xso-proc: Allow #[xml(flag)] without params

The namespace defaults to the parent’s namespace, and the name to the
field name, so we can avoid needlessly duplicating that info.

parsers/src/sm.rs

@@ -139,7 +139,7 @@ pub struct Resumed {
 #[xml(namespace = ns::SM, name = "sm")]
 pub struct StreamManagement {
     /// `<optional/>` flag.
-    #[xml(flag(name = "optional"))]
+    #[xml(flag)]
     pub optional: bool,
 }
 

parsers/src/starttls.rs

@@ -25,7 +25,7 @@ pub struct Proceed;
 #[xml(namespace = ns::TLS, name = "starttls")]
 pub struct StartTls {
     /// Marker for mandatory StartTLS.
-    #[xml(flag(name = "required"))]
+    #[xml(flag)]
     pub required: bool,
 }
 

parsers/src/util/macro_tests.rs

@@ -1977,7 +1977,7 @@ fn extract_ignore_unknown_stuff_roundtrip() {
 #[derive(FromXml, AsXml, PartialEq, Debug, Clone)]
 #[xml(namespace = NS1, name = "foo")]
 struct Flag {
-    #[xml(flag(namespace = NS1, name = "flag"))]
+    #[xml(flag)]
     flag: bool,
 }
 

xso-proc/src/meta.rs

@@ -1055,10 +1055,12 @@ impl XmlFieldMeta {
     /// Parse a `#[xml(flag)]` meta.
     fn flag_from_meta(meta: ParseNestedMeta<'_>) -> Result<Self> {
         let mut qname = QNameRef::default();
-        meta.parse_nested_meta(|meta| match qname.parse_incremental_from_meta(meta)? {
-            None => Ok(()),
-            Some(meta) => Err(Error::new_spanned(meta.path, "unsupported key")),
-        })?;
+        if meta.input.peek(syn::token::Paren) {
+            meta.parse_nested_meta(|meta| match qname.parse_incremental_from_meta(meta)? {
+                None => Ok(()),
+                Some(meta) => Err(Error::new_spanned(meta.path, "unsupported key")),
+            })?;
+        }
         Ok(Self::Flag {
             span: meta.path.span(),
             qname,