Friendlier message when cxx::CxxString is used in a #[cxx::bridge].

syntax/parse.rs

@@ -1320,6 +1320,13 @@ fn parse_type_path(ty: &TypePath) -> Result<Type> {
         }
     }
 
+    if ty.qself.is_none() && path.segments.len() == 2 && path.segments[0].ident == "cxx" {
+        return Err(Error::new_spanned(
+            ty,
+            "unexpected `cxx::` qualifier found in a `#[cxx::bridge]`",
+        ));
+    }
+
     Err(Error::new_spanned(ty, "unsupported type"))
 }
 

tests/ui/cxx_crate_name_qualified_cxx_string.rs

@@ -0,0 +1,17 @@
+#[cxx::bridge]
+mod ffi {
+    extern "Rust" {
+        fn foo(x: CxxString);
+        fn bar(x: &cxx::CxxString);
+    }
+}
+
+fn foo(_: &cxx::CxxString) {
+    todo!()
+}
+
+fn bar(_: &cxx::CxxString) {
+    todo!()
+}
+
+fn main() {}

tests/ui/cxx_crate_name_qualified_cxx_string.stderr

@@ -0,0 +1,5 @@
+error: unexpected `cxx::` qualifier found in a `#[cxx::bridge]`
+ --> tests/ui/cxx_crate_name_qualified_cxx_string.rs:5:20
+  |
+5 |         fn bar(x: &cxx::CxxString);
+  |                    ^^^^^^^^^^^^^^