Fix T65148: Drivers can't access shape keys

It was impossible for drivers to use shape key properties, modifiers
generate a new mesh. After mesh evaluation the shape keys are no longer
necessary, and because of this the `key` pointer was not copied. As
drivers work on evaluated data, however, they do need this `key`
pointer.

This commit makes the `key` pointer available in evaluated meshes, but
this is somewhat dangerous. There was an explicit reason why the key on
result was kept at null pointer: to have the evaluated mesh in a
consistent state. Assigning this pointer makes it potentially
inconsistent, as the evaluated mesh and the original shape key may have
different topologies.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D7785

1 files changed, 6 insertions, 0 deletions

diff --git a/source/blender/blenkernel/intern/DerivedMesh.c b/source/blender/blenkernel/intern/DerivedMesh.c
index 63e7933dd56..0dc85dfaa18 100644
--- a/source/blender/blenkernel/intern/DerivedMesh.c
+++ b/source/blender/blenkernel/intern/DerivedMesh.c
@@ -1810,6 +1810,12 @@ static void mesh_build_data(struct Depsgraph *depsgraph,
 
   BKE_object_boundbox_calc_from_mesh(ob, mesh_eval);
 
+  /* Make sure that drivers can target shapekey properties.
+   * Note that this causes a potential inconsistency, as the shapekey may have a
+   * different topology than the evaluated mesh. */
+  BLI_assert(mesh->key == NULL || DEG_is_evaluated_id(&mesh->key->id));
+  mesh_eval->key = mesh->key;
+
   if ((ob->mode & OB_MODE_ALL_SCULPT) && ob->sculpt) {
     if (DEG_is_active(depsgraph)) {
       BKE_sculpt_update_object_after_eval(depsgraph, ob);