xso: discard xml:lang implicitly

Because this attribute may occur in random places, it makes no sense
failing on it. We discard it after attribute processing though, so it
can still be captured by structs which are explicitly interested in it.

parsers/src/util/macro_tests.rs

@@ -139,6 +139,23 @@ fn empty_unexpected_attribute() {
     }
 }
 
+#[test]
+#[cfg_attr(
+    feature = "disable-validation",
+    should_panic = "unexpected result: Ok("
+)]
+fn empty_ignores_xml_lang() {
+    #[allow(unused_imports)]
+    use core::{
+        option::Option::{None, Some},
+        result::Result::{Err, Ok},
+    };
+    match parse_str::<Empty>("<foo xmlns='urn:example:ns1' xml:lang='bar'/>") {
+        Ok(Empty) => (),
+        other => panic!("unexpected result: {:?}", other),
+    }
+}
+
 #[test]
 #[cfg_attr(
     feature = "disable-validation",