Implement transparent mode for serialization

instant-xml-macros/src/lib.rs

@@ -54,7 +54,7 @@ impl<'input> ContainerMeta<'input> {
                 }
                 MetaItem::Mode(new) => match mode {
                     None => mode = Some(new),
-                    Some(_) => return Err(syn::Error::new(span, "cannot have two enum modes")),
+                    Some(_) => return Err(syn::Error::new(span, "cannot have two modes")),
                 },
                 _ => {
                     return Err(syn::Error::new(
@@ -302,6 +302,7 @@ fn discard_path_lifetimes(
 enum Mode {
     Forward,
     Scalar,
+    Transparent,
 }
 
 #[cfg(test)]
@@ -334,7 +335,7 @@ mod tests {
             }
         })
         .to_string())
-        .find("compile_error ! { \"missing enum mode\" }")
+        .find("compile_error ! { \"missing mode\" }")
         .unwrap();
     }
 

instant-xml-macros/src/meta.rs

@@ -298,6 +298,9 @@ pub(crate) fn meta_items(attrs: &[syn::Attribute]) -> Vec<(MetaItem, Span)> {
                 } else if id == "direct" {
                     items.push((MetaItem::Direct, span));
                     MetaState::Comma
+                } else if id == "transparent" {
+                    items.push((MetaItem::Mode(Mode::Transparent), span));
+                    MetaState::Comma
                 } else if id == "ns" {
                     MetaState::Ns
                 } else if id == "rename" {

instant-xml-macros/src/ser.rs

@@ -15,13 +15,23 @@ pub fn to_xml(input: &syn::DeriveInput) -> proc_macro2::TokenStream {
 
     match (&input.data, meta.mode) {
         (syn::Data::Struct(data), None) => serialize_struct(input, data, meta),
+        (syn::Data::Struct(data), Some(Mode::Transparent)) => {
+            serialize_inline_struct(input, data, meta)
+        }
         (syn::Data::Enum(data), Some(Mode::Scalar)) => serialize_scalar_enum(input, data, meta),
         (syn::Data::Enum(data), Some(Mode::Forward)) => serialize_forward_enum(input, data, meta),
-        (syn::Data::Struct(_), _) => {
-            syn::Error::new(input.span(), "enum mode not allowed on struct type").to_compile_error()
-        }
-        (syn::Data::Enum(_), _) => {
-            syn::Error::new(input.span(), "missing enum mode").to_compile_error()
+        (syn::Data::Struct(_), Some(mode)) => syn::Error::new(
+            input.span(),
+            format_args!("{mode:?} mode not allowed on struct type"),
+        )
+        .to_compile_error(),
+        (syn::Data::Enum(_), Some(mode)) => syn::Error::new(
+            input.span(),
+            format_args!("{mode:?} mode not allowed on enum type"),
+        )
+        .to_compile_error(),
+        (syn::Data::Enum(_), None) => {
+            syn::Error::new(input.span(), "missing mode").to_compile_error()
         }
         _ => todo!(),
     }
@@ -233,6 +243,82 @@ fn serialize_struct(
     )
 }
 
+fn serialize_inline_struct(
+    input: &syn::DeriveInput,
+    data: &syn::DataStruct,
+    meta: ContainerMeta,
+) -> proc_macro2::TokenStream {
+    if !meta.ns.prefixes.is_empty() {
+        return syn::Error::new(
+            input.span(),
+            "inline structs cannot have namespace declarations",
+        )
+        .to_compile_error();
+    } else if let Some(ns) = meta.ns.uri {
+        return syn::Error::new(
+            ns.span(),
+            "inline structs cannot have namespace declarations",
+        )
+        .to_compile_error();
+    } else if let Some(rename) = meta.rename {
+        return syn::Error::new(rename.span(), "inline structs cannot be renamed")
+            .to_compile_error();
+    }
+
+    let mut body = TokenStream::new();
+    let mut attributes = TokenStream::new();
+    let mut borrowed = BTreeSet::new();
+    match &data.fields {
+        syn::Fields::Named(fields) => {
+            for field in &fields.named {
+                if let Err(err) =
+                    named_field(field, &mut body, &mut attributes, &mut borrowed, &meta)
+                {
+                    return err.to_compile_error();
+                }
+
+                if !attributes.is_empty() {
+                    return syn::Error::new(
+                        input.span(),
+                        "no attributes allowed on inline structs",
+                    )
+                    .to_compile_error();
+                }
+            }
+        }
+        syn::Fields::Unnamed(fields) => {
+            for (index, field) in fields.unnamed.iter().enumerate() {
+                if let Err(err) = unnamed_field(field, index, &mut body, &mut borrowed) {
+                    return err.to_compile_error();
+                }
+            }
+        }
+        syn::Fields::Unit => body.extend(quote!(serializer.end_empty()?;)),
+    }
+
+    let mut generics = input.generics.clone();
+    for param in generics.type_params_mut() {
+        param
+            .bounds
+            .push(syn::parse_str("::instant_xml::ToXml").unwrap());
+    }
+
+    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
+    let ident = &input.ident;
+    quote!(
+        impl #impl_generics ToXml for #ident #ty_generics #where_clause {
+            fn serialize<W: ::core::fmt::Write + ?::core::marker::Sized>(
+                &self,
+                field: Option<::instant_xml::Id<'_>>,
+                serializer: &mut instant_xml::Serializer<W>,
+            ) -> Result<(), instant_xml::Error> {
+                #body
+                Ok(())
+            }
+        };
+    )
+}
+
 fn named_field(
     field: &syn::Field,
     body: &mut TokenStream,

instant-xml/tests/transparent.rs

@@ -0,0 +1,37 @@
+use instant_xml::{to_string, ToXml};
+
+#[derive(Debug, Eq, PartialEq, ToXml)]
+struct Wrapper {
+    inline: Inline,
+}
+
+#[derive(Debug, Eq, PartialEq, ToXml)]
+#[xml(transparent)]
+struct Inline {
+    foo: Foo,
+    bar: Bar,
+}
+
+#[derive(Debug, Eq, PartialEq, ToXml)]
+struct Foo {
+    i: u8,
+}
+
+#[derive(Debug, Eq, PartialEq, ToXml)]
+struct Bar {
+    s: String,
+}
+
+#[test]
+fn inline() {
+    let v = Wrapper {
+        inline: Inline {
+            foo: Foo { i: 42 },
+            bar: Bar {
+                s: "hello".to_string(),
+            },
+        },
+    };
+    let xml = r#"<Wrapper><Foo><i>42</i></Foo><Bar><s>hello</s></Bar></Wrapper>"#;
+    assert_eq!(xml, to_string(&v).unwrap());
+}