xso-proc: add support for optional children

parsers/src/util/macro_tests.rs

@@ -521,3 +521,32 @@ fn parent_positive() {
             .unwrap();
     assert_eq!(v.child.foo, "hello world!");
 }
+
+#[derive(FromXml, AsXml, PartialEq, Debug, Clone)]
+#[xml(namespace = NS1, name = "parent")]
+struct OptionalChild {
+    #[xml(child(default))]
+    child: std::option::Option<RequiredAttribute>,
+}
+
+#[test]
+fn optional_child_roundtrip_present() {
+    #[allow(unused_imports)]
+    use std::{
+        option::Option::{None, Some},
+        result::Result::{Err, Ok},
+    };
+    roundtrip_full::<OptionalChild>(
+        "<parent xmlns='urn:example:ns1'><attr foo='hello world!'/></parent>",
+    )
+}
+
+#[test]
+fn optional_child_roundtrip_absent() {
+    #[allow(unused_imports)]
+    use std::{
+        option::Option::{None, Some},
+        result::Result::{Err, Ok},
+    };
+    roundtrip_full::<OptionalChild>("<parent xmlns='urn:example:ns1'/>")
+}

xso-proc/src/field.rs

@@ -152,7 +152,11 @@ enum FieldKind {
     },
 
     /// The field maps to a child
-    Child,
+    Child {
+        // Flag indicating whether the value should be defaulted if the
+        // child is absent.
+        default_: Flag,
+    },
 }
 
 impl FieldKind {
@@ -199,7 +203,7 @@ impl FieldKind {
 
             XmlFieldMeta::Text { codec } => Ok(Self::Text { codec }),
 
-            XmlFieldMeta::Child => Ok(Self::Child),
+            XmlFieldMeta::Child { default_ } => Ok(Self::Child { default_ }),
         }
     }
 }
@@ -341,7 +345,7 @@ impl FieldDef {
                 })
             }
 
-            FieldKind::Child => {
+            FieldKind::Child { ref default_ } => {
                 let FromEventsScope {
                     ref substate_result,
                     ..
@@ -353,6 +357,18 @@ impl FieldDef {
                 let from_events = from_events_fn(self.ty.clone());
                 let from_xml_builder = from_xml_builder_ty(self.ty.clone());
 
+                let on_absent = match default_ {
+                    Flag::Absent => quote! {
+                        return ::core::result::Result::Err(::xso::error::Error::Other(#missing_msg).into())
+                    },
+                    Flag::Present(_) => {
+                        let default_ = default_fn(self.ty.clone());
+                        quote! {
+                            #default_()
+                        }
+                    }
+                };
+
                 Ok(FieldBuilderPart::Nested {
                     value: FieldTempInit {
                         init: quote! { ::std::option::Option::None },
@@ -368,7 +384,7 @@ impl FieldDef {
                     finalize: quote! {
                         match #field_access {
                             ::std::option::Option::Some(value) => value,
-                            ::std::option::Option::None => return ::core::result::Result::Err(::xso::error::Error::Other(#missing_msg).into()),
+                            ::std::option::Option::None => #on_absent,
                         }
                     },
                 })
@@ -426,7 +442,7 @@ impl FieldDef {
                 Ok(FieldIteratorPart::Text { generator })
             }
 
-            FieldKind::Child => {
+            FieldKind::Child { default_: _ } => {
                 let AsItemsScope { ref lifetime, .. } = scope;
 
                 let as_xml_iter = as_xml_iter_fn(self.ty.clone());

xso-proc/src/meta.rs

@@ -318,7 +318,10 @@ pub(crate) enum XmlFieldMeta {
     },
 
     /// `#[xml(child)`
-    Child,
+    Child {
+        /// The `default` flag.
+        default_: Flag,
+    },
 }
 
 impl XmlFieldMeta {
@@ -424,8 +427,26 @@ impl XmlFieldMeta {
     }
 
     /// Parse a `#[xml(child)]` meta.
-    fn child_from_meta(_: ParseNestedMeta<'_>) -> Result<Self> {
-        Ok(Self::Child)
+    fn child_from_meta(meta: ParseNestedMeta<'_>) -> Result<Self> {
+        if meta.input.peek(syn::token::Paren) {
+            let mut default_ = Flag::Absent;
+            meta.parse_nested_meta(|meta| {
+                if meta.path.is_ident("default") {
+                    if default_.is_set() {
+                        return Err(Error::new_spanned(meta.path, "duplicate `default` key"));
+                    }
+                    default_ = (&meta.path).into();
+                    Ok(())
+                } else {
+                    Err(Error::new_spanned(meta.path, "unsupported key"))
+                }
+            })?;
+            Ok(Self::Child { default_ })
+        } else {
+            Ok(Self::Child {
+                default_: Flag::Absent,
+            })
+        }
     }
 
