Add support for serialize_with attribute

instant-xml-macros/src/lib.rs

@@ -45,12 +45,6 @@ impl<'input> ContainerMeta<'input> {
 
         for (item, span) in meta_items(&input.attrs) {
             match item {
-                MetaItem::Attribute => {
-                    return Err(syn::Error::new(
-                        span,
-                        "attribute key invalid in container xml attribute",
-                    ))
-                }
                 MetaItem::Ns(namespace) => ns = namespace,
                 MetaItem::Rename(lit) => rename = Some(lit),
                 MetaItem::RenameAll(lit) => {
@@ -63,6 +57,12 @@ impl<'input> ContainerMeta<'input> {
                     None => mode = Some(new),
                     Some(_) => return Err(syn::Error::new(span, "cannot have two enum modes")),
                 },
+                _ => {
+                    return Err(syn::Error::new(
+                        span,
+                        "invalid field in container xml attribute",
+                    ))
+                }
             }
         }
 
@@ -111,6 +111,7 @@ struct FieldMeta {
     attribute: bool,
     ns: NamespaceMeta,
     tag: TokenStream,
+    serialize_with: Option<Literal>,
 }
 
 impl FieldMeta {
@@ -129,6 +130,7 @@ impl FieldMeta {
                 MetaItem::Attribute => meta.attribute = true,
                 MetaItem::Ns(ns) => meta.ns = ns,
                 MetaItem::Rename(lit) => meta.tag = quote!(#lit),
+                MetaItem::SerializeWith(lit) => meta.serialize_with = Some(lit),
                 MetaItem::RenameAll(_) => {
                     return Err(syn::Error::new(
                         span,
@@ -514,6 +516,8 @@ fn meta_items(attrs: &[syn::Attribute]) -> Vec<(MetaItem, Span)> {
                 } else if id == "wrapped" {
                     items.push((MetaItem::Mode(Mode::Wrapped), span));
                     MetaState::Comma
+                } else if id == "serialize_with" {
+                    MetaState::SerializeWith
                 } else {
                     panic!("unexpected key in xml attribute");
                 }
@@ -541,6 +545,13 @@ fn meta_items(attrs: &[syn::Attribute]) -> Vec<(MetaItem, Span)> {
                 items.push((MetaItem::RenameAll(lit), span));
                 MetaState::Comma
             }
+            (MetaState::SerializeWith, TokenTree::Punct(punct)) if punct.as_char() == '=' => {
+                MetaState::SerializeWithValue
+            }
+            (MetaState::SerializeWithValue, TokenTree::Literal(lit)) => {
+                items.push((MetaItem::SerializeWith(lit), span));
+                MetaState::Comma
+            }
             (state, tree) => {
                 panic!(
                     "invalid state transition while parsing xml attribute ({}, {tree})",
@@ -562,6 +573,8 @@ enum MetaState {
     RenameValue,
     RenameAll,
     RenameAllValue,
+    SerializeWith,
+    SerializeWithValue,
 }
 
 impl MetaState {
@@ -574,6 +587,8 @@ impl MetaState {
             MetaState::RenameValue => "RenameValue",
             MetaState::RenameAll => "RenameAll",
             MetaState::RenameAllValue => "RenameAllValue",
+            MetaState::SerializeWith => "SerializeWith",
+            MetaState::SerializeWithValue => "SerializeWithValue",
         }
     }
 }
@@ -635,6 +650,7 @@ enum MetaItem {
     Rename(Literal),
     Mode(Mode),
     RenameAll(Literal),
+    SerializeWith(Literal),
 }
 
 enum Namespace {

instant-xml-macros/src/ser.rs

@@ -166,9 +166,11 @@ fn serialize_struct(
 
     match data.fields {
         syn::Fields::Named(ref fields) => {
-            fields.named.iter().for_each(|field| {
+            for field in &fields.named {
-                process_named_field(field, &mut body, &mut attributes, &meta);
+                if let Err(err) = process_named_field(field, &mut body, &mut attributes, &meta) {
-            });
+                    return err.to_compile_error();
+                }
+            }
         }
         syn::Fields::Unnamed(_) => todo!(),
         syn::Fields::Unit => {}
@@ -238,16 +240,29 @@ fn process_named_field(
     body: &mut TokenStream,
     attributes: &mut TokenStream,
     meta: &ContainerMeta,
-) {
+) -> Result<(), syn::Error> {
     let field_name = field.ident.as_ref().unwrap();
     let field_meta = match FieldMeta::from_field(field, meta) {
         Ok(meta) => meta,
         Err(err) => {
             body.extend(err.into_compile_error());
-            return;
+            return Ok(());
         }
     };
 
+    if let Some(with) = field_meta.serialize_with {
+        let path = with.to_string();
+        let path = syn::parse_str::<syn::Path>(path.trim_matches('"')).map_err(|err| {
+            syn::Error::new(
+                with.span(),
+                format!("failed to parse serialize_with as path: {err}"),
+            )
+        })?;
+
+        body.extend(quote!(#path(&self.#field_name, serializer)?;));
+        return Ok(());
+    }
+
     let tag = field_meta.tag;
     let default_ns = match &meta.ns.uri {
         Some(ns) => quote!(#ns),
@@ -292,7 +307,7 @@ fn process_named_field(
             #error
             serializer.write_attr(#tag, #ns, &self.#field_name)?;
         ));
-        return;
+        return Ok(());
     }
 
     let ns = match field_meta.ns.uri {
@@ -315,4 +330,6 @@ fn process_named_field(
             }
         }
     ));
+
+    Ok(())
 }

instant-xml/tests/with.rs

@@ -0,0 +1,23 @@
+use std::fmt;
+
+use instant_xml::{to_string, Error, Serializer, ToXml};
+
+#[derive(ToXml)]
+struct Foo {
+    #[xml(serialize_with = "serialize_foo")]
+    foo: u8,
+}
+
+fn serialize_foo<W: fmt::Write + ?Sized>(
+    value: &u8,
+    serializer: &mut Serializer<'_, W>,
+) -> Result<(), Error> {
+    serializer.write_str(&format_args!("foo: {value}"))
+}
+
+#[test]
+fn serialize_with() {
+    let v = Foo { foo: 42 };
+    let xml = r#"<Foo>foo: 42</Foo>"#;
+    assert_eq!(xml, to_string(&v).unwrap());
+}