand_empty_elements(true);
+    ///
+    /// let data = Struct {
+    ///   question: None,
+    /// };
+    ///
+    /// data.serialize(ser).unwrap();
+    /// assert_eq!(
+    ///     buffer,
+    ///     "<Struct><question></question></Struct>"
+    /// );
+    /// ```
+    pub fn expand_empty_elements(&mut self, expand: bool) -> &mut Self {
+        self.ser.expand_empty_elements = expand;
+        self
+    }
+
     /// Configure indent for a serializer
     pub fn indent(&mut self, indent_char: char, indent_size: usize) -> &mut Self {
         self.ser.indent = Indent::Owned(Indentation::new(indent_char as u8, indent_size));