Implement API to add newlines between attributes when writing XML

Changelog.md

@@ -30,6 +30,7 @@ to get an offset of the error position. For `SyntaxError`s the range
 - [#362]: Added `BytesCData::minimal_escape()` which escapes only `&` and `<`.
 - [#362]: Added `Serializer::set_quote_level()` which allow to set desired level of escaping.
 - [#705]: Added `NsReader::prefixes()` to list all the prefixes currently declared.
+- [#275]: Added `ElementWriter::new_line()` which enables pretty printing elements with multiple attributes.
 
 ### Bug Fixes
 
@@ -61,6 +62,7 @@ to get an offset of the error position. For `SyntaxError`s the range
 - [#689]: `buffer_position()` now always report the position the parser last seen.
   To get an error position use `error_position()`.
 
+[#275]: https://github.com/tafia/quick-xml/issues/275
 [#362]: https://github.com/tafia/quick-xml/issues/362
 [#513]: https://github.com/tafia/quick-xml/issues/513
 [#622]: https://github.com/tafia/quick-xml/issues/622

src/events/mod.rs

@@ -242,13 +242,8 @@ impl<'a> BytesStart<'a> {
     where
         A: Into<Attribute<'b>>,
     {
-        let a = attr.into();
-        let bytes = self.buf.to_mut();
-        bytes.push(b' ');
-        bytes.extend_from_slice(a.key.as_ref());
-        bytes.extend_from_slice(b"=\"");
-        bytes.extend_from_slice(a.value.as_ref());
-        bytes.push(b'"');
+        self.buf.to_mut().push(b' ');
+        self.push_attr(attr.into());
     }
 
     /// Remove all attributes from the ByteStart
@@ -287,6 +282,26 @@ impl<'a> BytesStart<'a> {
         }
         Ok(None)
     }
+
+    /// Adds an attribute to this element.
+    pub(crate) fn push_attr<'b>(&mut self, attr: Attribute<'b>) {
+        let bytes = self.buf.to_mut();
+        bytes.extend_from_slice(attr.key.as_ref());
+        bytes.extend_from_slice(b"=\"");
+        // FIXME: need to escape attribute content
+        bytes.extend_from_slice(attr.value.as_ref());
+        bytes.push(b'"');
+    }
+
+    /// Adds new line in existing element
+    pub(crate) fn push_newline(&mut self) {
+        self.buf.to_mut().push(b'\n');
+    }
+
+    /// Adds indentation bytes in existing element
+    pub(crate) fn push_indent(&mut self, indent: &[u8]) {
+        self.buf.to_mut().extend_from_slice(indent);
+    }
 }
 
 impl<'a> Debug for BytesStart<'a> {