Track xml:lang throughout parsing

xso-proc/src/field/child.rs

@@ -68,7 +68,7 @@ impl Field for ChildField {
 
                 let from_events = from_events_fn(element_ty.clone());
 
-                let matcher = quote! { #from_events(name, attrs) };
+                let matcher = quote! { #from_events(name, attrs, ctx) };
                 let builder = from_xml_builder_ty(element_ty.clone());
 
                 (
@@ -307,7 +307,7 @@ impl ExtractDef {
         )?;
         let from_xml_builder_ty = ty_from_ident(from_xml_builder_ty_ident.clone()).into();
 
-        let matcher = quote! { #state_ty_ident::new(name, attrs).map(|x| #from_xml_builder_ty_ident(::core::option::Option::Some(x))) };
+        let matcher = quote! { #state_ty_ident::new(name, attrs, ctx).map(|x| #from_xml_builder_ty_ident(::core::option::Option::Some(x))) };
 
         let inner_ty = self.parts.to_single_or_tuple_ty();
 

xso-proc/src/field/element.rs

@@ -80,7 +80,7 @@ impl Field for ElementField {
                         if #field_access.is_some() {
                             ::core::result::Result::Err(::xso::error::FromEventsError::Mismatch { name, attrs })
                         } else {
-                            #from_events(name, attrs)
+                            #from_events(name, attrs, ctx)
                         }
                     }),
                     builder,

xso-proc/src/field/lang.rs

@@ -0,0 +1,72 @@
+// Copyright (c) 2025 Jonas Schäfer <jonas@zombofant.net>
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+//! This module concerns the processing of inherited `xml:lang` values.
+//!
+//! In particular, it provides the `#[xml(lang)]` implementation.
+
+use proc_macro2::Span;
+use quote::quote;
+use syn::*;
+
+use crate::error_message::ParentRef;
+use crate::scope::{AsItemsScope, FromEventsScope};
+use crate::types::{as_optional_xml_text_fn, option_ty, string_ty};
+
+use super::{Field, FieldBuilderPart, FieldIteratorPart, FieldTempInit};
+
+/// The field maps to a potentially inherited `xml:lang` value.
+pub(super) struct LangField;
+
+impl Field for LangField {
+    fn make_builder_part(
+        &self,
+        _scope: &FromEventsScope,
+        _container_name: &ParentRef,
+        _member: &Member,
+        _ty: &Type,
+    ) -> Result<FieldBuilderPart> {
+        let string_ty = string_ty(Span::call_site());
+        let ty = option_ty(string_ty.clone());
+
+        Ok(FieldBuilderPart::Init {
+            value: FieldTempInit {
+                ty,
+                init: quote! {
+                    ctx.language().map(#string_ty::from).into()
+                },
+            },
+        })
+    }
+
+    fn make_iterator_part(
+        &self,
+        _scope: &AsItemsScope,
+        _container_name: &ParentRef,
+        bound_name: &Ident,
+        _member: &Member,
+        ty: &Type,
+    ) -> Result<FieldIteratorPart> {
+        let as_optional_xml_text = as_optional_xml_text_fn(ty.clone());
+
+        Ok(FieldIteratorPart::Header {
+            generator: quote! {
+                #as_optional_xml_text(#bound_name)?.map(|#bound_name|
+                    ::xso::Item::Attribute(
+                        ::xso::exports::rxml::Namespace::XML,
+                        // SAFETY: `lang` is a known-good NcName
+                        unsafe {
+                            ::xso::exports::alloc::borrow::Cow::Borrowed(
+                                ::xso::exports::rxml::NcNameStr::from_str_unchecked("lang"),
+                            )
+                        },
+                        #bound_name,
+                    )
+                )
+            },
+        })
+    }
+}

xso-proc/src/field/mod.rs

@@ -21,6 +21,7 @@ mod child;
 #[cfg(feature = "minidom")]
 mod element;
 mod flag;
+mod lang;
 mod text;
 
 use self::attribute::AttributeField;
@@ -28,6 +29,7 @@ use self::child::{ChildField, ExtractDef};
 #[cfg(feature = "minidom")]
 use self::element::ElementField;
 use self::flag::FlagField;
+use self::lang::LangField;
 use self::text::TextField;
 
 /// Code slices necessary for declaring and initializing a temporary variable
@@ -444,6 +446,8 @@ fn new_field(
                 xml_name,
             }))
         }
+
+        XmlFieldMeta::Language { span: _ } => Ok(Box::new(LangField)),
     }
 }