Remove coords argument in compile_expression_dual_evaluation

First step towards dealing with #232.

Use fake coordinates coefficient when required instead of data carrying
.coordinates firedrake Function.

Author: ReubenHill

tsfc/driver.py

@@ -267,7 +267,7 @@ def name_multiindex(multiindex, name):
     return builder.construct_kernel(kernel_name, impero_c, index_names, quad_rule)
 
 
-def compile_expression_dual_evaluation(expression, to_element, coordinates, *,
+def compile_expression_dual_evaluation(expression, to_element, *,
                                        domain=None, interface=None,
                                        parameters=None, coffee=False):
     """Compile a UFL expression to be evaluated against a compile-time known reference element's dual basis.
@@ -276,8 +276,7 @@ def compile_expression_dual_evaluation(expression, to_element, coordinates, *,
 
     :arg expression: UFL expression
     :arg to_element: A FInAT element for the target space
-    :arg coordinates: the coordinate function
-    :arg domain: optional UFL domain the expression is defined on (useful when expression contains no domain).
+    :arg domain: optional UFL domain the expression is defined on (required when expression contains no domain).
     :arg interface: backend module for the kernel interface
     :arg parameters: parameters object
     :arg coffee: compile coffee kernel instead of loopy kernel
@@ -328,25 +327,28 @@ def compile_expression_dual_evaluation(expression, to_element, coordinates, *,
     argument_multiindices = tuple(builder.create_element(arg.ufl_element()).get_indices()
                                   for arg in arguments)
 
-    # Replace coordinates (if any)
-    domain = expression.ufl_domain()
-    if domain:
-        assert coordinates.ufl_domain() == domain
-        builder.domain_coordinate[domain] = coordinates
-        builder.set_cell_sizes(domain)
+    # Replace coordinates (if any) unless otherwise specified by kwarg
+    if domain is None:
+        domain = expression.ufl_domain()
+    assert domain is not None
+
+    # Create a fake coordinate coefficient for a domain.
+    coords_coefficient = ufl.Coefficient(ufl.FunctionSpace(domain, domain.ufl_coordinate_element()))
+    builder.domain_coordinate[domain] = coords_coefficient
+    builder.set_cell_sizes(domain)
 
     # Collect required coefficients
     coefficients = extract_coefficients(expression)
     if has_type(expression, GeometricQuantity) or any(fem.needs_coordinate_mapping(c.ufl_element()) for c in coefficients):
-        coefficients = [coordinates] + coefficients
+        coefficients = [coords_coefficient] + coefficients
     builder.set_coefficients(coefficients)
 
     # Split mixed coefficients
     expression = ufl_utils.split_coefficients(expression, builder.coefficient_split)
 
     # Translate to GEM
     kernel_cfg = dict(interface=builder,
-                      ufl_cell=coordinates.ufl_domain().ufl_cell(),
+                      ufl_cell=domain.ufl_cell(),
                       argument_multiindices=argument_multiindices,
                       index_cache={},
                       scalar_type=parameters["scalar_type"])