More driver linting messages - procedural animation and no-variables for average,etc.

Inspired by T39315, this commit adds a few more driver "linting" messages used for
providing users with tips on how to use drivers better. This time, we specifically
address 2 cases:

1) Drivers being abused for procedural animation, due to the misconception that
   procedurally generating F-Curves using F-Modifiers means that drivers are needed
   to wire such procedural motion-sources to properties.
2) Setting up Average/Sum/Min/Max driver types without any input variables - you can't
   expect anything to happen (unless of course, your intention was to lock the property
   to 0.0)

1 files changed, 15 insertions, 0 deletions

diff --git a/source/blender/editors/space_graph/graph_buttons.c b/source/blender/editors/space_graph/graph_buttons.c
index 671a4c3afd3..437e3bef0d4 100644
--- a/source/blender/editors/space_graph/graph_buttons.c
+++ b/source/blender/editors/space_graph/graph_buttons.c
@@ -671,6 +671,21 @@ static void graph_panel_drivers(const bContext *C, Panel *pa)
 		/* errors? */
 		if (driver->flag & DRIVER_FLAG_INVALID)
 			uiItemL(col, IFACE_("ERROR: Invalid target channel(s)"), ICON_ERROR);
+			
+		/* Warnings about a lack of variables
+		 * NOTE: The lack of variables is generally a bad thing, since it indicates
+		 *       that the driver doesn't work at all. This particular scenario arises
+		 *       primarily when users mistakenly try to use drivers for procedural
+		 *       property animation
+		 */
+		if (BLI_listbase_is_empty(&driver->variables)) {
+			uiItemL(col, IFACE_("ERROR: Driver is useless without any inputs"), ICON_ERROR);
+			
+			if (!BLI_listbase_is_empty(&fcu->modifiers)) {
+				uiItemL(col, IFACE_("TIP: Use F-Curves for procedural animation instead"), ICON_INFO);
+				uiItemL(col, IFACE_("F-Modifiers can generate curves for those too"), ICON_INFO);
+			}
+		}
 	}
 		
 	col = uiLayoutColumn(pa->layout, TRUE);