     /// Parse [`Self`] from a nestd meta, switching on the identifier

xso/ChangeLog

@@ -14,7 +14,8 @@ Version NEXT:
         All this is to avoid triggering the camel case lint on the types we
         generate.
     * Added
-      - Support for child elements in derive macros.
+      - Support for child elements in derive macros. Child elements may be
+        wrapped in Option.
 
 Version 0.1.2:
 2024-07-26 Jonas Schäfer <jonas@zombofant.net>

xso/src/from_xml_doc.md

@@ -139,8 +139,19 @@ assert_eq!(foo, Foo {
 #### `child` meta
 
 The `child` meta causes the field to be mapped to a child element of the
-element. It supports no options. The field's type must implement [`FromXml`]
-in order to derive `FromXml` and [`AsXml`] in order to derive `AsXml`.
+element.
+
+| Key | Value type | Description |
+| --- | --- | --- |
+| `default` | flag | If present, an absent child will substitute the default value instead of raising an error. |
+
+The field's type must implement [`FromXml`] in order to derive `FromXml` and
+[`AsXml`] in order to derive `AsXml`.
+
+If `default` is specified and the child is absent in the source, the value
+is generated using [`std::default::Default`], requiring the field type to
+implement the `Default` trait for a `FromXml` derivation. `default` has no
+influence on `AsXml`.
 
 ##### Example
 

xso/src/lib.rs

@@ -83,6 +83,28 @@ pub trait AsXml {
     fn as_xml_iter(&self) -> Result<Self::ItemIter<'_>, self::error::Error>;
 }
 
+/// Helper iterator to convert an `Option<T>` to XML.
+pub struct OptionAsXml<T: Iterator>(Option<T>);
+
+impl<'x, T: Iterator<Item = Result<Item<'x>, self::error::Error>>> Iterator for OptionAsXml<T> {
+    type Item = Result<Item<'x>, self::error::Error>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.0.as_mut()?.next()
+    }
+}
+
+impl<T: AsXml> AsXml for Option<T> {
+    type ItemIter<'x> = OptionAsXml<T::ItemIter<'x>> where T: 'x;
+
+    fn as_xml_iter(&self) -> Result<Self::ItemIter<'_>, self::error::Error> {
+        match self {
+            Some(ref value) => Ok(OptionAsXml(Some(T::as_xml_iter(value)?))),
+            None => Ok(OptionAsXml(None)),
+        }
+    }
+}
+
 /// Trait for a temporary object allowing to construct a struct from
 /// [`rxml::Event`] items.
 ///
@@ -109,6 +131,17 @@ pub trait FromEventsBuilder {
     fn feed(&mut self, ev: rxml::Event) -> Result<Option<Self::Output>, self::error::Error>;
 }
 
+/// Helper struct to construct an `Option<T>` from XML events.
+pub struct OptionBuilder<T: FromEventsBuilder>(T);
+
+impl<T: FromEventsBuilder> FromEventsBuilder for OptionBuilder<T> {
+    type Output = Option<T::Output>;
+
+    fn feed(&mut self, ev: rxml::Event) -> Result<Option<Self::Output>, self::error::Error> {
+        self.0.feed(ev).map(|ok| ok.map(|value| Some(value)))
+    }
+}
+
 /// Trait allowing to construct a struct from a stream of
 /// [`rxml::Event`] items.
 ///
@@ -146,6 +179,17 @@ pub trait FromXml {
     ) -> Result<Self::Builder, self::error::FromEventsError>;
 }
 
+impl<T: FromXml> FromXml for Option<T> {
+    type Builder = OptionBuilder<T::Builder>;
+
+    fn from_events(
+        name: rxml::QName,
+        attrs: rxml::AttrMap,
+    ) -> Result<Self::Builder, self::error::FromEventsError> {
+        Ok(OptionBuilder(T::from_events(name, attrs)?))
+    }
+}
+
 /// Trait allowing to convert XML text to a value.
 ///
 /// This trait is similar to [`std::str::FromStr`], however, due to