Fix T91691: Selecting "Remove unused slots" in Materials panel removes slots that are assigned to particle systems/hair.

`BKE_object_material_slot_used` would only check obdata usages, but
particle settings can also (weirdly enough) use objects' material slots.

So now, as its name suggests, `BKE_object_material_slot_used` does take
an object as parameter, and also checks for potential slot usage from
psys in the object.

source/blender/blenkernel/BKE_material.h

@@ -90,7 +90,7 @@ void BKE_object_material_array_assign(struct Main *bmain,
 short BKE_object_material_slot_find_index(struct Object *ob, struct Material *ma);
 bool BKE_object_material_slot_add(struct Main *bmain, struct Object *ob);
 bool BKE_object_material_slot_remove(struct Main *bmain, struct Object *ob);
-bool BKE_object_material_slot_used(struct ID *id, short actcol);
+bool BKE_object_material_slot_used(struct Object *object, short actcol);
 
 struct Material *BKE_gpencil_material(struct Object *ob, short act);
 struct MaterialGPencilStyle *BKE_gpencil_material_settings(struct Object *ob, short act);

source/blender/blenkernel/intern/gpencil_curve.c

@@ -543,7 +543,7 @@ void BKE_gpencil_convert_curve(Main *bmain,
   int actcol = ob_gp->actcol;
 
   for (int slot = 1; slot <= ob_gp->totcol; slot++) {
-    while (slot <= ob_gp->totcol && !BKE_object_material_slot_used(ob_gp->data, slot)) {
+    while (slot <= ob_gp->totcol && !BKE_object_material_slot_used(ob_gp, slot)) {
       ob_gp->actcol = slot;
       BKE_object_material_slot_remove(bmain, ob_gp);
 

source/blender/blenkernel/intern/material.c

@@ -46,6 +46,7 @@
 #include "DNA_meta_types.h"
 #include "DNA_node_types.h"
 #include "DNA_object_types.h"
+#include "DNA_particle_types.h"
 #include "DNA_pointcloud_types.h"
 #include "DNA_scene_types.h"
 #include "DNA_volume_types.h"
@@ -73,6 +74,7 @@
 #include "BKE_material.h"
 #include "BKE_mesh.h"
 #include "BKE_node.h"
+#include "BKE_object.h"
 #include "BKE_scene.h"
 
 #include "DEG_depsgraph.h"
@@ -462,21 +464,33 @@ static void material_data_index_remove_id(ID *id, short index)
   }
 }
 
-bool BKE_object_material_slot_used(ID *id, short actcol)
+bool BKE_object_material_slot_used(Object *object, short actcol)
 {
-  /* ensure we don't try get materials from non-obdata */
-  BLI_assert(OB_DATA_SUPPORT_ID(GS(id->name)));
+  if (!BKE_object_supports_material_slots(object)) {
+    return false;
+  }
 
-  switch (GS(id->name)) {
+  LISTBASE_FOREACH (ParticleSystem *, psys, &object->particlesystem) {
+    if (psys->part->omat == actcol) {
+      return true;
+    }
+  }
+
+  ID *ob_data = object->data;
+  if (ob_data == NULL || !OB_DATA_SUPPORT_ID(GS(ob_data->name))) {
+    return false;
+  }
+
+  switch (GS(ob_data->name)) {
     case ID_ME:
-      return BKE_mesh_material_index_used((Mesh *)id, actcol - 1);
+      return BKE_mesh_material_index_used((Mesh *)ob_data, actcol - 1);
     case ID_CU:
-      return BKE_curve_material_index_used((Curve *)id, actcol - 1);
+      return BKE_curve_material_index_used((Curve *)ob_data, actcol - 1);
     case ID_MB:
       /* Meta-elements don't support materials at the moment. */
       return false;
     case ID_GD:
-      return BKE_gpencil_material_index_used((bGPdata *)id, actcol - 1);
+      return BKE_gpencil_material_index_used((bGPdata *)ob_data, actcol - 1);
     default:
       return false;
   }

source/blender/editors/gpencil/gpencil_edit.c

@@ -4729,7 +4729,7 @@ static int gpencil_stroke_separate_exec(bContext *C, wmOperator *op)
   /* Remove unused slots. */
   int actcol = ob_dst->actcol;
   for (int slot = 1; slot <= ob_dst->totcol; slot++) {
-    while (slot <= ob_dst->totcol && !BKE_object_material_slot_used(ob_dst->data, slot)) {
+    while (slot <= ob_dst->totcol && !BKE_object_material_slot_used(ob_dst, slot)) {
       ob_dst->actcol = slot;
       BKE_object_material_slot_remove(bmain, ob_dst);
       if (actcol >= slot) {

source/blender/editors/gpencil/gpencil_mesh.c

@@ -345,7 +345,7 @@ static int gpencil_bake_mesh_animation_exec(bContext *C, wmOperator *op)
   /* Remove unused materials. */
   int actcol = ob_gpencil->actcol;
   for (int slot = 1; slot <= ob_gpencil->totcol; slot++) {
-    while (slot <= ob_gpencil->totcol && !BKE_object_material_slot_used(ob_gpencil->data, slot)) {
+    while (slot <= ob_gpencil->totcol && !BKE_object_material_slot_used(ob_gpencil, slot)) {
       ob_gpencil->actcol = slot;
       BKE_object_material_slot_remove(CTX_data_main(C), ob_gpencil);
 

source/blender/editors/object/object_add.c

@@ -2828,8 +2828,7 @@ static int object_convert_exec(bContext *C, wmOperator *op)
       /* Remove unused materials. */
       int actcol = ob_gpencil->actcol;
       for (int slot = 1; slot <= ob_gpencil->totcol; slot++) {
-        while (slot <= ob_gpencil->totcol &&
-               !BKE_object_material_slot_used(ob_gpencil->data, slot)) {
+        while (slot <= ob_gpencil->totcol && !BKE_object_material_slot_used(ob_gpencil, slot)) {
           ob_gpencil->actcol = slot;
           BKE_object_material_slot_remove(CTX_data_main(C), ob_gpencil);
 

source/blender/editors/render/render_shading.c

@@ -690,7 +690,7 @@ static int material_slot_remove_unused_exec(bContext *C, wmOperator *op)
     Object *ob = objects[ob_index];
     int actcol = ob->actcol;
     for (int slot = 1; slot <= ob->totcol; slot++) {
-      while (slot <= ob->totcol && !BKE_object_material_slot_used(ob->data, slot)) {
+      while (slot <= ob->totcol && !BKE_object_material_slot_used(ob, slot)) {
         ob->actcol = slot;
         BKE_object_material_slot_remove(bmain, ob);