Store language identifier inside ExternFn for diagnostics
diff --git a/syntax/mod.rs b/syntax/mod.rs
index 5b51648..79a4a7b 100644
--- a/syntax/mod.rs
+++ b/syntax/mod.rs
@@ -44,6 +44,7 @@
 }
 
 pub struct ExternFn {
+    pub lang: Lang,
     pub doc: Doc,
     pub fn_token: Token![fn],
     pub ident: Ident,
@@ -87,6 +88,12 @@
 }
 
 #[derive(Copy, Clone, PartialEq)]
+pub enum Lang {
+    Cxx,
+    Rust,
+}
+
+#[derive(Copy, Clone, PartialEq)]
 pub enum Derive {
     Clone,
     Copy,