Suggestions from review

Author: pbrubeck

firedrake/assemble.py

@@ -594,7 +594,8 @@ def base_form_assembly_visitor(self, expr, tensor, *args):
                                           v1: v1.reconstruct(number=v0.number())})
                 v, operand = expr.argument_slots()
 
-            # Assemble the interpolator matrix if the meshes are different
+            # Matrix-free adjoint interpolation is only implemented by SameMeshInterpolator
+            # so we need assemble the interpolator matrix if the meshes are different
             target_mesh = V.mesh()
             source_mesh = extract_unique_domain(operand) or target_mesh
             if is_adjoint and rank < 2 and source_mesh is not target_mesh:
@@ -605,6 +606,9 @@ def base_form_assembly_visitor(self, expr, tensor, *args):
             interp_data = expr.interp_data.copy()
             default_missing_val = interp_data.pop('default_missing_val', None)
             if matfree and ((is_adjoint and rank == 1) or rank == 0):
+                # Adjoint interpolation of a Cofunction or the action of a
+                # Cofunction on an interpolated Function require INC access
+                # on the output tensor
                 interp_data["access"] = op2.INC
 
             if rank == 1 and matfree and isinstance(tensor, firedrake.Function):

firedrake/interpolation.py

@@ -389,7 +389,9 @@ def __init__(
             )
 
         if isinstance(expr, ufl.Interpolate):
-            expr, = expr.ufl_operands
+            dual_arg, expr = expr.argument_slots()
+            if not isinstance(dual_arg, Coargument):
+                raise NotImplementedError(f"{type(self).__name__} does not support matrix-free adjoint interpolation.")
         super().__init__(expr, V, subset, freeze_expr, access, bcs, allow_missing_dofs, matfree)
 
         self.arguments = extract_arguments(expr)
@@ -749,7 +751,7 @@ def _interpolate(self, *function, output=None, transpose=None, adjoint=False, **
                     # Interpolation action
                     self.frozen_assembled_interpolator = assembled_interpolator.copy()
 
-        if len(self.arguments) == 2 and len(function):
+        if len(self.arguments) == 2 and len(function) > 0:
             function, = function
             if not hasattr(function, "dat"):
                 raise ValueError("The expression had arguments: we therefore need to be given a Function (not an expression) to interpolate!")
@@ -865,7 +867,7 @@ def make_interpolator(expr, V, subset, access, bcs=None, matfree=True):
             tensor = op2.Mat(sparsity)
         f = tensor
     else:
-        raise ValueError("Cannot interpolate an expression with %d arguments" % rank)
+        raise ValueError(f"Cannot interpolate an expression with {rank} arguments")
 
     if vom_onto_other_vom:
         wrapper = VomOntoVomWrapper(V, source_mesh, target_mesh, operand, matfree)