Properly use xsi:nil to deserialize null values via serde

.github/workflows/cifuzz.yml

@@ -17,7 +17,7 @@ jobs:
        language: rust
        fuzz-seconds: 600
    - name: Upload Crash
-     uses: actions/upload-artifact@v3
+     uses: actions/upload-artifact@v4
      if: failure() && steps.build.outcome == 'success'
      with:
        name: artifacts

Cargo.toml

@@ -231,6 +231,11 @@ name = "serde-de-seq"
 required-features = ["serialize"]
 path = "tests/serde-de-seq.rs"
 
+[[test]]
+name = "serde-de-xsi"
+required-features = ["serialize"]
+path = "tests/serde-de-xsi.rs"
+
 [[test]]
 name = "serde-se"
 required-features = ["serialize"]

Changelog.md

@@ -15,10 +15,17 @@
 
 ### New Features
 
+- [#850]: Add `Attribute::as_bool()` method to get an attribute value as a boolean.
+- [#850]: Add `Attributes::has_nil()` method to check if attributes has `xsi:nil` attribute set to `true`.
+- [#497]: Handle `xsi:nil` attribute in serde Deserializer to better process optional fields.
+
 ### Bug Fixes
 
 ### Misc Changes
 
+[#497]: https://github.com/tafia/quick-xml/issues/497
+[#850]: https://github.com/tafia/quick-xml/pull/850
+
 
 ## 0.37.2 -- 2024-12-29
 

src/de/map.rs

@@ -215,6 +215,21 @@ where
             has_value_field: fields.contains(&VALUE_KEY),
         })
     }
+
+    /// Determines if subtree started with the specified event shoould be skipped.
+    ///
+    /// Used to map elements with `xsi:nil` attribute set to true to `None` in optional contexts.
+    ///
+    /// We need to handle two attributes:
+    /// - on parent element: <map xsi:nil="true"><foo/></map>
+    /// - on this element:   <map><foo xsi:nil="true"/></map>
+    ///
+    /// We check parent element too because `xsi:nil` affects only nested elements of the
+    /// tag where it is defined. We can map structure with fields mapped to attributes to
+    /// the `<map>` element and set to `None` all its optional elements.
+    fn should_skip_subtree(&self, start: &BytesStart) -> bool {
+        self.de.reader.reader.has_nil_attr(&self.start) || self.de.reader.reader.has_nil_attr(start)
+    }
 }
 
 impl<'de, 'd, R, E> MapAccess<'de> for ElementMapAccess<'de, 'd, R, E>
@@ -540,8 +555,14 @@ where
     where
         V: Visitor<'de>,
     {
-        match self.map.de.peek()? {
+        // We cannot use result of `peek()` directly because of borrow checker
+        let _ = self.map.de.peek()?;
+        match self.map.de.last_peeked() {
             DeEvent::Text(t) if t.is_empty() => visitor.visit_none(),
+            DeEvent::Start(start) if self.map.should_skip_subtree(start) => {
+                self.map.de.skip_next_tree()?;
+                visitor.visit_none()
+            }
             _ => visitor.visit_some(self),
         }
     }