UFL classes are already immutable.

Directly return them.

python/dune/perftool/preambles.py

@@ -50,6 +50,10 @@ def generate_cache_tuple(*args):
         if isinstance(data, str):
             return data
 
+        import ufl.classes
+        if isinstance(data, ufl.classes.Expr):
+            return data
+
         # Check if the given data is already hashable
         if isinstance(data, Hashable):
             if isinstance(data, Iterable):