check free indices after calling fem.compile_ufl (#241)

Add asserts that we get the free indices we expect after calling
fem.compile_ufl. This is based on the understanding that GEM can
optimise away unnecessary free indices early, e.g. when interpolating an
expression which has no spatial variation (a ufl Constant).

tsfc/driver.py

@@ -378,6 +378,9 @@ def compile_expression_dual_evaluation(expression, to_element, *,
             config["quadrature_rule"] = quad_rule
 
         expr, = fem.compile_ufl(expression, **config, point_sum=False)
+        # In some cases point_set.indices may be dropped from expr, but nothing
+        # new should now appear
+        assert set(expr.free_indices) <= set(chain(point_set.indices, *argument_multiindices))
         shape_indices = tuple(gem.Index() for _ in expr.shape)
         basis_indices = point_set.indices
         ir = gem.Indexed(expr, shape_indices)
@@ -396,6 +399,9 @@ def compile_expression_dual_evaluation(expression, to_element, *,
                 config = kernel_cfg.copy()
                 config.update(point_set=point_set)
                 expr, = fem.compile_ufl(expression, **config, point_sum=False)
+                # In some cases point_set.indices may be dropped from expr, but
+                # nothing new should now appear
+                assert set(expr.free_indices) <= set(chain(point_set.indices, *argument_multiindices))
                 expr = gem.partial_indexed(expr, shape_indices)
                 expr_cache[pts] = expr, point_set
             weights = collections.defaultdict(list)